Delete the oldest report files to stay under the size cap (closes #54)

When a report would take the report files past DATA_DIR_MAX_BYTES,
reportbuf now deletes the oldest report files until it fits, and does
the same at start when files left by an earlier run are already past
it. The buffer keeps the files it may delete in a list, oldest first; a
file joins it only once it is completely written, so a file still being
written is never deleted. A report is refused with 507 only when the
reports not yet written leave no room for it on their own, and then no
file is deleted. A file whose deletion fails keeps counting; one
already deleted by hand counts as freed.

Model: opus-5-5

.dockerignore

@@ -4,3 +4,6 @@ tmp
 .DS_Store
 *.log
 .claude
+
+# .git is sent so the build can stamp the version, without its config.
+.git/config

Dockerfile

@@ -20,7 +20,7 @@ RUN make lint
 # golang:1.25-alpine (2026-02-27)
 FROM golang:1.25-alpine@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
 
-RUN apk add --no-cache make
+RUN apk add --no-cache git make
 
 WORKDIR /src
 
@@ -37,11 +37,24 @@ RUN make test
 
 # make build is a shim around backend/script/build, the one definition
 # of the build command:
-# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
+# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=..."
 # That script reads VERSION from the environment, so it is handed over
 # there rather than as a make variable.
-ARG VERSION=dev
+#
-RUN VERSION="${VERSION}" make build
+# The version is the VERSION build argument when one is given, otherwise
+# `git describe --tags --always` of the repo's .git: the tag on a tagged
+# commit, tag-N-gHASH on a commit after one, the short commit when no
+# tag is reachable. A version that still comes out empty, dev or unknown
+# fails the build. .git goes to /git, not /src/.git, where go build would
+# find it and record VCS details of a work tree holding only backend/.
+COPY .git /git
+ARG VERSION
+RUN version="${VERSION:-$(git --git-dir=/git describe --tags --always)}"; \
+    case "$version" in ""|dev|unknown) \
+        echo "version is '$version' although .git is present" >&2; \
+        exit 1 ;; \
+    esac; \
+    VERSION="$version" make build
 
 # Frontend stage
 # node:22-alpine as of 2026-02-22

README.md

@@ -212,7 +212,7 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
     - `REPORTS_PER_MINUTE`, default `60`: reports each client address may send a
       minute
     - `DATA_DIR_MAX_BYTES`, default `1073741824` (1 GiB): the most room the
-      report files may take
+      report files may take; the oldest are deleted to stay under it
     - `CORS_ALLOWED_ORIGINS`, default empty: other origins whose pages may call
       the API
     - `DEBUG`, default `false`: debug logging

TODO.md

@@ -23,6 +23,14 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-10-03: `DATA_DIR_MAX_BYTES` is now how much of the report files is kept
+  (issue #54): when a report would take them past it, the oldest report files
+  are deleted to make room, each deletion logged, and at start files already
+  past it are deleted the same way. A file still being written is never deleted.
+  A report is refused with 507 only when the reports not yet written leave no
+  room for it on their own, and then no file is deleted. A file that cannot be
+  deleted still counts until the next start; one already deleted by hand counts
+  as freed
 - 2026-09-29: the container sets up its own data directory (issue #75):
   `bin/entrypoint.sh`, still as root, creates `DATA_DIR` if missing and gives it
   and `/data` to the `netwatch` user with mode 750 before starting the backend

backend/README.md

@@ -32,7 +32,7 @@ pattern as the repo root: the targets in `backend/Makefile` are thin shims over
 `test`, `fmt` and `fmt-check`:
 
