Report file writes run one at a time (closes #150)

Two writes that failed at the same time put their reports back in the
wrong order when the older one failed first. A mutex is now held
across each write, from taking the waiting reports out of the buffer
to putting them back if the write fails, so the next write takes them
in order. The write Append starts at the 10 MiB threshold waits for
one under way, and Append starts no other while it waits. Files
written since the start are now deleted to make room in the order they
were written. A test that needed two writes at once is removed.

Model: opus-5-5

README.md

@@ -86,7 +86,9 @@ either one is broken. We provide:
 - `script/frontend-fmt-check` — check prettier formatting (read-only)
 - `script/frontend-viewport-test` — responsive-layout verification of the built
   frontend in a containerised headless Chrome (see
-  [test/viewport/README.md](test/viewport/README.md)). Not part of
+  [test/viewport/README.md](test/viewport/README.md)). Before the browser
+  starts, it checks that the page, and a path that falls back to it, are served
+  with `Cache-Control: no-cache` and `Content-Security-Policy`. Not part of
   `script/check`: it takes minutes.
 - `script/docker` — build the image from `Dockerfile` without the build cache,
   tagged `netwatch` via `script/projectname`
@@ -153,8 +155,26 @@ report a delta, so only new samples are sent; the mark advances only on a
 delivered report, and while paused nothing is sent. Changing the interval clears
 each host's history on the page; the samples in it not yet reported still go in
 the next report. Delivery failure is quiet — one debug-log line per outage,
-retried at the next interval, never blocking probing. The report-building step
+never blocking probing. The report-building step is a pure function of host
-is a pure function of host state.
+state.
+
+A sample in the history that no report has delivered is sent only as long as the
+history holds it, so after a refused or failed report the next report carries
+the samples the history still has. Samples an interval change cleared from the
+history are kept apart, the last 100 per host, until they are delivered. The
+history keeps each host's last 100 rounds: 100 seconds at the 1-second interval,
+200 seconds at 2 seconds, 5 minutes at the default of 3 seconds and 100 minutes
+at 60 seconds. At 1 second, one refused or failed report loses the oldest 20
+seconds of each host's samples, as the next delivered report comes 2 minutes
+after the last. At 2 seconds and longer one loses nothing; samples are lost from
+the third refused report in a row at 2 seconds, from the fifth at 3 seconds, and
+later still at the longer intervals.
+
+No report is sent while the page is paused, but each paused round still adds an
+entry to the history, so each report due during a pause counts as a refused one.
+A pause longer than the history spans loses the samples in the history recorded
+between the last delivered report and the pause: up to a minute's worth for each
+host if the report before the pause was delivered.
 
 ### Backend
 
@@ -212,8 +232,15 @@ replaces the default; set and empty, the server trusts no proxy. Loopback is not
 in the default, so a reverse proxy on the same host has to be named, as in
 `TRUSTED_PROXIES=127.0.0.1/32`. A request whose direct peer is outside this set
 has its forwarded headers ignored, and the direct peer is logged and
-rate-limited instead. The container image does not use this default; see
+rate-limited instead. From a direct peer inside the set, the client address is
-[Deployment](#deployment).
+the right-most `X-Forwarded-For` entry that is not in the set: each proxy
+appends the address it took the request from, so the entries further left were
+written by the client and are not used. An entry written as an address with a
+port, such as `198.51.100.7:5000` or `[2001:db8::1]:443`, is read as that
+address. If the search from the right finds only addresses in the set, or meets
+an entry that is not an address at all, such as `unknown`, the direct peer is
+used. `X-Real-IP` is read only when `X-Forwarded-For` is missing or empty. The
+container image does not use this default; see [Deployment](#deployment).
 
 A variable set to a value the server cannot use, such as `PORT=abc`,
 `DEBUG=maybe` or a `BIND_ADDRESS` that is not an IP address, stops it from
@@ -232,18 +259,22 @@ toward `DATA_DIR_MAX_BYTES` from the next start, only if removing it fails too.
 Files are created with `O_EXCL` to prevent overwrites.
 
 The reports of a failed write stay in memory, and the next flush, within a
-minute, tries again to write them. From the failed write until a write succeeds,
+minute, tries again to write them. The server writes one file at a time, so a
-`POST /api/v1/reports` refuses every report with 500 and keeps nothing of it, so
+file holds its reports in the order they arrived, those of a failed write
-the page keeps its samples and sends them again at its next report. Reports
+included. From the failed write until a write succeeds, `POST /api/v1/reports`
-already answered 200 are lost only if the server stops before a write succeeds;
+refuses every report with 500 and keeps nothing of it. The page sends the
-the final flush at stop tries them too, and if it fails, the process exits
+refused samples again in its next report, as long as it still holds them: at the
-non-zero.
+1-second interval one refused report already loses some of those in the history
+(see [Reporting](#reporting)). Reports already answered 200 are lost only if the
+server stops before a write succeeds; the final flush at stop tries them too,
+and if it fails, the process exits non-zero.
 
 #### Report limits
 
 `POST /api/v1/reports` takes reports from anyone who can reach it, without
 credentials, so it is bounded instead. Each refusal below answers with the same
-`{"status":"error"}` body as any other error.
+`{"status":"error"}` body as any other error, except nginx's refusal of a body
+over the limit in the image.
 
 - **JSON only.** A report must be sent with `Content-Type: application/json`,
   parameters such as `charset` allowed, as the page sends it; any other is
@@ -251,6 +282,16 @@ credentials, so it is bounded instead. Each refusal below answers with the same
   send a `text/plain` POST, or a form, without a CORS preflight; refusing them
   keeps any site from making its visitors' browsers store reports (see
   [CORS](#cors)).
+- **Body limit.** A report body may be at most 1 MiB, the backend's limit for
+  any request. Run on its own, the backend answers 413 before the route to a
+  request that declares a longer body. Without a declared length, the route
+  answers 413 only when the report it reads runs past 1 MiB; nothing after the
+  end of the report is read, so a longer body whose report ends sooner is
+  stored, and one that stops being valid JSON sooner is answered 400. In the
+  image, nginx refuses a body over 1 MiB first, with or without a declared
+  length, as its `client_max_body_size`, which `configs/nginx.conf` does not
+  set, is 1 MiB by default. nginx answers 413 with its own HTML error page, and
+  the backend never sees the request.
 - **Rate limit.** Each client address, resolved through `TRUSTED_PROXIES`, may
   send `REPORTS_PER_MINUTE` reports a minute; past that it gets 429 with
   `Retry-After: 60`. The minute slides: reports from the minute before still
@@ -274,6 +315,12 @@ credentials, so it is bounded instead. Each refusal below answers with the same
   how much of the newest reports is kept: the default of 1 GiB is small enough
   for any host; set it to the space you can give `DATA_DIR`.
 
+At the defaults, one address may send 60 reports of up to 1 MiB a minute, and a
+report made up to compress badly takes about as much room in the report files as
+its request body, so in about 17 minutes one address can fill the 1 GiB cap with
+its own reports, every report stored before them deleted to make room. Lowering
+`REPORTS_PER_MINUTE` or raising `DATA_DIR_MAX_BYTES` makes that take longer.
+
 #### CORS
 
 The page calls the API from the origin it is served from, so by default the
@@ -295,12 +342,16 @@ the metrics record its duration and response size, labelled with its path,
 method and status; they also count those requests in progress, and include Go's
 runtime and process metrics. No other request is recorded: not those to
 `/metrics` itself, and not those answered before they reach either route, such
-as a CORS preflight, or a request refused with 404 for a path no route has, 405
+as a CORS preflight, or a request refused with 404 for a path no route has or
-for a method its route does not take, or 413 for declaring a body length over
+405 for a method its route does not take. Run on its own, the backend also
-the 1 MiB limit. A report whose body goes over the limit without declaring its
+answers 413 before the route to a request declaring a body length over the 1 MiB
-length reaches the route, is answered 413 there, and is recorded with that
+limit, which is not recorded. A body over the limit sent without a declared
-status. Clients can make up any number of paths and methods, and each would add
+length reaches the route and is recorded with the status the route answers,
-labels to the metrics for as long as the server runs.
+which is 413 only when the report it reads runs past the limit: nothing after
+the end of the report is read. In the image neither reaches the backend: nginx
+answers both with 413 itself (see [Report limits](#report-limits)), and nothing
+of them is recorded. Clients can make up any number of paths and methods, and
+each would add labels to the metrics for as long as the server runs.
 
