main() is one call into internal/cli (closes #160)

cmd/netwatch-server/main.go parsed the command line, ran the check-cidr
and prepare-data-dir subcommands and built the fx application itself;
the Go style guide puts all of that in internal/ or pkg/. The new
package internal/cli does it, with the same arguments, messages and
exit codes. main() is now os.Exit(cli.Main(os.Stderr, Appname, Version,
os.Args[1:], cli.RunServer)): Main takes the writer for errors and the
function that runs the server, so its tests give it a buffer and a
function that fails them. Appname and Version stay in main, where the
build sets Version. prepare-data-dir also takes the volume and the
user's name, so its own tests give it a temporary directory and the
user they run as.

Model: opus-5-5

README.md

@@ -199,7 +199,9 @@ stops. Its routes:
   address may send a limited number a minute, and the report files are capped in
   size, the oldest deleted first
 - `GET /.well-known/healthcheck` — answers 200 with `"status":"ok"`, the
-  server's version and its uptime
+  server's version and its uptime; while reports cannot be stored, as the last
+  write of a report file failed, it answers 503 with `"status":"error"` and the
+  same fields (see [Report storage](#report-storage))
 - `GET /metrics` — Prometheus metrics behind basic auth, only when
   `METRICS_USERNAME` and `METRICS_PASSWORD` are set; each client address may
   make a limited number of requests to it a minute
@@ -207,6 +209,9 @@ stops. Its routes:
 It is an `fx`-wired Go application: `cmd/netwatch-server/` starts it, and its
 packages are in `internal/`:
 
+- **`cli`**: Runs `netwatch-server` from its command line: the `check-cidr` and
+  `prepare-data-dir` subcommands, which `bin/entrypoint.sh` runs before it
+  starts the server, and otherwise the server, with the `fx` wiring.
 - **`config`**: Loads configuration from environment variables and config files
   via Viper.
 - **`handlers`**: HTTP request handlers for the API (health check, report
@@ -274,10 +279,11 @@ The reports of a failed write stay in memory, and the next flush, within a
 minute, tries again to write them. The server writes one file at a time, so a
 file holds its reports in the order they arrived, those of a failed write
 included. From the failed write until a write succeeds, `POST /api/v1/reports`
-refuses every report with 500 and keeps nothing of it. The page sends the
-refused samples again in its next report, as long as it still holds them: at the
-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
+refuses every report with 500 and keeps nothing of it, and the health check
+answers 503 with `"status":"error"`. The page sends the refused samples again in
+its next report, as long as it still holds them: at the 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.
 
@@ -551,9 +557,16 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
       stops the container. Empty, the backend sends nothing to Sentry
 - **Health check:** the image's `HEALTHCHECK` requests
   `/.well-known/healthcheck` through nginx every 30 seconds, so it fails unless
-  both nginx and the backend answer. upaas reads the container's health 60
-  seconds after a deploy and fails the deploy unless it is `healthy`. The
-  container also stops when either process exits.
+  both nginx and the backend answer. It also fails while reports cannot be
+  stored, as when the volume is full or `DATA_DIR` cannot be written: the health
+  check answers 503 from a failed write of a report file until a write succeeds,
+  and the backend tries the write again every minute. Three failed checks in a
+  row make the container `unhealthy`, and one that passes makes it `healthy`
+  again. upaas reads the container's health once, 60 seconds after a deploy, and
+  fails the deploy unless it is `healthy`. A container that turns `unhealthy`
+  after that is neither restarted nor reported by upaas, whose `unless-stopped`
+  restart policy ignores health; only `docker ps` and `docker inspect` show it.
+  The container also stops when either process exits.
 