 - `script/build` — compile the static `netwatch-server` binary with its version
-  and architecture stamped in. The version is `VERSION` from the environment;
+  stamped in. The version is `VERSION` from the environment;
   when that is unset or empty, it falls back to `git describe` inside a git
   checkout, then to `dev`
 - `script/test` — run the Go tests under a 30-second timeout
@@ -80,7 +80,7 @@ Internal packages in `internal/` follow standard Go project layout:
 | `BIND_ADDRESS`         | empty                | IP address to listen on; empty listens on every interface                                                |
 | `PORT`                 | `8080`               | HTTP listen port                                                                                         |
 | `DATA_DIR`             | `./data/reports`     | Directory for compressed reports                                                                         |
-| `DATA_DIR_MAX_BYTES`   | `1073741824` (1 GiB) | Largest total size of the report files in `DATA_DIR`; see [Report limits](#report-limits)                |
+| `DATA_DIR_MAX_BYTES`   | `1073741824` (1 GiB) | Most bytes of report files kept in `DATA_DIR`, oldest deleted first; see [Report limits](#report-limits) |
 | `DEBUG`                | `false`              | Enable debug logging                                                                                     |
 | `TRUSTED_PROXIES`      | loopback + RFC1918   | Comma-separated CIDRs whose `X-Forwarded-For` / `X-Real-IP` headers are trusted for client IP resolution |
 | `REPORTS_PER_MINUTE`   | `60`                 | Reports each client address may send a minute; see [Report limits](#report-limits)                       |
@@ -148,11 +148,15 @@ credentials, so it is bounded instead. Both refusals below answer with the same
   `X-RateLimit-Reset` headers.
 - **Size cap.** The report files in `DATA_DIR` may total at most
   `DATA_DIR_MAX_BYTES`, counting the files already there at start. Reports
-  waiting in memory count at their uncompressed size until they are written, so
+  waiting in memory count at their uncompressed size until they are written.
-  a report that would take the total past the cap is refused with 507, and
+  When a report would take the total past the cap, the oldest report files are
-  nothing of it is stored. Deleting report files frees room only at the next
+  deleted to make room, and each deletion is logged with the file's name and
-  start, when the files are counted again. The default of 1 GiB is small enough
+  size; a file still being written is never deleted. A report is refused with
-  for any host; set it to the space you can give `DATA_DIR`.
+  507, and nothing of it is stored, only when the reports not yet written leave
+  no room for it on their own, and then no file is deleted. At start, report
+  files past the cap, as after lowering it, are deleted the same way. So the cap
+  is 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`.
 
 ### CORS
 
@@ -170,7 +174,6 @@ starting, with an error naming `CORS_ALLOWED_ORIGINS`.
 - Add integration test that POSTs a report and verifies the compressed output
 - Add report decompression/query endpoint
 - Add metrics (Prometheus) for buffer size, flush count, report count
-- Add retention policy to prune old report files
 
 ## License
 

backend/cmd/netwatch-server/main.go

@@ -19,9 +19,8 @@ import (
 
 //nolint:gochecknoglobals // set via ldflags at build time
 var (
-	Appname   = "netwatch-server"
+	Appname = "netwatch-server"
-	Version   string
+	Version string
-	Buildarch string
 )
 
 func main() {
@@ -40,7 +39,6 @@ func main() {
 
 	globals.Appname = Appname
 	globals.Version = Version
-	globals.Buildarch = Buildarch
 
 	fx.New(
 		fx.Provide(

backend/internal/globals/globals.go

@@ -10,22 +10,18 @@ var (
 	Appname string
 	// Version is the git version tag.
 	Version string
-	// Buildarch is the build architecture.
-	Buildarch string
 )
 
 // Globals holds build-time metadata for the application.
 type Globals struct {
-	Appname   string
+	Appname string
-	Version   string
+	Version string
-	Buildarch string
 }
 
 // New creates a Globals instance from package-level variables.
 func New(_ fx.Lifecycle) (*Globals, error) {
 	return &Globals{
-		Appname:   Appname,
+		Appname: Appname,
-		Buildarch: Buildarch,
+		Version: Version,
-		Version:   Version,
 	}, nil
 }

backend/internal/logger/logger.go

@@ -5,6 +5,7 @@ package logger
 import (
 	"log/slog"
 	"os"
+	"runtime"
 
 	"sneak.berlin/go/netwatch/internal/globals"
 