 `/metrics` is rate limited, so that its password cannot be guessed quickly: each
 client address, resolved through `TRUSTED_PROXIES`, may make 60 requests to it a
@@ -428,7 +479,10 @@ address nginx passes on and no other. The image:
   each entry with `netwatch-server check-cidr`, which parses it as the backend
   parses its own `TRUSTED_PROXIES`
 - Sends access logs to stdout
-- Caches static assets with immutable headers
+- Sends the page with `Cache-Control: no-cache`, so a browser that keeps a copy
+  checks with the server that it is current before using it again; only the
+  files under `/assets/`, whose names change whenever their contents do, are
+  cached as immutable
 - Sends the security headers `REPO_POLICIES.md` requires on every response, as
   `configs/security-headers.conf` sets them, in place of the backend's own
 - Stores reports in `DATA_DIR`, `/data/reports` by default, on the `/data`

TODO.md

@@ -21,6 +21,108 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: report file writes no longer overlap
+  ([#150](https://git.eeqj.de/sneak/netwatch/issues/150)), so the reports of
+  failed writes are written in the order they arrived. Two writes that failed at
+  the same time put their reports back in the wrong order when the older failed
+  first. One write runs at a time, and each takes the waiting reports only when
+  it starts, after a failed write has put its own back. The write that 10 MiB
+  waiting starts waits for one under way, and only one such write waits at a
+  time. A unit test holds a write open, starts a second at 10 MiB, fails both
+  and checks the order in the next file. Files written since the start are
+  deleted to make room in the order they were written, after those found at
+  start
+- 2026-10-18: when the backend runs on its own, a `DATA_DIR` that cannot be
+  created or read, or where no file can be created, stops the start with an
+  error naming `DATA_DIR` and its value, as the other settings' errors do
+  ([#148](https://git.eeqj.de/sneak/netwatch/issues/148)); the first two named
+  only the path, and the third was found only at the first write of a report
+  file. At start the server creates a file in `DATA_DIR` and removes it. Unit
+  tests start the buffer with `DATA_DIR` below a regular file and with
+  `DATA_DIR` set to `/proc`, and check the error. The report file number is an
+  `atomic.Int64`, as the Go style guide asks for int types
+  ([#161](https://git.eeqj.de/sneak/netwatch/issues/161)); file names are
+  unchanged
+- 2026-10-18: the image's nginx sends the page with `Cache-Control: no-cache`
+  ([#141](https://git.eeqj.de/sneak/netwatch/issues/141)), so a browser that
+  keeps a copy checks with the server that it is current before using it again.
+  The page went out with no `Cache-Control`, so a browser could keep it after a
+  deploy and ask for files under `/assets/` that the new build no longer has,
+  and show an empty page. A path that `location /` answers with the page through
+  its `try_files` fallback gets the same header. `location /` now includes the
+  security headers again, as `/assets/` does, since its own `add_header` drops
+  the server's. `/assets/` keeps its immutable caching.
+  `make frontend-viewport-test` fails unless `/` and a path that falls back to
+  the page carry `Cache-Control: no-cache` and `Content-Security-Policy`, which
+  comes from the same file as the other security headers
+- 2026-10-18: `README.md` says what the page loses while reports are refused or
+  the page is paused ([#145](https://git.eeqj.de/sneak/netwatch/issues/145)),
+  how fast one address can replace the stored reports
+  ([#146](https://git.eeqj.de/sneak/netwatch/issues/146)), and that in the image
+  nginx answers a body over 1 MiB itself
+  ([#147](https://git.eeqj.de/sneak/netwatch/issues/147)). A sample in the
+  history that no report has delivered is sent only as long as the history holds
+  it: at the 1-second interval one refused or failed report loses the oldest 20
+  seconds of each host's samples, and a pause longer than the history loses
+  every sample between the last delivered report and the pause. Samples an
+  interval change cleared are kept apart, the last 100 per host, until they are
+  delivered. At the defaults, one address sending 1 MiB reports made up to
+  compress badly can fill the 1 GiB cap in about 17 minutes. nginx's
+  `client_max_body_size` is also 1 MiB by default, so in the image it answers a
+  larger body with 413 and its own HTML page, and the backend never sees or
+  records it; the backend's own 413 sentences now say they apply to it run on
+  its own. The reporter, the body limit and nginx are unchanged
+- 2026-10-18: a browser that cannot format the header's clocks still probes
+  ([#149](https://git.eeqj.de/sneak/netwatch/issues/149)). The clocks name the
+  local time zone through `Intl.DateTimeFormat`'s `shortGeneric` option, which
+  Chrome before 95, Firefox before 91 and Safari before 15.4 reject with an
+  error; it came before the first round started, so such a browser never checked
+  a target. The clocks now start last, after the rounds, and an error formatting
+  them leaves both blank. The footer's color legend reads `>=500ms` for red,
+  where it read `>500ms`, so 500 ms, which is red, is in one of its colors
+  ([#152](https://git.eeqj.de/sneak/netwatch/issues/152)). Unit tests check the
+  clocks with a working `Intl.DateTimeFormat` and with one that rejects
+  `shortGeneric`, and draw the page and check the legend against every entry of
+  `CONFIG.latencyColors`
+- 2026-10-18: `netwatch-server prepare-data-dir` no longer changes the owner of
+  a file outside `/data` through a hard link in the volume
+  ([#143](https://git.eeqj.de/sneak/netwatch/issues/143)). A file with a second
+  name (hard link) keeps its owner, and the start logs its path. Owners are now
+  set on the opened directory, file or link, never by name, so a hard link
+  swapped in after the check is caught too; a symbolic link is opened without
+  following it and given to the netwatch user itself, so the server can still
+  follow one in a sticky directory that anyone can write to. Devices, named
+  pipes and sockets keep their owner. Setting the owner on an open file that is
+  not a directory is built only for Linux: `prepare-data-dir` runs only in the
+  Linux image, and macOS, where the backend is also built for development, has
+  neither `O_PATH` nor `AT_EMPTY_PATH`. On any other system the package still
+  builds, and `prepare-data-dir` fails at the first file that is not a
+  directory. The test that runs as root hard-links a file, a symbolic link and a
+  named pipe from outside the volume into it and finds their owners unchanged
+- 2026-10-18: from a trusted proxy, the backend takes the client address from
+  the right-most `X-Forwarded-For` entry that is not in `TRUSTED_PROXIES`
+  ([#144](https://git.eeqj.de/sneak/netwatch/issues/144)). It took the left-most
+  entry, which the client writes itself behind a proxy that appends to the
+  header, so a client could choose the address the rate limit counts and the
+  request log records. An entry written as an address with a port is read as
+  that address. With only trusted addresses in the header, or an entry that is
+  not an address at all (such as `unknown`) met on the way, the direct peer is
+  used. `X-Real-IP` is read only without `X-Forwarded-For`, and every
+  `X-Forwarded-For` header line is read, not only the first. The image's nginx,
+  and the `TRUSTED_PROXIES=127.0.0.1/32` it starts the backend with, are
+  unchanged; the comment in `configs/nginx.conf` that said the backend takes the
+  first entry now says it takes the one address nginx passes on. Unit tests
+  resolve the address from chains ending in an untrusted address, in trusted
+  ones, of only trusted ones, with ports, with an entry that is not an address,
+  and split over two header lines
+- 2026-10-18: after an interval change, the summary's Reachable slot reads
+  `--/--` in white, as at page load, until the second round since the change
+  ends ([#140](https://git.eeqj.de/sneak/netwatch/issues/140)). It used to read
+  0/26 in red from the change on, as no WAN host had a check recorded, for up to
+  two intervals with the network fine. The summary update now writes `--/--`
+  while every WAN host still waits for its first check. A unit test resets the
+  hosts as the interval menu does and checks the slot; another checks that it
+  reads 0/26 in red once every WAN host's check has failed
 - 2026-10-14: stopping the server waits for a report file write already under
   way, the periodic flush's or the one `Append` starts when 10 MiB are waiting,
   before the final flush

bin/entrypoint.sh

@@ -66,10 +66,13 @@ done > /etc/nginx/trusted-proxies.conf
 # netwatch-server keeps its report files in DATA_DIR, on the /data
 # volume, which may be a host directory owned by root or by another
 # uid. Here, as root, netwatch-server prepare-data-dir creates DATA_DIR
-# and gives /data and everything in it to the netwatch user, so the
+# and gives /data and the directories, regular files and symbolic links
-# host directory needs no preparing. It stops the start, naming
+# in it to the netwatch user, so the host directory needs no preparing.
-# DATA_DIR, unless DATA_DIR is /data or a path below it, and it acts on
+# A file with a second name (hard link) keeps its owner, as that name
-# nothing outside /data, whatever symbolic links it meets there.
+# could be outside /data, and its path is logged. It stops the start,
+# naming DATA_DIR, unless DATA_DIR is /data or a path below it, and it
+# acts on nothing outside /data, whatever symbolic or hard links it
+# meets there.
 export DATA_DIR="${DATA_DIR:-/data/reports}"
 netwatch-server prepare-data-dir "$DATA_DIR" || exit 1
 

configs/nginx.conf

@@ -30,8 +30,13 @@ server {
     access_log /dev/stdout combined;
     error_log  /dev/stderr warn;
 