 ## Browser Compatibility
 

TODO.md

@@ -21,6 +21,28 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: `main()` in `cmd/netwatch-server/main.go` is one call into a new
+  package, `internal/cli`, as the Go style guide asks
+  ([#160](https://git.eeqj.de/sneak/netwatch/issues/160)). `internal/cli` runs
+  the `check-cidr` and `prepare-data-dir` subcommands, with the same arguments,
+  messages and exit codes, and otherwise the server, with the `fx` wiring.
+  `Appname` and `Version` stay in `main`, where the build sets `Version`, and
+  `main` gives `cli.Main` stderr and the function that runs the server, which
+  the tests replace with a buffer and a function that fails them. Unit tests
+  call `cli.Main` with `check-cidr` and a CIDR it takes and one it refuses, and
+  check the exit code, and with `prepare-data-dir` and a path outside `/data`,
+  and check that it exits 1 with the error from looking up the `netwatch` user,
+  which the test image does not have. Others give each subcommand input it takes
+  and input it refuses, and check the exit code and what it writes to stderr
+- 2026-10-19: while report files cannot be written, the health check answers 503
+  with `"status":"error"` and the same fields, so the image's `HEALTHCHECK`
+  reports the container `unhealthy`
+  ([#142](https://git.eeqj.de/sneak/netwatch/issues/142)). It answered 200
+  `"status":"ok"` whatever happened, so the platform saw a healthy container
+  while every report was refused with 500. It answers 200 at start, before any
+  write has failed, and again from the first write that succeeds. A unit test
+  removes `DATA_DIR` to make a write fail and checks the 503, then puts it back
+  and checks the 200 after the next write
 - 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

cmd/netwatch-server/main.go

@@ -1,22 +1,11 @@
-// Package main is the entry point for netwatch-server.
+// Package main is the entry point for netwatch-server; package cli
+// does the work.
 package main
 
 import (
-	"fmt"
 	"os"
-	"os/user"
 
-	"sneak.berlin/go/netwatch/internal/config"
-	"sneak.berlin/go/netwatch/internal/globals"
-	"sneak.berlin/go/netwatch/internal/handlers"
-	"sneak.berlin/go/netwatch/internal/healthcheck"
-	"sneak.berlin/go/netwatch/internal/logger"
-	"sneak.berlin/go/netwatch/internal/middleware"
-	"sneak.berlin/go/netwatch/internal/reportbuf"
-	"sneak.berlin/go/netwatch/internal/server"
-
-	"go.uber.org/fx"
-	"go.uber.org/fx/fxevent"
+	"sneak.berlin/go/netwatch/internal/cli"
 )
 
 //nolint:gochecknoglobals // set via ldflags at build time
@@ -26,64 +15,5 @@ var (
 )
 
 func main() {
-	// "netwatch-server check-cidr CIDR" exits 1, with the error, if
-	// this server would refuse CIDR in its TRUSTED_PROXIES.
-	// bin/entrypoint.sh runs it on each entry it gives nginx.
-	if len(os.Args) == 3 && os.Args[1] == "check-cidr" {
-		_, err := middleware.ParseTrustedProxies(os.Args[2:])
-		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
-			os.Exit(1)
-		}
-
-		return
-	}
-
-	// "netwatch-server prepare-data-dir DATA_DIR" gets DATA_DIR ready
-	// for the netwatch user, or exits 1 with the error; see
-	// reportbuf.PrepareDataDir. bin/entrypoint.sh runs it as root
-	// before it starts this server as that user.
-	if len(os.Args) == 3 && os.Args[1] == "prepare-data-dir" {
-		netwatch, err := user.Lookup("netwatch")
-		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
-			os.Exit(1)
-		}
-
-		err = reportbuf.PrepareDataDir("/data", os.Args[2], netwatch)
-		if err != nil {
-			fmt.Fprintf(os.Stderr, "DATA_DIR '%s': %v\n", os.Args[2], err)
-			os.Exit(1)
-		}
-
-		return
-	}
-
-	globals.Appname = Appname
-	globals.Version = Version
-
-	fx.New(
-		// fx logs each step of starting and stopping through the
-		// server's own logger, so off a terminal those lines are
-		// JSON like every other line.
-		fx.WithLogger(func(log *logger.Logger) fxevent.Logger {
-			return &fxevent.SlogLogger{Logger: log.Get()}
-		}),
-		fx.Provide(
-			config.New,
-			globals.New,
-			handlers.New,
-			healthcheck.New,
-			logger.New,
-			middleware.New,
-			reportbuf.New,
-			server.New,
-		),
-		fx.Invoke(
-			// First, so the name and version are logged even when
-			// a setting stops the start.
-			func(log *logger.Logger) { log.Identify() },
-			func(*server.Server) {},
-		),
-	).Run()
+	os.Exit(cli.Main(os.Stderr, Appname, Version, os.Args[1:], cli.RunServer))
 }