@@ -95,6 +96,6 @@ func (l *Logger) Identify() {
 	l.log.Info("starting",
 		"appname", l.params.Globals.Appname,
 		"version", l.params.Globals.Version,
-		"buildarch", l.params.Globals.Buildarch,
+		"arch", runtime.GOARCH,
 	)
 }

backend/internal/reportbuf/reportbuf.go

@@ -1,5 +1,6 @@
 // Package reportbuf accumulates telemetry reports in memory
-// and periodically flushes them to zstd-compressed JSONL files.
+// and periodically flushes them to zstd-compressed JSONL files,
+// deleting the oldest files to keep them under a size cap.
 package reportbuf
 
 import (
@@ -37,8 +38,9 @@ const (
 	fileSuffix = ".jsonl.zst"
 )
 
-// ErrFull is returned by Append when storing the report would
+// ErrFull is returned by Append when the reports not yet written
-// take the report files past the configured maximum size.
+// leave no room for the report under the configured maximum size,
+// however many report files are deleted.
 var ErrFull = errors.New("report files at their size cap")
 
 // Params defines the dependencies for Buffer.
@@ -49,15 +51,28 @@ type Params struct {
 	Logger *logger.Logger
 }
 
+// reportFile is a report file that may be deleted to make room, with
+// the size it counts for in usedBytes.
+type reportFile struct {
+	name string
+	size int64
+}
+
 // Buffer accumulates JSON lines in memory and flushes them
 // to zstd-compressed files on disk.
 type Buffer struct {
-	buf      bytes.Buffer
+	buf     bytes.Buffer
-	dataDir  string
+	dataDir string
-	done     chan struct{}
+	done    chan struct{}
-	log      *slog.Logger
+	// files are the report files that may be deleted to make room,
-	maxBytes int64
+	// oldest first: those in dataDir at start, then each one this
-	mu       sync.Mutex
+	// buffer writes, once it is complete. A file still being written
+	// is not among them. filesBytes is their total size.
+	files      []reportFile
+	filesBytes int64
+	log        *slog.Logger
+	maxBytes   int64
+	mu         sync.Mutex
 	// now is the clock report files are named by: time.Now, except
 	// in tests that need two flushes to share a timestamp.
 	now func() time.Time
@@ -97,12 +112,28 @@ func New(
 				return fmt.Errorf("create data dir: %w", err)
 			}
 
-			// Report files left by earlier runs count too.
+			// Report files left by earlier runs count too, and are
-			b.usedBytes, err = reportFilesSize(b.dataDir)
+			// the first to be deleted to make room.
+			files, err := reportFiles(b.dataDir)
 			if err != nil {
 				return err
 			}
 
+			b.mu.Lock()
+
+			b.files = files
+			for _, f := range files {
+				b.filesBytes += f.size
+			}
+
+			b.usedBytes = b.filesBytes
+
+			// The files may be past the cap, if it was lowered since
+			// the last run.
+			b.deleteOldestFiles(0)
+
+			b.mu.Unlock()
+
 			go b.flushLoop()
 
 			return nil
@@ -128,9 +159,10 @@ func New(
 }
 
 // Append marshals v as a single JSON line and appends it to
-// the buffer. It stores nothing and returns ErrFull if the line
+// the buffer. If the line would take usedBytes past maxBytes, the
-// would take usedBytes past maxBytes. If the buffer reaches the
+// oldest report files are deleted to make room; it stores nothing
-// size threshold, it is drained and written to disk
+// and returns ErrFull if that cannot make room. If the buffer
+// reaches the size threshold, it is drained and written to disk
 // asynchronously.
 func (b *Buffer) Append(v any) error {
 	line, err := json.Marshal(v)
@@ -142,6 +174,8 @@ func (b *Buffer) Append(v any) error {
 
 	b.mu.Lock()
 
+	b.deleteOldestFiles(lineBytes)
+
 	if b.usedBytes+lineBytes > b.maxBytes {
 		b.mu.Unlock()
 