+    # With no-cache, a browser never uses a copy of the page that names
+    # assets a newer build no longer has without first checking with the
+    # server that it is current.
     location / {
         try_files $uri $uri/ /index.html;
+        add_header Cache-Control no-cache;
+        include /etc/nginx/security-headers.conf;
     }
 
     # Cache static assets aggressively
@@ -46,7 +51,9 @@ server {
     # 127.0.0.1:8081. These headers go with every request passed to it.
     # X-Forwarded-For carries only the client address, as resolved by
     # the real IP settings above, and not the chain the request came
-    # with: the backend takes the first entry, which a client can write.
+    # with. bin/entrypoint.sh starts the backend with
+    # TRUSTED_PROXIES=127.0.0.1/32, the address nginx connects from, so
+    # the backend takes that one entry as the client address.
     proxy_set_header Host $host;
     proxy_set_header X-Real-IP $remote_addr;
     proxy_set_header X-Forwarded-For $remote_addr;

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/viper v1.21.0
 	go.uber.org/fx v1.24.0
+	golang.org/x/sys v0.47.0
 )
 
 require (
@@ -41,7 +42,6 @@ require (
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/sys v0.47.0 // indirect
 	golang.org/x/text v0.40.0 // indirect
 	google.golang.org/protobuf v1.36.11 // indirect
 )

internal/middleware/middleware.go

@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"net/netip"
 	"runtime/debug"
+	"slices"
 	"strings"
 	"time"
 
@@ -133,6 +134,17 @@ func ipFromHostPort(hostPort string) string {
 // X-Real-IP are honoured only when the direct peer is a
 // trusted proxy; otherwise the direct peer is returned so a
 // spoofed header cannot forge the logged address.
+//
+// Each proxy appends to X-Forwarded-For the address it took the
+// request from, so the entries on the right are written by the
+// trusted proxies and those further left by the client. The
+// client's address is therefore the right-most entry that is not
+// a trusted proxy. An entry written as an address with a port is
+// read as that address, both for the trust check and as the
+// result. When every entry is a trusted proxy, or the walk from
+// the right meets an entry that is not an address at all (such as
+// "unknown"), the peer is used. X-Real-IP is read only when there
+// is no X-Forwarded-For entry at all.
 func clientIP(
 	remoteAddr string,
 	header http.Header,
@@ -144,28 +156,49 @@ func clientIP(
 		return peer
 	}
 
-	if xff := firstForwardedFor(header.Get("X-Forwarded-For")); xff != "" {
+	forwarded := forwardedFor(header)
-		return xff
+
+	if len(forwarded) == 0 {
+		if xr := strings.TrimSpace(header.Get("X-Real-IP")); validIP(xr) {
+			return xr
+		}
+
+		return peer
 	}
 
-	if xr := strings.TrimSpace(header.Get("X-Real-IP")); validIP(xr) {
+	for _, entry := range slices.Backward(forwarded) {
-		return xr
+		if !validIP(entry) {
+			return peer
+		}
+
+		if !addrInAny(entry, trusted) {
+			return entry
+		}
 	}
 
 	return peer
 }
 
-// firstForwardedFor returns the left-most valid address in an
+// forwardedFor returns the entries of X-Forwarded-For in order,
-// X-Forwarded-For list (the original client), or "" if none.
+// leaving out blank ones. A proxy may add its entry as a header
-func firstForwardedFor(value string) string {
+// line of its own rather than append it to the line the client
+// sent, so every line is read, not only the first. Some proxies
+// write an entry as an address with a port, such as
+// 198.51.100.7:5000 or [2001:db8::1]:443; the port is removed so
+// that the entry is read as its address.
+func forwardedFor(header http.Header) []string {
+	value := strings.Join(header.Values("X-Forwarded-For"), ",")
+
+	var entries []string
+
 	for part := range strings.SplitSeq(value, ",") {
-		candidate := strings.TrimSpace(part)
+		entry := strings.TrimSpace(part)
-		if validIP(candidate) {
+		if entry != "" {
-			return candidate
+			entries = append(entries, ipFromHostPort(entry))
 		}
 	}
 
-	return ""
+	return entries
 }
 
 func validIP(s string) bool {

internal/middleware/middleware_test.go

@@ -25,10 +25,16 @@ import (
 const (
 	// loopbackPeer is a remote address inside the trusted-proxy allowlist.
 	loopbackPeer = "127.0.0.1:5000"
+	// loopbackIP is the address of loopbackPeer, which ClientIP returns
+	// when it uses the peer.
+	loopbackIP = "127.0.0.1"
 	// forwardedIP is the client address presented via X-Forwarded-For.
 	forwardedIP = "203.0.113.7"
 	// realIP is the client address presented via X-Real-IP.
 	realIP = "203.0.113.9"
+	// appendedIP is the X-Forwarded-For entry a trusted proxy appended
+	// after the entries the client wrote.
+	appendedIP = "198.51.100.7"
 )
 
 func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
@@ -85,10 +91,35 @@ func clientIPCases() []clientIPCase {
 			want:       forwardedIP,
 		},
 		{
-			name:       "trusted proxy uses left-most of chain",
+			name:       "trusted proxy uses right-most of chain",
-			remoteAddr: "10.1.2.3:5000",
+			remoteAddr: loopbackPeer,
-			xff:        forwardedIP + ", 10.1.2.3",
+			xff:        "203.0.113.9, " + appendedIP,
-			want:       forwardedIP,
+			want:       appendedIP,
+		},
+		{
+			name:       "trusted proxy entries are skipped from the right",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, 10.0.0.5",
+			want:       "203.0.113.9",
+		},
+		{
+			name:       "chain of only trusted proxies uses peer",
+			remoteAddr: loopbackPeer,
+			xff:        "10.0.0.5, 10.1.2.3",
+			want:       loopbackIP,
+		},
+		{
+			name:       "entry that is not an address uses peer",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, unknown",
+			want:       loopbackIP,
+		},
+		{
+			name:       "x-real-ip ignored when forwarded-for has entries",
+			remoteAddr: loopbackPeer,
+			xff:        "10.0.0.5",
+			xRealIP:    realIP,
+			want:       loopbackIP,
 		},
 		{
 			name:       "trusted proxy falls back to x-real-ip",
@@ -117,7 +148,7 @@ func clientIPCases() []clientIPCase {
 			name:       "trusted proxy with garbage header uses peer",
 			remoteAddr: loopbackPeer,
 			xff:        "not-an-ip",
-			want:       "127.0.0.1",
+			want:       loopbackIP,
 		},
 	}
 }
@@ -148,6 +179,47 @@ func TestClientIP(t *testing.T) {
 	}
 }
 
+// TestClientIPReadsEveryForwardedForLine checks that an entry a proxy
+// adds as an X-Forwarded-For line of its own is read, and the line the
+// client sent before it is not taken.
+func TestClientIPReadsEveryForwardedForLine(t *testing.T) {
+	t.Parallel()
+
+	header := http.Header{}
+	header.Add("X-Forwarded-For", "203.0.113.9")
+	header.Add("X-Forwarded-For", appendedIP)
+
+	got := middleware.ClientIP(loopbackPeer, header,
+		mustPrefixes(t, "127.0.0.1/32"))
+	if got != appendedIP {
+		t.Errorf("ClientIP() = %q, want %q", got, appendedIP)
+	}
+}
+
+// TestClientIPReadsAddressWithPort checks that an X-Forwarded-For entry
+// written as an address with a port is read as that address, both for
+// the trust check and as the result.
+func TestClientIPReadsAddressWithPort(t *testing.T) {
+	t.Parallel()
+
+	trusted := mustPrefixes(t, "127.0.0.1/32", "10.0.0.0/8")
+
+	for xff, want := range map[string]string{
+		"203.0.113.9, " + appendedIP + ":5000": appendedIP,
+		"203.0.113.9, [2001:db8::1]:443":       "2001:db8::1",
+		"203.0.113.9, 10.0.0.5:5000":           "203.0.113.9",
+	} {
+		header := http.Header{}
+		header.Set("X-Forwarded-For", xff)
+
+		got := middleware.ClientIP(loopbackPeer, header, trusted)
+		if got != want {
+			t.Errorf("X-Forwarded-For %q: ClientIP() = %q, want %q",
+				xff, got, want)
+		}
+	}
+}
+
 func TestSecurityHeaders(t *testing.T) {
 	t.Parallel()
 