internal/cli/cli.go

@@ -0,0 +1,129 @@
+// Package cli runs netwatch-server from its command line: the
+// check-cidr and prepare-data-dir subcommands, which bin/entrypoint.sh
+// runs before it starts the server, and otherwise the server, with the
+// fx wiring.
+package cli
+
+import (
+	"fmt"
+	"io"
+	"os/user"
+
+	"sneak.berlin/go/netwatch/internal/config"
+	"sneak.berlin/go/netwatch/internal/globals"
+	"sneak.berlin/go/netwatch/internal/handlers"
+	"sneak.berlin/go/netwatch/internal/healthcheck"
+	"sneak.berlin/go/netwatch/internal/logger"
+	"sneak.berlin/go/netwatch/internal/middleware"
+	"sneak.berlin/go/netwatch/internal/reportbuf"
+	"sneak.berlin/go/netwatch/internal/server"
+
+	"go.uber.org/fx"
+	"go.uber.org/fx/fxevent"
+)
+
+// Main runs netwatch-server with args, its command line after the
+// program's name, and returns the exit code; the subcommands write
+// their errors to stderr. Unless args are "check-cidr CIDR" or
+// "prepare-data-dir DATA_DIR", Main runs the server by calling serve
+// with appname and version, the name and version the server logs and
+// reports. main passes os.Stderr and RunServer; the tests pass a
+// buffer and a function that fails them, so that no test starts the
+// server. When serve is RunServer and the server fails to start or to
+// stop, fx's Run exits the process with status 1 and Main never
+// returns.
+func Main(
+	stderr io.Writer,
+	appname, version string,
+	args []string,
+	serve func(appname, version string),
+) int {
+	// "netwatch-server check-cidr CIDR" exits 1, with the error, if
+	// this server would refuse CIDR in its TRUSTED_PROXIES.
+	// bin/entrypoint.sh runs it on each entry it gives nginx.
+	if len(args) == 2 && args[0] == "check-cidr" {
+		return checkCIDR(stderr, args[1])
+	}
+
+	// "netwatch-server prepare-data-dir DATA_DIR" gets DATA_DIR ready
+	// for the netwatch user, or exits 1 with the error; see
+	// reportbuf.PrepareDataDir. bin/entrypoint.sh runs it as root
+	// before it starts this server as that user.
+	if len(args) == 2 && args[0] == "prepare-data-dir" {
+		return prepareDataDir(stderr, "/data", args[1], "netwatch")
+	}
+
+	serve(appname, version)
+
+	return 0
+}
+
+// checkCIDR returns 0 if the server would take cidr in its
+// TRUSTED_PROXIES, and otherwise writes the error to stderr and
+// returns 1.
+func checkCIDR(stderr io.Writer, cidr string) int {
+	_, err := middleware.ParseTrustedProxies([]string{cidr})
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, err)
+
+		return 1
+	}
+
+	return 0
+}
+
+// prepareDataDir gets dataDir ready for the user named username with
+// reportbuf.PrepareDataDir, volume being where the data volume is
+// mounted, and returns 0. Otherwise it writes the error to stderr and
+// returns 1.
+func prepareDataDir(stderr io.Writer, volume, dataDir, username string) int {
+	owner, err := user.Lookup(username)
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, err)
+
+		return 1
+	}
+
+	err = reportbuf.PrepareDataDir(volume, dataDir, owner)
+	if err != nil {
+		_, _ = fmt.Fprintf(stderr, "DATA_DIR '%s': %v\n", dataDir, err)
+
+		return 1
+	}
+
+	return 0
+}
+
+// RunServer runs the server until it is told to stop, with appname and
+// version as the name and version it logs and reports. If the server
+// fails to start or to stop, fx's Run exits the process with status 1
+// itself.
+func RunServer(appname, version string) {
+	globals.Appname = appname
+	globals.Version = version
+
+	fx.New(
+		// fx logs each step of starting and stopping through the
+		// server's own logger, so off a terminal those lines are
+		// JSON like every other line.
+		fx.WithLogger(func(log *logger.Logger) fxevent.Logger {
+			return &fxevent.SlogLogger{Logger: log.Get()}
+		}),
+		fx.Provide(
+			config.New,
+			globals.New,
+			handlers.New,
+			healthcheck.New,
+			logger.New,
+			middleware.New,
+			reportbuf.New,
+			server.New,
+		),
+		fx.Invoke(
+			// First, so the name and version are logged even when
+			// a setting stops the start.
+			func(log *logger.Logger) { log.Identify() },
+			func(*server.Server) {},
+		),
+	).Run()
+}