@@ -171,6 +205,37 @@ func (b *Buffer) Append(v any) error {
 	return nil
 }
 
+// deleteOldestFiles deletes report files, oldest first, until n more
+// bytes fit under maxBytes. It deletes none when the reports not yet
+// written leave no room for n even with every file gone, since that
+// would lose the files for nothing. The caller must hold b.mu.
+func (b *Buffer) deleteOldestFiles(n int64) {
+	for b.usedBytes+n > b.maxBytes && len(b.files) > 0 {
+		if b.usedBytes-b.filesBytes+n > b.maxBytes {
+			return
+		}
+
+		f := b.files[0]
+		b.files = b.files[1:]
+		b.filesBytes -= f.size
+
+		// A file already gone, deleted by hand, has freed its room too.
+		err := os.Remove(filepath.Join(b.dataDir, f.name))
+		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+			// The file is still there, so it still counts. It is
+			// not tried again until the next start.
+			b.log.Error("delete report file failed",
+				"file", f.name, "error", err)
+
+			continue
+		}
+
+		b.usedBytes -= f.size
+		b.log.Info("deleted report file to make room",
+			"file", f.name, "bytes", f.size)
+	}
+}
+
 // flushLoop runs a ticker that periodically flushes buffered
 // data to disk until the done channel is closed.
 func (b *Buffer) flushLoop() {
@@ -270,25 +335,28 @@ func (b *Buffer) writeFile(data []byte) error {
 	}
 
 	// The reports counted at their uncompressed size while they
-	// waited; now they count as the file. After a failed write they
+	// waited; now they count as the file, which from here on may be
-	// stay counted as they were, which errs toward refusing reports
+	// deleted to make room. After a failed write they stay counted as
-	// early rather than letting the files pass the cap.
+	// they were, which errs toward refusing reports early rather than
+	// letting the files pass the cap.
 	b.mu.Lock()
 	b.usedBytes += info.Size() - int64(len(data))
+	b.files = append(b.files, reportFile{name: name, size: info.Size()})
+	b.filesBytes += info.Size()
 	b.mu.Unlock()
 
 	return nil
 }
 
-// reportFilesSize returns the total size of the report files in
+// reportFiles returns the report files in dir, oldest first:
-// dir.
+// os.ReadDir sorts them by name, and the names sort by time.
-func reportFilesSize(dir string) (int64, error) {
+func reportFiles(dir string) ([]reportFile, error) {
 	entries, err := os.ReadDir(dir)
 	if err != nil {
-		return 0, fmt.Errorf("read data dir: %w", err)
+		return nil, fmt.Errorf("read data dir: %w", err)
 	}
 
-	var total int64
+	files := make([]reportFile, 0, len(entries))
 
 	for _, entry := range entries {
 		name := entry.Name()
@@ -299,11 +367,11 @@ func reportFilesSize(dir string) (int64, error) {
 
 		info, err := entry.Info()
 		if err != nil {
-			return 0, fmt.Errorf("stat report file: %w", err)
+			return nil, fmt.Errorf("stat report file: %w", err)
 		}
 
-		total += info.Size()
+		files = append(files, reportFile{name: name, size: info.Size()})
 	}
 
-	return total, nil
+	return files, nil
 }

backend/internal/reportbuf/reportbuf_test.go

@@ -139,11 +139,19 @@ func lineBytes(t *testing.T, report any) int {
 	return len(line) + 1
 }
 
+// TestAppendPastCapIsRefused fills the cap with a report not yet
+// written. The next report is refused, and the report file already in
+// DATA_DIR is kept: it is smaller than a report, so deleting it could
+// not make room.
 func TestAppendPastCapIsRefused(t *testing.T) {
 	report := map[string]string{"id": "cap"}
+	dir := t.TempDir()
+	earlier := reportFilePath(dir, 1)
 
-	t.Setenv("DATA_DIR", t.TempDir())
+	writeBytes(t, earlier, 1)
-	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(lineBytes(t, report)))
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(1+lineBytes(t, report)))
 
 	buf := startBuffer(t)
 
@@ -156,24 +164,38 @@ func TestAppendPastCapIsRefused(t *testing.T) {
 	if !errors.Is(err, reportbuf.ErrFull) {
 		t.Fatalf("report past the cap: error = %v, want ErrFull", err)
 	}
+
+	if !exists(t, earlier) {
+		t.Fatal("report file deleted, though that could not make room")
+	}
 }
 