internal/reportbuf/datadir.go

@@ -18,9 +18,15 @@ var ErrDataDirOutsideVolume = errors.New(
 // PrepareDataDir gets dir, the server's DATA_DIR, ready for owner, the
 // user the server runs as, so that a host directory mounted at volume,
 // /data in the image, needs no preparing: it creates dir, gives volume
-// and everything in it to owner, and gives volume and dir the mode the
+// and the directories, regular files and symbolic links in it to
-// server gives a directory it creates. dir must be volume or a path
+// owner, and gives volume and dir the mode the server gives a directory
-// below it, with no '.', '..', empty part or '/' at the end.
+// it creates. It leaves a file with a second name (hard link) as it
+// is, as that name could be outside volume, and logs its path; it
+// leaves a device, named pipe or socket as it is too (see chownFile).
+// dir must be volume or a path below it, with no '.', '..', empty part
+// or '/' at the end. On any system but Linux, the system of the image, it
+// fails at the first file in volume that is not a directory (see
+// chownFile).
 //
 // bin/entrypoint.sh runs this as root, which would follow a symbolic
 // link anywhere, so every step goes through an os.Root opened on
@@ -32,10 +38,9 @@ var ErrDataDirOutsideVolume = errors.New(
 // open, never following a link at the name it creates, and follows a
 // link on the path only as the os.Root allows, so a relative link
 // inside volume can lead it to create directories elsewhere inside
-// volume. Lchown never changes what a link points to, and the modes
+// volume. The owners and modes are set on files already opened (see
-// are set on directories already opened (see chmodDir), so the most
+// chownAll and chmodDir), so the most that process can do is make a
-// that process can do is make a step fail or wait, or act on
+// step fail or wait, or act on something else inside volume.
-// something else inside volume.
 func PrepareDataDir(volume, dir string, owner *user.User) error {
 	// rel is dir as a path from volume; IsLocal is false for one that
 	// leads out of it.
@@ -66,7 +71,7 @@ func PrepareDataDir(volume, dir string, owner *user.User) error {
 		return err
 	}
 
-	err = lchownAll(root, ".", uid, gid)
+	err = chownAll(root, ".", uid, gid)
 	if err != nil {
 		return err
 	}
@@ -79,28 +84,23 @@ func PrepareDataDir(volume, dir string, owner *user.User) error {
 	return chmodDir(root, rel)
 }
 
-// lchownAll gives name, a directory inside root, and everything in it
+// chownAll gives name, a directory inside root, and the directories,
-// to uid and gid. It reads each directory opened through root, not
+// regular files and symbolic links in it to uid and gid, but not the
-// through root.FS(), which refuses a name that is not valid UTF-8, and
+// files chownFile leaves. It opens each through root and gives the
-// calls Lchown on every entry, which gives a symbolic link itself to
+// open file to them, as chmodDir sets a mode on the open directory.
-// them, not what it points to. It goes into an entry only when the
+// Root.Lchown would not hold: it acts on whatever is at the name by
-// read found a directory there, so it follows no link it finds; one
+// then, and a hard link swapped in there shares its owner with a file
-// swapped in for that directory afterwards is followed only as the
+// that could be outside root. A symbolic link is given to them itself,
-// os.Root allows.
+// never what it points to, because in a directory that anyone can
-func lchownAll(root *os.Root, name string, uid, gid int) error {
+// write to and that has the sticky bit set, as /tmp has, the kernel
-	err := root.Lchown(name, uid, gid)
+// follows a link only for the link's owner or when the link and the
-	if err != nil {
+// directory have the same owner. chownAll goes into an entry only when
-		return err
+// the read found a directory there, so it follows no link it finds;
-	}
+// one swapped in afterwards for a directory is followed only as the
-
+// os.Root allows, and one swapped in for any other file is not
-	dir, err := root.Open(name)
+// followed at all (see chownFile).
-	if err != nil {
+func chownAll(root *os.Root, name string, uid, gid int) error {
-		return err
+	entries, err := chownDir(root, name, uid, gid)
-	}
-
-	entries, err := dir.ReadDir(-1)
-	_ = dir.Close()
-
 	if err != nil {
 		return err
 	}
@@ -108,9 +108,9 @@ func lchownAll(root *os.Root, name string, uid, gid int) error {
 	for _, entry := range entries {
 		entryName := filepath.Join(name, entry.Name())
 		if entry.IsDir() {
-			err = lchownAll(root, entryName, uid, gid)
+			err = chownAll(root, entryName, uid, gid)
 		} else {
-			err = root.Lchown(entryName, uid, gid)
+			err = chownFile(root, entryName, uid, gid)
 		}
 
 		if err != nil {
@@ -121,6 +121,36 @@ func lchownAll(root *os.Root, name string, uid, gid int) error {
 	return nil
 }
 
+// chownDir gives name, a directory inside root, to uid and gid, and
+// returns what it holds. Like chmodDir, it opens name through root and
+// refuses anything but a directory, which has no second name (hard
+// link). It reads the open directory, not root.FS(), which refuses a
+// name that is not valid UTF-8.
+func chownDir(root *os.Root, name string, uid, gid int) ([]fs.DirEntry, error) {
+	dir, err := root.Open(name)
+	if err != nil {
+		return nil, err
+	}
+
+	defer func() { _ = dir.Close() }()
+
+	info, err := dir.Stat()
+	if err != nil {
+		return nil, err
+	}
+
+	if !info.IsDir() {
+		return nil, &fs.PathError{Op: "chown", Path: name, Err: syscall.ENOTDIR}
+	}
+
+	err = dir.Chown(uid, gid)
+	if err != nil {
+		return nil, err
+	}
+
+	return dir.ReadDir(-1)
+}
+
 // chmodDir gives name, a directory inside root, the mode the server
 // gives a directory it creates. Root.Chmod would not hold: on Linux it
 // checks that name is not a symbolic link, then sets the mode by name,

internal/reportbuf/datadir_linux.go

@@ -0,0 +1,65 @@
+//go:build linux
+
+package reportbuf
+
+import (
+	"io/fs"
+	"log/slog"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// chownFile gives name, a file inside root that is not a directory, to
+// uid and gid if it is a regular file or symbolic link with one name.
+// It leaves, and logs, a file of any kind with a second name (hard
+// link): all the names of a file share its owner, and the other could
+// be outside root. It leaves any other file with one name, such as a
+// device, named pipe or socket: the server needs to own none of them.
+// It opens name through root with O_PATH, which reads nothing from the
+// file, so opening a device or named pipe neither waits nor acts on
+// it; os.Root adds O_NOFOLLOW, and the two together open a symbolic
+// link itself, never what it points to. It reads the kind of file and
+// its number of names from the open file and gives that file to them,
+// so a hard link swapped in at name is caught too.
+//
+// This file is built only on Linux: prepare-data-dir runs only in the
+// Linux image, and macOS, where the backend is also built for
+// development, has neither O_PATH nor AT_EMPTY_PATH. On any other
+// system chownFile returns an error (see datadir_other.go).
+func chownFile(root *os.Root, name string, uid, gid int) error {
+	file, err := root.OpenFile(name, unix.O_PATH, 0)
+	if err != nil {
+		return err
+	}
+
+	defer func() { _ = file.Close() }()
+
+	info, err := file.Stat()
+	if err != nil {
+		return err
+	}
+
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok || stat.Nlink > 1 {
+		slog.Warn("owner not changed: file has a second name (hard link)",
+			"path", file.Name())
+
+		return nil
+	}
+
+	if !info.Mode().IsRegular() && info.Mode().Type() != fs.ModeSymlink {
+		return nil
+	}
+
+	// File.Chown would fail: fchown refuses a file opened with O_PATH.
+	// Fchownat with an empty name and AT_EMPTY_PATH acts on the open
+	// file instead.
+	err = unix.Fchownat(int(file.Fd()), "", uid, gid, unix.AT_EMPTY_PATH)
+	if err != nil {
+		return &fs.PathError{Op: "chown", Path: file.Name(), Err: err}
+	}
+
+	return nil
+}

internal/reportbuf/datadir_other.go

@@ -0,0 +1,21 @@
+//go:build !linux
+
+package reportbuf
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+)
+
+// chownFile returns an error on every system but Linux. Opening a file
+// that is not a directory without reading it or following it, and
+// giving it to a user on the open file, never by name, is built only
+// for Linux (see datadir_linux.go): prepare-data-dir runs only in the
+// Linux image, and macOS, where the backend is also built for
+// development, has neither O_PATH nor AT_EMPTY_PATH. This file lets
+// the package build elsewhere, such as on macOS, where a developer
+// runs make build and make run.
+func chownFile(_ *os.Root, name string, _, _ int) error {
+	return &fs.PathError{Op: "chown", Path: name, Err: errors.ErrUnsupported}
+}

internal/reportbuf/datadir_test.go

@@ -254,10 +254,13 @@ func TestPrepareDataDirTakesDirectoryNamedInLatin1(t *testing.T) {
 	requireOwner(t, filepath.Join(latin1, "f"), uint32(uid), uint32(gid))
 }
 
-// TestPrepareDataDirGivesVolumeToOwner gives everything in the volume
+// TestPrepareDataDirGivesVolumeToOwner gives the directories, regular
-// to a uid and gid that own nothing, which only root can do. A
+// files and symbolic links in the volume to a uid and gid that own
-// symbolic link in the volume to a directory outside it is given to
+// nothing, which only root can do. A symbolic link in the volume to a
-// them itself; what it points to is left as it was.
+// directory outside it is given to them itself; what it points to is
+// left as it was. A hard link to a file outside the volume, one to a
+// symbolic link outside it and one to a named pipe outside it are left
+// as they were, and so is a named pipe in the volume.
 func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 	t.Parallel()
 