internal/cli/cli_test.go

@@ -0,0 +1,152 @@
+package cli_test
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"os/user"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/netwatch/internal/cli"
+	"sneak.berlin/go/netwatch/internal/reportbuf"
+)
+
+// TestMainRunsCheckCIDR: Main given check-cidr returns 0 for a CIDR the
+// server takes in its TRUSTED_PROXIES and 1 for one it refuses, and
+// does not start the server.
+func TestMainRunsCheckCIDR(t *testing.T) {
+	t.Parallel()
+
+	for cidr, want := range map[string]int{"10.0.0.0/8": 0, "1.2.3/32": 1} {
+		code := cli.Main(io.Discard, "netwatch", "test",
+			[]string{"check-cidr", cidr}, failIfServerStarts(t))
+		if code != want {
+			t.Errorf("check-cidr %q: exit %d, want %d", cidr, code, want)
+		}
+	}
+}
+
+// TestMainRunsPrepareDataDir: Main given prepare-data-dir runs the
+// prepare-data-dir subcommand for the netwatch user, and does not start
+// the server. The test image has no netwatch user, so that subcommand
+// exits 1 with the error from looking the user up;
+// TestPrepareDataDirRefusesPathOutsideVolume checks the path.
+func TestMainRunsPrepareDataDir(t *testing.T) {
+	t.Parallel()
+
+	var stderr bytes.Buffer
+
+	code := cli.Main(&stderr, "netwatch", "test",
+		[]string{"prepare-data-dir", "/etc"}, failIfServerStarts(t))
+
+	want := user.UnknownUserError("netwatch").Error() + "\n"
+	if code != 1 || stderr.String() != want {
+		t.Errorf("prepare-data-dir /etc: exit %d, stderr %q; want exit 1 and %q",
+			code, stderr.String(), want)
+	}
+}
+
+// TestCheckCIDRTakesCIDR: check-cidr exits 0, writing nothing, for a
+// CIDR the server takes in its TRUSTED_PROXIES.
+func TestCheckCIDRTakesCIDR(t *testing.T) {
+	t.Parallel()
+
+	for _, cidr := range []string{"10.0.0.0/8", "192.0.2.1/32", "2001:db8::/32"} {
+		var stderr bytes.Buffer
+
+		code := cli.CheckCIDR(&stderr, cidr)
+		if code != 0 || stderr.Len() != 0 {
+			t.Errorf("%q: exit %d, stderr %q; want exit 0 and no output",
+				cidr, code, stderr.String())
+		}
+	}
+}
+
+// TestCheckCIDRRefusesOtherInput: check-cidr exits 1, with an error
+// naming TRUSTED_PROXIES and the input, for anything the server would
+// refuse in its TRUSTED_PROXIES, such as 1.2.3/32, which nginx would
+// look up as a hostname.
+func TestCheckCIDRRefusesOtherInput(t *testing.T) {
+	t.Parallel()
+
+	for _, cidr := range []string{
+		"1.2.3/32", "example.com/32", "10.0.0.0/33", "10.0.0.1", "",
+	} {
+		var stderr bytes.Buffer
+
+		code := cli.CheckCIDR(&stderr, cidr)
+
+		want := "TRUSTED_PROXIES " + strconv.Quote(cidr) + ": "
+		if code != 1 || !strings.HasPrefix(stderr.String(), want) {