-// TestCapCountsReportFilesAlreadyInDataDir starts on a data
+// TestOldestReportFileDeletedFirst starts on a data directory holding
-// directory holding a report file from an earlier run, and a file
+// report files from an earlier run, and a file that is not a report,
-// that is not a report, which must not count.
+// which neither counts nor is ever deleted. Nothing is deleted while
-func TestCapCountsReportFilesAlreadyInDataDir(t *testing.T) {
+// there is room; then only the oldest report file is.
-	const earlierBytes = 100
+func TestOldestReportFileDeletedFirst(t *testing.T) {
+	const fileBytes = 100
 
-	report := map[string]string{"id": "cap"}
+	report := map[string]string{"id": "oldest"}
 	dir := t.TempDir()
+	oldest := reportFilePath(dir, 1)
+	kept := []string{
+		reportFilePath(dir, 2),
+		reportFilePath(dir, 3),
+		filepath.Join(dir, "notes.txt"),
+	}
 
-	writeBytes(t, filepath.Join(dir, "reports-2026-01-01T00-00-00.000Z.jsonl.zst"),
+	writeBytes(t, oldest, fileBytes)
-		earlierBytes)
+
-	writeBytes(t, filepath.Join(dir, "notes.txt"), 10*earlierBytes)
+	for _, path := range kept {
+		writeBytes(t, path, fileBytes)
+	}
 
 	t.Setenv("DATA_DIR", dir)
+	// Room for the three report files and one report.
 	t.Setenv("DATA_DIR_MAX_BYTES",
-		strconv.Itoa(earlierBytes+lineBytes(t, report)))
+		strconv.Itoa(3*fileBytes+lineBytes(t, report)))
 
 	buf := startBuffer(t)
 
@@ -182,9 +204,55 @@ func TestCapCountsReportFilesAlreadyInDataDir(t *testing.T) {
 		t.Fatalf("report that fills the cap exactly: %v", err)
 	}
 
+	if !exists(t, oldest) {
+		t.Fatal("oldest report file deleted while there was room")
+	}
+
 	err = buf.Append(report)
-	if !errors.Is(err, reportbuf.ErrFull) {
+	if err != nil {
-		t.Fatalf("report past the cap: error = %v, want ErrFull", err)
+		t.Fatalf("report past the cap: %v", err)
+	}
+
+	if exists(t, oldest) {
+		t.Fatal("oldest report file kept when room was needed")
+	}
+
+	for _, path := range kept {
+		if !exists(t, path) {
+			t.Fatalf("%s deleted; only the oldest report file should be", path)
+		}
+	}
+}
+
+// TestStartDeletesFilesPastCap starts on report files past the cap,
+// as after the cap is lowered: the oldest are deleted until the rest
+// fit.
+func TestStartDeletesFilesPastCap(t *testing.T) {
+	const fileBytes = 100
+
+	dir := t.TempDir()
+	oldest := reportFilePath(dir, 1)
+	kept := []string{reportFilePath(dir, 2), reportFilePath(dir, 3)}
+
+	writeBytes(t, oldest, fileBytes)
+
+	for _, path := range kept {
+		writeBytes(t, path, fileBytes)
+	}
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(len(kept)*fileBytes))
+
+	startBuffer(t)
+
+	if exists(t, oldest) {
+		t.Fatal("oldest report file kept, though the files were past the cap")
+	}
+
+	for _, path := range kept {
+		if !exists(t, path) {
+			t.Fatalf("%s deleted, though the rest fit without it", path)
+		}
 	}
 }
 
@@ -195,8 +263,9 @@ func TestWrittenReportsCountAtFileSize(t *testing.T) {
 	// Repetitive, so its file is far smaller than its JSON.
 	report := map[string]string{"id": strings.Repeat("a", 1000)}
 	size := lineBytes(t, report)
+	dir := t.TempDir()
 