@@ -274,6 +277,16 @@ func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	err = os.Symlink("f", filepath.Join(outside, "link"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = syscall.Mkfifo(filepath.Join(outside, "fifo"), 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	err = os.Mkdir(old, 0o700)
 	if err != nil {
 		t.Fatal(err)
@@ -289,6 +302,21 @@ func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	err = syscall.Mkfifo(filepath.Join(old, "fifo"), 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// os.Link does not follow a symbolic link: it gives the link itself
+	// a second name.
+	for _, name := range []string{"f", "link", "fifo"} {
+		err = os.Link(filepath.Join(outside, name),
+			filepath.Join(old, "hardlink-to-"+name))
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
 	err = reportbuf.PrepareDataDir(volume, filepath.Join(volume, reports),
 		&user.User{Uid: "4242", Gid: "4343"})
 	if err != nil {
@@ -296,12 +324,16 @@ func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 	}
 
 	for _, path := range []string{
-		volume, filepath.Join(volume, reports), old,
+		volume, filepath.Join(volume, reports), old, filepath.Join(old, "f"),
-		filepath.Join(old, "f"), filepath.Join(old, "link"),
+		filepath.Join(old, "link"),
 	} {
 		requireOwner(t, path, 4242, 4343)
 	}
 
-	requireOwner(t, outside, 0, 0)
+	for _, path := range []string{
-	requireOwner(t, filepath.Join(outside, "f"), 0, 0)
+		outside, filepath.Join(outside, "f"), filepath.Join(outside, "link"),
+		filepath.Join(outside, "fifo"), filepath.Join(old, "fifo"),
+	} {
+		requireOwner(t, path, 0, 0)
+	}
 }

internal/reportbuf/reportbuf.go

@@ -1,6 +1,9 @@
 // Package reportbuf accumulates telemetry reports in memory
 // and periodically flushes them to zstd-compressed JSONL files,
-// deleting the oldest files to keep them under a size cap.
+// deleting the oldest files to keep them under a size cap. Before the
+// server starts, PrepareDataDir gets the directory those files are
+// kept in, DATA_DIR, ready for the user the server runs as: it creates
+// it and gives it and the volume it is on to that user.
 package reportbuf
 
 import (
@@ -13,7 +16,6 @@ import (
 	"log/slog"
 	"os"
 	"path/filepath"
-	"slices"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -70,11 +72,9 @@ type Buffer struct {
 	// except in tests that hold the write open or make it fail.
 	fileCreated func(f *os.File)
 	// files are the report files that may be deleted to make room,
-	// in name order, which is oldest first: those in dataDir at
+	// oldest first: those in dataDir at start, in name order, then
-	// start, and each one this buffer writes, put in at its place by
+	// each one this buffer writes, once it is complete. A file still
-	// name once it is complete, even when an older file's write
+	// being written is not among them. filesBytes is their total size.
-	// completes after a newer one's. A file still being written is
-	// not among them. filesBytes is their total size.
 	files      []reportFile
 	filesBytes int64
 	log        *slog.Logger
@@ -85,8 +85,12 @@ type Buffer struct {
 	now func() time.Time
 	// seq numbers the report files, so that two named in the same
 	// millisecond still get different names.
-	seq      atomic.Uint64
+	seq      atomic.Int64
 	stopOnce sync.Once
+	// thresholdWriteWaiting is set while a write Append started at
+	// flushSizeThreshold waits for writeMu; Append starts no other
+	// while it is set.
+	thresholdWriteWaiting atomic.Bool
 	// usedBytes is what Append checks against maxBytes: the size
 	// of the report files in dataDir, plus the reports waiting to
 	// be written to one at their uncompressed size.
@@ -96,6 +100,11 @@ type Buffer struct {
 	// refuses reports, and the reports of that write wait in buf for
 	// the next flush.
 	writeErr error
+	// writeMu is held across each write of a report file, so writes
+	// never overlap: a failed write puts its reports back in buf before
+	// the next write takes them, which keeps them in the order they
+	// were appended.
+	writeMu sync.Mutex
 	// writers are the goroutines that write report files: flushLoop,
 	// and each one Append starts at flushSizeThreshold. OnStop waits
 	// for them before the final flush.
@@ -124,16 +133,33 @@ func New(
 
 	lc.Append(fx.Hook{
 		OnStart: func(_ context.Context) error {
+			// The errors name DATA_DIR, as config.New's do for the
+			// settings it checks.
 			err := os.MkdirAll(b.dataDir, dirPerms)
 			if err != nil {
-				return fmt.Errorf("create data dir: %w", err)
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
+			}
+
+			// Creating and removing a file stops the start when no
+			// file can be created in DATA_DIR, instead of the first
+			// write of a report file failing after reports were
+			// answered 200. A full disk is not caught: the empty
+			// file is still created.
+			f, err := os.CreateTemp(b.dataDir, "")
+			if err != nil {
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
+			}
+
+			err = errors.Join(f.Close(), os.Remove(f.Name()))
+			if err != nil {
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
 			}
 
 			// Report files left by earlier runs count too, and are
 			// the first to be deleted to make room.
 			files, err := reportFiles(b.dataDir)
 			if err != nil {
-				return err
+				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
 			}
 
 			b.mu.Lock()
@@ -187,8 +213,9 @@ func New(
 // write of a report file failed, until a write succeeds again. If the
 // line would take usedBytes past maxBytes, the oldest report files are
 // deleted to make room; it stores nothing and returns ErrFull if that
-// cannot make room. If the buffer reaches the size threshold, it is
+// cannot make room. If the buffer reaches the size threshold, it starts
-// drained and written to disk asynchronously.
+// a write of the reports in the background (see flushAtThreshold),
+// unless one it started is still waiting.
 func (b *Buffer) Append(v any) error {
 	line, err := json.Marshal(v)
 	if err != nil {
@@ -218,18 +245,9 @@ func (b *Buffer) Append(v any) error {
 	b.buf.Write(line)
 	b.buf.WriteByte('\n')
 
-	if b.buf.Len() >= flushSizeThreshold {
+	if b.buf.Len() >= flushSizeThreshold &&
-		data := b.drainBuf()
+		b.thresholdWriteWaiting.CompareAndSwap(false, true) {
-		b.mu.Unlock()
+		b.writers.Go(b.flushAtThreshold)
-
-		b.writers.Go(func() {
-			writeErr := b.writeFile(data)
-			if writeErr != nil {
-				b.log.Error("flush reports failed", "error", writeErr)
-			}
-		})
-
-		return nil
 	}
 
 	b.mu.Unlock()
@@ -287,21 +305,28 @@ func (b *Buffer) flushLoop() {
 	}
 }
 
-// flushLocked acquires the lock, drains the buffer, and
+// flushLocked waits for any write of a report file under way, then
-// writes the data to a compressed file.
+// writes the reports waiting in the buffer to a new one.
 func (b *Buffer) flushLocked() error {
-	b.mu.Lock()
+	b.writeMu.Lock()
+	defer b.writeMu.Unlock()
 
-	if b.buf.Len() == 0 {
+	return b.writeFile()
-		b.mu.Unlock()
+}
 
-		return nil
+// flushAtThreshold is the write Append starts when the buffer reaches
+// flushSizeThreshold. Once any write under way is done, it lets Append
+// start another and writes the reports waiting in the buffer.
+func (b *Buffer) flushAtThreshold() {
+	b.writeMu.Lock()
+	defer b.writeMu.Unlock()
+
+	b.thresholdWriteWaiting.Store(false)
+
+	err := b.writeFile()
+	if err != nil {
+		b.log.Error("flush reports failed", "error", err)
 	}
-
-	data := b.drainBuf()
-	b.mu.Unlock()
-
-	return b.writeFile(data)
 }
 
 // drainBuf copies the buffer contents and resets it.
@@ -314,10 +339,22 @@ func (b *Buffer) drainBuf() []byte {
 	return data
 }
 