+			t.Errorf("%q: exit %d, stderr %q; want exit 1 and %q...",
+				cidr, code, stderr.String(), want)
+		}
+	}
+}
+
+// TestPrepareDataDirTakesPathInVolume: prepare-data-dir exits 0,
+// writing nothing, for a DATA_DIR below the volume, and creates it.
+func TestPrepareDataDirTakesPathInVolume(t *testing.T) {
+	t.Parallel()
+
+	volume := t.TempDir()
+	dataDir := filepath.Join(volume, "reports")
+
+	var stderr bytes.Buffer
+
+	code := cli.PrepareDataDir(&stderr, volume, dataDir, currentUsername(t))
+	if code != 0 || stderr.Len() != 0 {
+		t.Errorf("exit %d, stderr %q; want exit 0 and no output",
+			code, stderr.String())
+	}
+
+	info, err := os.Stat(dataDir)
+	if err != nil || !info.IsDir() {
+		t.Errorf("DATA_DIR not created: %v", err)
+	}
+}
+
+// TestPrepareDataDirRefusesPathOutsideVolume: prepare-data-dir exits
+// 1, with an error naming DATA_DIR and its value, for a DATA_DIR
+// outside the volume.
+func TestPrepareDataDirRefusesPathOutsideVolume(t *testing.T) {
+	t.Parallel()
+
+	volume := t.TempDir()
+
+	var stderr bytes.Buffer
+
+	code := cli.PrepareDataDir(&stderr, volume, "/etc", currentUsername(t))
+
+	want := "DATA_DIR '/etc': " + reportbuf.ErrDataDirOutsideVolume.Error() + "\n"
+	if code != 1 || stderr.String() != want {
+		t.Errorf("exit %d, stderr %q; want exit 1 and %q",
+			code, stderr.String(), want)
+	}
+}
+
+// currentUsername is the name of the user the test runs as, the only
+// owner a test not run as root can give files to.
+func currentUsername(t *testing.T) string {
+	t.Helper()
+
+	current, err := user.Current()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return current.Username
+}
+
+// failIfServerStarts is what the tests give Main in place of
+// cli.RunServer: it fails the test instead of starting the server.
+func failIfServerStarts(t *testing.T) func(appname, version string) {
+	t.Helper()
+
+	return func(string, string) {
+		t.Error("Main started the server")
+	}
+}

internal/cli/export_test.go

@@ -0,0 +1,16 @@
+package cli
+
+import "io"
+
+// CheckCIDR exposes the check-cidr subcommand to the external tests,
+// which read what it writes to stderr.
+func CheckCIDR(stderr io.Writer, cidr string) int {
+	return checkCIDR(stderr, cidr)
+}
+
+// PrepareDataDir exposes the prepare-data-dir subcommand to the
+// external tests, which give it a volume and a user of their own in
+// place of /data and netwatch.
+func PrepareDataDir(stderr io.Writer, volume, dataDir, username string) int {
+	return prepareDataDir(stderr, volume, dataDir, username)
+}

internal/config/config_test.go

@@ -13,8 +13,8 @@ import (
 	"go.uber.org/fx"
 )
 
-// requireConfigError builds the config as main does and fails the
-// test unless that fails with an error naming setting. It uses
+// requireConfigError builds the config as cli.RunServer does and fails
+// the test unless that fails with an error naming setting. It uses
 // fx.New, because fxtest.New fails the test itself on an error.
 func requireConfigError(t *testing.T, setting string) {
 	t.Helper()
@@ -31,8 +31,8 @@ func requireConfigError(t *testing.T, setting string) {
 	}
 }
 
-// requireTrustedProxies builds the config as main does and fails the
-// test unless its TrustedProxies are want.
+// requireTrustedProxies builds the config as cli.RunServer does and
+// fails the test unless its TrustedProxies are want.
 func requireTrustedProxies(t *testing.T, want []string) {
 	t.Helper()
 

internal/globals/globals.go

@@ -4,7 +4,7 @@ package globals
 
 import "go.uber.org/fx"
 
-//nolint:gochecknoglobals // set from main before fx starts
+//nolint:gochecknoglobals // set by cli.RunServer before fx starts
 var (
 	// Appname is the application name.
 	Appname string

internal/handlers/healthcheck.go

@@ -3,9 +3,17 @@ package handlers
 import "net/http"
 
 // HandleHealthCheck returns a handler for the health check
-// endpoint.
+// endpoint. It answers 200 while the status is "ok" and 503 otherwise,
+// so that the image's HEALTHCHECK fails.
 func (s *Handlers) HandleHealthCheck() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		s.respondJSON(w, r, s.hc.Healthcheck(), http.StatusOK)
+		resp := s.hc.Healthcheck()
+
+		code := http.StatusOK
+		if resp.Status != "ok" {
+			code = http.StatusServiceUnavailable
+		}
+
+		s.respondJSON(w, r, resp, code)
 	}
 }

internal/handlers/healthcheck_test.go

@@ -9,17 +9,19 @@ import (
 	"testing"
 	"time"
 
+	"sneak.berlin/go/netwatch/internal/config"
 	"sneak.berlin/go/netwatch/internal/globals"
 	"sneak.berlin/go/netwatch/internal/handlers"
 	"sneak.berlin/go/netwatch/internal/healthcheck"
 	"sneak.berlin/go/netwatch/internal/logger"
+	"sneak.berlin/go/netwatch/internal/reportbuf"
 
 	"go.uber.org/fx/fxtest"
 )
 
 // newStartedHandlers builds Handlers with a real health check for the
-// server named in g, and starts them, which records the time the
-// uptime counts from.
+// server named in g, and a report buffer in a temporary directory, and
+// starts them, which records the time the uptime counts from.
 func newStartedHandlers(t *testing.T, g *globals.Globals) *handlers.Handlers {
 	t.Helper()
 
@@ -30,14 +32,23 @@ func newStartedHandlers(t *testing.T, g *globals.Globals) *handlers.Handlers {
 		t.Fatalf("logger: %v", err)
 	}
 
+	buf, err := reportbuf.New(lc, reportbuf.Params{
+		Config: &config.Config{DataDir: t.TempDir(), DataDirMaxBytes: 1 << 20},
+		Logger: log,
+	})
+	if err != nil {
+		t.Fatalf("report buffer: %v", err)
+	}
+
 	hc, err := healthcheck.New(lc,
-		healthcheck.Params{Globals: g, Logger: log})
+		healthcheck.Params{Buffer: buf, Globals: g, Logger: log})
 	if err != nil {
 		t.Fatalf("health check: %v", err)
 	}
 
-	h, err := handlers.New(lc,
-		handlers.Params{Globals: g, Healthcheck: hc, Logger: log})
+	h, err := handlers.New(lc, handlers.Params{
+		Buffer: buf, Globals: g, Healthcheck: hc, Logger: log,
+	})
 	if err != nil {
 		t.Fatalf("handlers: %v", err)
 	}
@@ -48,10 +59,12 @@ func newStartedHandlers(t *testing.T, g *globals.Globals) *handlers.Handlers {
 	return h
 }
 