-// writeFile writes data, reports drained from the buffer, to a new
+// writeFile takes the reports waiting in the buffer, if any, and writes
-// timestamped zstd-compressed JSONL file in the data directory. If the
+// them to a new timestamped zstd-compressed JSONL file in the data
-// write fails, it puts the reports back in the buffer.
+// directory. If the write fails, it puts the reports back in the
-func (b *Buffer) writeFile(data []byte) error {
+// buffer. The caller must hold b.writeMu.
+func (b *Buffer) writeFile() error {
+	b.mu.Lock()
+
+	if b.buf.Len() == 0 {
+		b.mu.Unlock()
+
+		return nil
+	}
+
+	data := b.drainBuf()
+	b.mu.Unlock()
+
 	// The timestamp comes first, so the names sort by time; the number
 	// after it tells apart files named in the same millisecond.
 	ts := b.now().UTC().Format("2006-01-02T15-04-05.000Z")
@@ -348,14 +385,7 @@ func (b *Buffer) writeFile(data []byte) error {
 	b.usedBytes -= int64(len(data))
 	b.usedBytes += size
 	b.filesBytes += size
-
+	b.files = append(b.files, reportFile{name: name, size: size})
-	// At its place by name, not at the end: another write, of a newer
-	// file, may have completed while this one was being written.
-	i, _ := slices.BinarySearchFunc(b.files, name,
-		func(f reportFile, target string) int {
-			return strings.Compare(f.name, target)
-		})
-	b.files = slices.Insert(b.files, i, reportFile{name: name, size: size})
 
 	return nil
 }

internal/reportbuf/reportbuf_test.go

@@ -108,6 +108,68 @@ func TestFailedFinalFlushFailsStop(t *testing.T) {
 	}
 }
 
+// TestUncreatableDataDirStopsStart sets DATA_DIR below a regular file,
+// where no directory can be created, even by root, as the tests run in
+// the backend image. The start must fail with an error naming DATA_DIR
+// and its value, as the errors for the other settings do.
+func TestUncreatableDataDirStopsStart(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "file")
+	writeBytes(t, file, 1)
+
+	dir := filepath.Join(file, "reports")
+	t.Setenv("DATA_DIR", dir)
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Invoke(func(*reportbuf.Buffer) {}),
+	)
+
+	err := app.Start(t.Context())
+	if err == nil {
+		t.Fatal("start succeeded with DATA_DIR below a regular file")
+	}
+
+	want := fmt.Sprintf("DATA_DIR %q: ", dir)
+	if !strings.Contains(err.Error(), want) {
+		t.Fatalf("start error = %q, want it to contain %q", err, want)
+	}
+}
+
+// TestUnwritableDataDirStopsStart sets DATA_DIR to /proc, which exists
+// and can be read, but where no file can be created, even by root, as
+// the tests run in the backend image: root ignores a directory the test
+// makes read-only. The start must fail with an error naming DATA_DIR and
+// its value, rather than the first write of a report file.
+func TestUnwritableDataDirStopsStart(t *testing.T) {
+	dir := "/proc"
+	t.Setenv("DATA_DIR", dir)
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Invoke(func(*reportbuf.Buffer) {}),
+	)
+
+	err := app.Start(t.Context())
+	if err == nil {
+		t.Fatal("start succeeded with DATA_DIR /proc")
+	}
+
+	want := fmt.Sprintf("DATA_DIR %q: ", dir)
+	if !strings.Contains(err.Error(), want) {
+		t.Fatalf("start error = %q, want it to contain %q", err, want)
+	}
+}
+
 // TestStopWaitsForPeriodicWrite holds open the write the periodic flush
 // starts, then stops the app. The stop must wait until the write is
 // released, and the report must then be in its file. The test runs in
@@ -604,18 +666,23 @@ func TestFileDeletedByHandFreesRoom(t *testing.T) {
 	}
 }
 
-// TestFileBeingWrittenIsNeverDeleted holds the write of one report file
+// TestFileBeingWrittenIsNeverDeleted holds the write of a report file
-// open while a second write completes, then sends a report that needs
+// open, then sends a report that needs room. The report file from an
-// room. Deleting either file would make it, and the one being written is
+// earlier run is deleted to make it, and the one being written is not.
-// the older, but only the complete one may be deleted. Once the first
+// Once the write is complete, its file is deleted when room is needed.
-// write is complete, its file is deleted when room is needed.
 func TestFileBeingWrittenIsNeverDeleted(t *testing.T) {
-	report := map[string]string{"id": "writing"}
+	const fileBytes = 100
 
-	t.Setenv("DATA_DIR", t.TempDir())
+	report := map[string]string{"id": "writing"}
-	// Room for two reports waiting to be written, but not for two
+	dir := t.TempDir()
-	// beside a report file.
+	earlier := reportFilePath(dir, 1)
-	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(2*lineBytes(t, report)))
+
+	writeBytes(t, earlier, fileBytes)
+
+	t.Setenv("DATA_DIR", dir)
+	// Room for the earlier file and one report.
+	t.Setenv("DATA_DIR_MAX_BYTES",
+		strconv.Itoa(fileBytes+lineBytes(t, report)))
 
 	buf := startBuffer(t)
 
@@ -639,24 +706,8 @@ func TestFileBeingWrittenIsNeverDeleted(t *testing.T) {
 
 	writing := <-created
 
-	// Only the first write is held; the second goes through, and
+	// Errorf, not Fatalf, until the write is released, so that a
-	// so do the writes after it, the final one at stop included.
-	var complete string
-
-	buf.OnFileCreated(func(f *os.File) { complete = f.Name() })
-
-	// Errorf, not Fatalf, until the first write is released, so that a
 	// failure here does not leave it held.
-	err = buf.Append(report)
-	if err != nil {
-		t.Errorf("second report: %v", err)
-	}
-
-	err = buf.Flush()
-	if err != nil {
-		t.Errorf("flush of the second report: %v", err)
-	}
-
 	err = buf.Append(report)
 	if err != nil {
 		t.Errorf("report that needs room: %v", err)
@@ -666,10 +717,14 @@ func TestFileBeingWrittenIsNeverDeleted(t *testing.T) {
 		t.Error("report file deleted while it was being written")
 	}
 
-	if exists(t, complete) {
+	if exists(t, earlier) {
-		t.Error("complete report file kept, though room was needed")
+		t.Error("earlier report file kept, though room was needed")
 	}
 
+	// Only this write is held; the writes after it, the final one at
+	// stop included, go through.
+	buf.OnFileCreated(func(*os.File) {})
+
 	close(release)
 
 	err = <-flushed
@@ -677,9 +732,13 @@ func TestFileBeingWrittenIsNeverDeleted(t *testing.T) {
 		t.Fatalf("flush of the first report: %v", err)
 	}
 
-	err = buf.Append(report)
+	// As large as the earlier file, so it fits only once the file just
+	// written is deleted too.
+	pad := fileBytes - lineBytes(t, map[string]string{"id": ""})
+
+	err = buf.Append(map[string]string{"id": strings.Repeat("a", pad)})
 	if err != nil {
-		t.Fatalf("report after the first write was complete: %v", err)
+		t.Fatalf("report after the write was complete: %v", err)
 	}
 
 	if exists(t, writing) {
@@ -821,6 +880,82 @@ func TestFailedWriteKeepsReportsAppendedDuringIt(t *testing.T) {
 	}
 }
 
+// TestFailedWritesKeepReportsInOrder holds a write open, then appends a
+// report that takes the buffer to FlushSizeThreshold, so that Append
+// starts a second write. The held write fails first, then the second,
+// which waited for it and took both reports. The next flush writes them
+// in the order they were appended, the held write's first.
+func TestFailedWritesKeepReportsInOrder(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	buf := startBuffer(t)
+
+	created := make(chan struct{})
+	release := make(chan struct{})
+
+	// Each write is held until release is closed, then fails, as its
+	// file is closed under it.
+	buf.OnFileCreated(func(f *os.File) {
+		created <- struct{}{}
+
+		<-release
+
+		_ = f.Close()
+	})
+
+	err := buf.Append(map[string]string{"id": "held"})
+	if err != nil {
+		t.Fatalf("report of the held write: %v", err)
+	}
+
+	flushed := make(chan error)
+
+	go func() { flushed <- buf.Flush() }()
+
+	<-created
+
+	big := strings.Repeat("a", reportbuf.FlushSizeThreshold)
+
+	// Errorf, not Fatalf, until the held write is released, so that a
+	// failure here does not leave it held.
+	err = buf.Append(map[string]string{"id": big})
+	if err != nil {
+		t.Errorf("report that starts the second write: %v", err)
+	}
+
+	close(release)
+
+	err = <-flushed
+	if err == nil {
+		t.Error("held write succeeded, though its file was closed under it")
+	}
+
+	// The second write has created its file, so it has taken the
+	// reports.
+	<-created
+
+	// Writes from here on, the final one at stop included, succeed.
+	buf.OnFileCreated(func(*os.File) {})
+
+	// Flush waits for the second write to fail before it writes.
+	err = buf.Flush()
+	if err != nil {
+		t.Fatalf("flush after the failed writes: %v", err)
+	}
+
+	files, err := readReportFiles(dir)
+	if err != nil {
+		t.Fatalf("read report files: %v", err)
+	}
+
+	want := `{"id":"held"}` + "\n" + `{"id":"` + big + `"}` + "\n"
+	if len(files) != 1 || files[0] != want {
+		t.Fatalf("%d report files, want one holding the held write's "+
+			"report, then the second one", len(files))
+	}
+}
+
 // TestReportsRefusedWhileWritesFail posts reports through the report
 // handler, as the page does. Once a write has failed, a report is
 // refused with 500 until a write succeeds again; the reports taken
@@ -1233,85 +1368,3 @@ func hasReportFile(t *testing.T, dir string) bool {
 
 	return false
 }
-
-// TestFilesDeletedOldestFirstWhenWritesOverlap holds the write of an
-// older report file open until a newer one's write completes, then
-// releases it. When room is needed, the older file is deleted first,
-// though its write was the last to complete.
-func TestFilesDeletedOldestFirstWhenWritesOverlap(t *testing.T) {
-	const maxBytes = 1000
-
-	report := map[string]string{"id": "overlap"}
-
-	t.Setenv("DATA_DIR", t.TempDir())
-	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(maxBytes))
-
-	buf := startBuffer(t)
-
-	created := make(chan string)
-	release := make(chan struct{})
-
-	buf.OnFileCreated(func(f *os.File) {
-		created <- f.Name()
-
-		<-release
-	})
-
-	err := buf.Append(report)
-	if err != nil {
-		t.Fatalf("older report: %v", err)
-	}
-
-	flushed := make(chan error)
-
-	go func() { flushed <- buf.Flush() }()
-
-	older := <-created
-
-	// Only the older write is held; the newer one goes through, and
-	// so do the writes after it, the final one at stop included.
-	var newer string
-
-	buf.OnFileCreated(func(f *os.File) { newer = f.Name() })
-
-	// Errorf, not Fatalf, until the older write is released, so that a
-	// failure here does not leave it held.
-	err = buf.Append(report)
-	if err != nil {
-		t.Errorf("newer report: %v", err)
-	}
-
-	err = buf.Flush()
-	if err != nil {
-		t.Errorf("flush of the newer report: %v", err)
-	}
-
-	close(release)
-
-	err = <-flushed
-	if err != nil {
-		t.Fatalf("flush of the older report: %v", err)
-	}
-
-	info, err := os.Stat(newer)
-	if err != nil {
-		t.Fatalf("stat %s: %v", newer, err)
-	}
-
-	// A report that fits beside the newer file alone, so deleting the
-	// older one makes exactly the room it needs.
-	pad := maxBytes - int(info.Size()) - lineBytes(t, map[string]string{"id": ""})
-
-	err = buf.Append(map[string]string{"id": strings.Repeat("a", pad)})
-	if err != nil {
-		t.Fatalf("report that needs room: %v", err)
-	}
-
-	if exists(t, older) {
-		t.Fatal("older report file kept when room was needed")
-	}
-
-	if !exists(t, newer) {
-		t.Fatal("newer report file deleted before the older one")
-	}
-}

script/frontend-viewport-test

@@ -7,6 +7,8 @@
 # headless Chrome at it over CDP, and asserts on computed layout at every
 # viewport width derived from the app's own CSS. See
 # test/viewport/README.md for what this covers and what it cannot.
+# Before the browser starts, it checks that the page is served with
+# Cache-Control: no-cache and Content-Security-Policy.
 #
 # Deliberately not part of script/check: it needs Docker and takes far
 # longer than the 20s budget make test has to stay inside.
@@ -75,6 +77,32 @@ main() {
         -v "$ROOT/configs/security-headers.conf:/etc/nginx/security-headers.conf:ro" \
         "$SERVER_IMAGE" > /dev/null
 
+    # The page, and /no-such-page, which location / answers with the
+    # page through its try_files fallback, must go out with
+    # Cache-Control: no-cache. That add_header in its location drops the
+    # server's own unless the location includes the security headers
+    # again; only Content-Security-Policy is checked, as it comes from
+    # the same file as the rest. curl tries up to ten more times, a
+    # second apart, while nginx starts. It runs without -S, which would
+    # print an error for every failed attempt, even when a later one
+    # succeeds.
+    for path in / /no-such-page; do
+        if ! headers="$(docker exec "$SERVER" curl -s -o /dev/null -D - \
+            --retry 10 --retry-connrefused --retry-delay 1 \
+            "http://127.0.0.1:8080$path")"; then
+            echo "frontend-viewport-test: nginx did not answer $path" >&2
+            exit 1
+        fi
+        headers="$(printf '%s\n' "$headers" | tr -d '\r')"
+        if ! printf '%s\n' "$headers" | grep -qix 'Cache-Control: no-cache' ||
+            ! printf '%s\n' "$headers" | grep -qi '^Content-Security-Policy:'; then
+            echo "frontend-viewport-test: $path needs" \
+                "Cache-Control: no-cache and Content-Security-Policy; got:" >&2
+            printf '%s\n' "$headers" >&2
+            exit 1
+        fi
+    done
+
     # The image's own entrypoint already exposes CDP on 9222 and passes
     # --no-sandbox, so only extra flags belong here; re-specifying the
     # debugging port collides with it and leaves the endpoint bound to

test/unit/main.test.js

@@ -6,6 +6,7 @@ import { beforeEach, test } from "node:test";
 import assert from "node:assert/strict";
 import {
     AppState,
+    buildUI,
     CONFIG,
     greyOutUI,
     hostRowHTML,
@@ -17,6 +18,8 @@ import {
     measureLatency,
     Reporter,
     tick,
+    updateClocks,
+    updateSummary,
 } from "../../web/src/main.js";
 
 // There is no page here, so the tests stand in for it. The debug log looks
@@ -27,8 +30,10 @@ import {
 // selector until the next test starts. As on a page, writing its text
 // replaces its markup; the status dot greyOutUI looks for in it is not
 // there. Drawing a sparkline does nothing; it looks for the pixel ratio on
-// window and finds none. In each test, console.error and console.info
+// window and finds none. The commit the page's footer names, which
-// print nothing and keep what they are given.
+// vite.config.js fills in when it builds the page, is blank. In each test,
+// console.error and console.info print nothing and keep what they are
+// given.
 const doNothing = () => {};
 let elements;
 beforeEach((t) => {
@@ -50,6 +55,8 @@ const canvasContext = {
 globalThis.window = {};
 globalThis.requestAnimationFrame = doNothing;
 globalThis.cancelAnimationFrame = doNothing;
+globalThis.__COMMIT_HASH__ = "";
+globalThis.__COMMIT_FULL__ = "";
 globalThis.document = {
     getElementById: () => null,
     createElement: () => ({}),
@@ -578,6 +585,32 @@ test("a check that timed out or found its target unreachable has a grey figure a
     assert.equal(latencyHex(null), "#6b7280");
 });
 
+// buildUI draws the page into #app, here a plain object. Each entry in the
+// footer's color legend is a colored dot and the latencies it stands for,
+// with < and > written as &lt; and &gt;. A latency takes the color of the
+// first entry in CONFIG.latencyColors whose limit it is below, so the
+// legend has, for each of those entries in order, a dot of its color (a
+// bg- class where CONFIG.latencyColors has a text- one) and the latencies
+// below its limit, or, for the last, which has no limit, the latencies
+// from the limit before it up. A gray dot for offline comes after them.
+test("the footer's color legend has each entry in CONFIG.latencyColors, with its color and limit, then offline", (t) => {
+    const app = {};
+    t.mock.method(globalThis.document, "getElementById", () => app);
+    buildUI(new AppState([]));
+
+    const legend = [
+        ...app.innerHTML.matchAll(/(bg-\w+-500) [^"]*"><\/span>(\S+)/g),
+    ].map(([, dot, latencies]) => [dot, latencies]);
+    const colors = CONFIG.latencyColors;
+    const expected = colors.map((color, i) => [
+        color.className.replace("text-", "bg-"),
+        color.below === Infinity
+            ? `&gt;=${colors[i - 1].below}ms`
+            : `&lt;${color.below}ms`,
+    ]);
+    assert.deepEqual(legend, [...expected, ["bg-gray-500", "offline"]]);
+});
+
 // A target whose checks, in turn, answered after each of latencies ms, or,
 // for null, found it unreachable.
 function hostAfter(latencies) {
@@ -654,6 +687,102 @@ test("the summary's min, max, median and average latency over the WAN targets' l
     });
 });
 