-// TestHandleHealthCheck checks the health check's answer: 200, a JSON
-// content type, and a JSON object with exactly the fields of
-// healthcheck.HealthcheckResponse, carrying this server's name and
-// version and an uptime counted from its start.
+// TestHandleHealthCheck checks the health check's answer at start, when
+// no write of a report file has failed: 200, a JSON content type, and a
+// JSON object with exactly the fields of healthcheck.HealthcheckResponse,
+// carrying "status":"ok", this server's name and version and an uptime
+// counted from its start. The answer while writes fail is checked in
+// internal/reportbuf, whose tests can make a write fail.
 func TestHandleHealthCheck(t *testing.T) {
 	t.Parallel()
 

internal/healthcheck/healthcheck.go

@@ -10,6 +10,7 @@ import (
 	"sneak.berlin/go/netwatch/internal/config"
 	"sneak.berlin/go/netwatch/internal/globals"
 	"sneak.berlin/go/netwatch/internal/logger"
+	"sneak.berlin/go/netwatch/internal/reportbuf"
 
 	"go.uber.org/fx"
 )
@@ -18,6 +19,7 @@ import (
 type Params struct {
 	fx.In
 
+	Buffer  *reportbuf.Buffer
 	Config  *config.Config
 	Globals *globals.Globals
 	Logger  *logger.Logger
@@ -68,12 +70,18 @@ func New(
 	return s, nil
 }
 
-// Healthcheck returns the current health status of the application.
+// Healthcheck returns the current health status of the application:
+// "ok", or "error" while report files cannot be written.
 func (s *Healthcheck) Healthcheck() *HealthcheckResponse {
+	status := "ok"
+	if s.params.Buffer.LastWriteFailed() {
+		status = "error"
+	}
+
 	return &HealthcheckResponse{
 		Appname:       s.params.Globals.Appname,
 		Now:           time.Now().UTC().Format(time.RFC3339Nano),
-		Status:        "ok",
+		Status:        status,
 		UptimeHuman:   s.uptime().String(),
 		UptimeSeconds: int64(s.uptime().Seconds()),
 		Version:       s.params.Globals.Version,

internal/reportbuf/reportbuf.go

@@ -255,6 +255,16 @@ func (b *Buffer) Append(v any) error {
 	return nil
 }
 
+// LastWriteFailed reports whether the last write of a report file to
+// finish failed: false at start until a write fails, and false again
+// once a write succeeds. While it is true, Append refuses reports.
+func (b *Buffer) LastWriteFailed() bool {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	return b.writeErr != nil
+}
+
 // deleteOldestFiles deletes report files, oldest first, until n more
 // bytes fit under maxBytes. It deletes none when the reports waiting
 // to be written leave no room for n even with every file gone, since

internal/reportbuf/reportbuf_test.go

@@ -334,9 +334,9 @@ func stopWhileHeld(
 	return <-stopped
 }
 
-// startApp starts a Buffer through fx, as main does, with the DATA_DIR
-// and DATA_DIR_MAX_BYTES the calling test has set, and returns it with
-// the app, for tests that stop the app themselves.
+// startApp starts a Buffer through fx, as cli.RunServer does, with the
+// DATA_DIR and DATA_DIR_MAX_BYTES the calling test has set, and returns
+// it with the app, for tests that stop the app themselves.
 func startApp(t *testing.T) (*reportbuf.Buffer, *fxtest.App) {
 	t.Helper()
 
@@ -358,8 +358,8 @@ func startApp(t *testing.T) (*reportbuf.Buffer, *fxtest.App) {
 	return buf, app
 }
 
-// startBuffer starts a Buffer through fx, as main does, with the
-// DATA_DIR and DATA_DIR_MAX_BYTES the calling test has set.
+// startBuffer starts a Buffer through fx, as cli.RunServer does, with
+// the DATA_DIR and DATA_DIR_MAX_BYTES the calling test has set.
 func startBuffer(t *testing.T) *reportbuf.Buffer {
 	t.Helper()
 