+// In the next two tests each element of the summary that updateSummary
+// writes into is a plain object, made the first time it is looked up and
+// kept in elements under "#" and its id.
+
+// The interval menu resets each target and counts the rounds from 0 again,
+// then updates the summary, before any check since the change is recorded.
+test("after an interval change, the summary's Reachable slot reads --/-- in white, as at page load", (t) => {
+    t.mock.method(
+        globalThis.document,
+        "getElementById",
+        (id) => (elements[`#${id}`] ??= {}),
+    );
+    const state = new AppState([]);
+    for (const host of state.wan) {
+        host.pushSample(Date.now(), { latency: 30, error: null });
+    }
+    state.tickCount = 5;
+
+    for (const host of state.allHosts) host.reset();
+    state.tickCount = 0;
+    updateSummary(state);
+
+    assert.equal(elements["#summary-reachable"].textContent, "--/--");
+    assert.equal(elements["#summary-reachable"].className, "text-white");
+});
+
+test("once every WAN target's check has failed, the summary's Reachable slot reads 0 of them, in red", (t) => {
+    t.mock.method(
+        globalThis.document,
+        "getElementById",
+        (id) => (elements[`#${id}`] ??= {}),
+    );
+    const state = new AppState([]);
+    for (const host of state.wan) {
+        host.pushSample(Date.now(), { latency: null, error: "timeout" });
+    }
+
+    updateSummary(state);
+
+    assert.equal(
+        elements["#summary-reachable"].textContent,
+        `0/${state.wan.length}`,
+    );
+    assert.equal(elements["#summary-reachable"].className, "text-red-400");
+});
+
+// In the next two tests each of the header's clocks is a plain object, made
+// the first time updateClocks looks it up and kept in elements under "#" and
+// its id.
+
+// The mocked clock reads 1970-01-01T00:00:00.000Z.
+test("the header's clocks show the local time with its time zone's name, and the UTC time", (t) => {
+    t.mock.method(
+        globalThis.document,
+        "getElementById",
+        (id) => (elements[`#${id}`] ??= {}),
+    );
+    t.mock.timers.enable({ apis: ["Date"] });
+
+    updateClocks();
+
+    assert.match(
+        elements["#clock-local"].textContent,
+        /^Local: \d{4}-\d\d-\d\d \d\d:\d\d:\d\d \S/,
+    );
+    assert.equal(
+        elements["#clock-utc"].textContent,
+        "UTC: 1970-01-01 00:00:00 UTC",
+    );
+});
+
+// Here Intl.DateTimeFormat is as in Chrome before 95, Firefox before 91 or
+// Safari before 15.4: it throws a RangeError when asked for the time zone's
+// "shortGeneric" name.
+test("in a browser that cannot name the time zone, updating the clocks throws nothing and leaves them blank", (t) => {
+    t.mock.method(
+        globalThis.document,
+        "getElementById",
+        (id) => (elements[`#${id}`] ??= {}),
+    );
+    const DateTimeFormat = Intl.DateTimeFormat;
+    t.mock.method(Intl, "DateTimeFormat", function (locales, options) {
+        if (options?.timeZoneName === "shortGeneric") {
+            throw new RangeError(
+                "Value shortGeneric out of range for Intl.DateTimeFormat options property timeZoneName",
+            );
+        }
+        return new DateTimeFormat(locales, options);
+    });
+
+    updateClocks();
+
+    assert.equal(elements["#clock-local"].textContent, "");
+    assert.equal(elements["#clock-utc"].textContent, "");
+});
+
 // An app state in which, of the WAN targets, the first timedOut timed out,
 // the next unreachable were found unreachable, the next answered answered
 // after latency ms, and the rest have not been checked yet.

web/src/main.js

@@ -829,7 +829,7 @@ function wanRowsHTML(wanHosts) {
     return rows.join("");
 }
 
-function buildUI(state) {
+export function buildUI(state) {
     const app = document.getElementById("app");
     app.innerHTML = `
     <div class="mx-auto px-[5%] py-8">
@@ -911,7 +911,7 @@ function buildUI(state) {
           <span class="inline-block w-3 h-3 rounded-full bg-lime-500 mr-1 ml-3 align-middle"></span>&lt;100ms
           <span class="inline-block w-3 h-3 rounded-full bg-yellow-500 mr-1 ml-3 align-middle"></span>&lt;200ms
           <span class="inline-block w-3 h-3 rounded-full bg-orange-500 mr-1 ml-3 align-middle"></span>&lt;500ms
-          <span class="inline-block w-3 h-3 rounded-full bg-red-500 mr-1 ml-3 align-middle"></span>&gt;500ms
+          <span class="inline-block w-3 h-3 rounded-full bg-red-500 mr-1 ml-3 align-middle"></span>&gt;=500ms
           <span class="inline-block w-3 h-3 rounded-full bg-gray-500 mr-1 ml-3 align-middle"></span>offline
         </p>
         <p class="mt-2"><a href="https://git.eeqj.de/sneak/netwatch/commit/${__COMMIT_FULL__}" target="_blank" rel="noopener" class="text-gray-600 hover:text-gray-400">${__COMMIT_HASH__}</a></p>
@@ -995,7 +995,7 @@ function updateHostRow(host, index) {
     SparklineRenderer.draw(canvas, host.history);
 }
 
-function updateSummary(state) {
+export function updateSummary(state) {
     const stats = state.wanStats();
     const hstats = state.wanHistoryStats();
 
@@ -1014,13 +1014,20 @@ function updateSummary(state) {
     const hmaxUnitEl = document.getElementById("summary-hmax-unit");
     if (!reachableEl) return;
 
-    reachableEl.textContent = `${stats.reachable}/${stats.total}`;
+    // Before any WAN host's check is recorded, as after an interval change,
-    reachableEl.className =
+    // no host has answered or failed, so the slot reads as at page load.
-        stats.reachable === stats.total
+    if (state.wan.every((h) => h.status === "pending")) {
-            ? "text-green-400"
+        reachableEl.textContent = "--/--";
-            : stats.reachable === 0
+        reachableEl.className = "text-white";
-              ? "text-red-400"
+    } else {
-              : "text-yellow-400";
+        reachableEl.textContent = `${stats.reachable}/${stats.total}`;
+        reachableEl.className =
+            stats.reachable === stats.total
+                ? "text-green-400"
+                : stats.reachable === 0
+                  ? "text-red-400"
+                  : "text-yellow-400";
+    }
 
     if (stats.min !== null) {
         minEl.textContent = `${stats.min}`;
@@ -1107,6 +1114,33 @@ function updateHealthBox(state) {
     else if (health === "healthy") log.notice(`Health: ${labels[health]}`);
 }
 
+// Writes the local time, with its time zone's name, and the UTC time into
+// the header's clocks. The clocks only decorate the page: in a browser
+// whose Intl.DateTimeFormat rejects the "shortGeneric" time zone name
+// (Chrome before 95, Firefox before 91, Safari before 15.4) they are left
+// blank.
+export function updateClocks() {
+    let local = "";
+    let utc = "";
+    try {
+        const now = new Date();
+        const zone = new Intl.DateTimeFormat("en-US", {
+            timeZoneName: "shortGeneric",
+        })
+            .formatToParts(now)
+            .find((p) => p.type === "timeZoneName").value;
+        const pad = (n) => String(n).padStart(2, "0");
+        local = `Local: ${now.toLocaleString("sv-SE", { hour12: false })} ${zone}`;
+        utc =
+            `UTC: ${now.getUTCFullYear()}-${pad(now.getUTCMonth() + 1)}-${pad(now.getUTCDate())} ` +
+            `${pad(now.getUTCHours())}:${pad(now.getUTCMinutes())}:${pad(now.getUTCSeconds())} UTC`;
+    } catch {
+        // Both clocks are left blank.
+    }
+    document.getElementById("clock-local").textContent = local;
+    document.getElementById("clock-utc").textContent = utc;
+}
+
 // --- Debug Log Renderer ------------------------------------------------------
 
 // One line of the debug log panel, its message set as text.
@@ -1502,24 +1536,6 @@ async function init() {
         sortAndRebuildWAN(state);
     });
 
-    function updateClocks() {
-        const now = new Date();
-        const pad = (n) => String(n).padStart(2, "0");
-        const utc =
-            `${now.getUTCFullYear()}-${pad(now.getUTCMonth() + 1)}-${pad(now.getUTCDate())} ` +
-            `${pad(now.getUTCHours())}:${pad(now.getUTCMinutes())}:${pad(now.getUTCSeconds())} UTC`;
-        const local =
-            now.toLocaleString("sv-SE", { hour12: false }) +
-            " " +
-            new Intl.DateTimeFormat("en-US", { timeZoneName: "shortGeneric" })
-                .formatToParts(now)
-                .find((p) => p.type === "timeZoneName").value;
-        document.getElementById("clock-local").textContent = "Local: " + local;
-        document.getElementById("clock-utc").textContent = "UTC: " + utc;
-    }
-    updateClocks();
-    setInterval(updateClocks, 1000);
-
     // Rounds never overlap: a round first gives up the last round's checks
     // if they are still waiting, and the last round then records nothing
     // more. At a steady interval they never are, as they time out at 80% of
@@ -1599,6 +1615,11 @@ async function init() {
 
     window.addEventListener("resize", () => handleResize(state));
     setTimeout(() => handleResize(state), CONFIG.resizeDelay);
+
+    // The clocks start last, after the rounds: they only decorate the page,
+    // so nothing in them may stop the monitor from probing.
+    updateClocks();
+    setInterval(updateClocks, 1000);
 }
 
 // Bootstrap only when loaded as the page: a real DOM containing the #app