@@ -1074,9 +1074,54 @@ func TestReportsRefusedWhileWritesFail(t *testing.T) {
 	}
 }
 
+// TestHealthCheckFailsWhileWritesFail asks the health check handler
+// for its answer around a failed write. It answers 200 with
+// "status":"ok" at start, 503 with "status":"error" from the failed
+// write until a write succeeds, and 200 with "status":"ok" again after.
+func TestHealthCheckFailsWhileWritesFail(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	buf, h := startBufferWithHandlers(t)
+
+	checkHealthCheck(t, h, "at start", http.StatusOK, "ok")
+
+	// A flush writes a file only when a report is waiting.
+	err := buf.Append(map[string]string{"id": "kept"})
+	if err != nil {
+		t.Fatalf("append: %v", err)
+	}
+
+	// With the data directory gone, the write has nowhere to go.
+	err = os.Remove(dir)
+	if err != nil {
+		t.Fatalf("remove data dir: %v", err)
+	}
+
+	err = buf.Flush()
+	if err == nil {
+		t.Fatal("flush succeeded with the data directory gone")
+	}
+
+	checkHealthCheck(t, h, "after the failed write",
+		http.StatusServiceUnavailable, "error")
+
+	err = os.Mkdir(dir, 0o750)
+	if err != nil {
+		t.Fatalf("make data dir: %v", err)
+	}
+
+	err = buf.Flush()
+	if err != nil {
+		t.Fatalf("flush with the data directory back: %v", err)
+	}
+
+	checkHealthCheck(t, h, "after a write succeeded", http.StatusOK, "ok")
+}
+
 // startBufferWithHandlers starts a Buffer and the Handlers that store
-// reports in it through fx, as main does, with the DATA_DIR the calling
-// test has set.
+// reports in it through fx, as cli.RunServer does, with the DATA_DIR
+// the calling test has set.
 func startBufferWithHandlers(
 	t *testing.T,
 ) (*reportbuf.Buffer, *handlers.Handlers) {
@@ -1121,6 +1166,39 @@ func postReport(t *testing.T, h *handlers.Handlers, clientID string) int {
 	return rec.Code
 }
 
+// checkHealthCheck asks the health check handler for its answer, and
+// fails the test, saying when it asked, unless the answer has status
+// code wantCode and a "status" field of wantStatus.
+func checkHealthCheck(
+	t *testing.T,
+	h *handlers.Handlers,
+	when string,
+	wantCode int,
+	wantStatus string,
+) {
+	t.Helper()
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodGet, "/.well-known/healthcheck", http.NoBody)
+	h.HandleHealthCheck().ServeHTTP(rec, req)
+
+	var body struct {
+		Status string `json:"status"`
+	}
+
+	err := json.Unmarshal(rec.Body.Bytes(), &body)
+	if err != nil {
+		t.Fatalf("health check %s: body not JSON: %v (%q)",
+			when, err, rec.Body.String())
+	}
+
+	if rec.Code != wantCode || body.Status != wantStatus {
+		t.Fatalf("health check %s: %d with status %q, want %d with %q",
+			when, rec.Code, body.Status, wantCode, wantStatus)
+	}
+}
+
 // TestConcurrentAppendsStopAtCap appends from many goroutines at once
 // with room for exactly roomFor reports: exactly that many must be
 // taken, which holds only if Append checks and counts each report

internal/server/routes_test.go

@@ -22,9 +22,9 @@ import (
 	"go.uber.org/fx/fxtest"
 )
 
-// newServer builds a Server from the same constructors as main,
-// configured from the environment. It is never started, so nothing
-// listens.
+// newServer builds a Server from the same constructors as
+// cli.RunServer, configured from the environment. It is never started,
+// so nothing listens.
 func newServer(t *testing.T) *server.Server {
 	t.Helper()