-	t.Setenv("DATA_DIR", t.TempDir())
+	t.Setenv("DATA_DIR", dir)
 	// Room for the report twice over only if the first one counts
 	// at its file's size by the time the second arrives.
 	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(2*size-1))
@@ -217,13 +286,22 @@ func TestWrittenReportsCountAtFileSize(t *testing.T) {
 	if err != nil {
 		t.Fatalf("second report, after the first was written: %v", err)
 	}
+
+	if !hasReportFile(t, dir) {
+		t.Fatal("first report's file deleted, though the second fit beside it")
+	}
 }
 
-// TestWrittenReportsKeepCounting writes one report file after another
+// TestWrittenFilesDeletedToMakeRoom writes one report file after
-// under a small cap: each report must be taken while the files on disk
+// another under a small cap. Every report must be taken; files are
-// leave room for it, and refused once they do not.
+// deleted only when the report would not fit beside them, and the
-func TestWrittenReportsKeepCounting(t *testing.T) {
+// files kept leave room for it.
-	const maxBytes = 200
+func TestWrittenFilesDeletedToMakeRoom(t *testing.T) {
+	const (
+		maxBytes = 200
+		// One file each, which take far more than maxBytes together.
+		reports = 50
+	)
 
 	report := map[string]string{"id": "written"}
 	size := int64(lineBytes(t, report))
@@ -234,23 +312,24 @@ func TestWrittenReportsKeepCounting(t *testing.T) {
 
 	buf := startBuffer(t)
 
-	// Every file takes at least a byte, so they fill the cap within
+	for range reports {
-	// maxBytes rounds.
+		before := reportFilesBytes(t, dir)
-	for range maxBytes {
-		used := reportFilesBytes(t, dir)
 
 		err := buf.Append(report)
-		if used+size > maxBytes {
+		if err != nil {
-			if !errors.Is(err, reportbuf.ErrFull) {
+			t.Fatalf("with %d bytes of report files: %v", before, err)
-				t.Fatalf("with %d bytes of report files: error = %v, "+
-					"want ErrFull", used, err)
-			}
-
-			return
 		}
 
-		if err != nil {
+		after := reportFilesBytes(t, dir)
-			t.Fatalf("with %d bytes of report files: %v", used, err)
+
+		if before+size <= maxBytes && after != before {
+			t.Fatalf("files deleted, though the report fit beside "+
+				"their %d bytes", before)
+		}
+
+		if after+size > maxBytes {
+			t.Fatalf("%d bytes of report files kept, leaving no room "+
+				"for the report", after)
 		}
 
 		err = buf.Flush()
@@ -258,8 +337,124 @@ func TestWrittenReportsKeepCounting(t *testing.T) {
 			t.Fatalf("flush: %v", err)
 		}
 	}
+}
 
-	t.Fatal("the report files never filled the cap")
+// TestFailedDeletionStillCounts makes deleting the oldest report file
+// fail. It is still there, so it still takes room, and the next oldest
+// is deleted in its place.
+func TestFailedDeletionStillCounts(t *testing.T) {
+	const fileBytes = 100
+
+	report := map[string]string{"id": "stuck"}
+	dir := t.TempDir()
+	stuck := reportFilePath(dir, 1)
+	next := reportFilePath(dir, 2)
+	newest := reportFilePath(dir, 3)
+
+	for _, path := range []string{stuck, next, newest} {
+		writeBytes(t, path, fileBytes)
+	}
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(3*fileBytes))
+
+	buf := startBuffer(t)
+
+	// A directory that is not empty cannot be deleted, even by root,
+	// which the tests run as in the backend image.
+	err := os.Remove(stuck)
+	if err != nil {
+		t.Fatalf("remove %s: %v", stuck, err)
+	}
+
+	err = os.Mkdir(stuck, 0o750)
+	if err != nil {
+		t.Fatalf("make directory %s: %v", stuck, err)
+	}
+
+	writeBytes(t, filepath.Join(stuck, "file"), 1)
+
+	err = buf.Append(report)
+	if err != nil {
+		t.Fatalf("report past the cap: %v", err)
+	}
+
+	if exists(t, next) {
+		t.Fatal("next oldest report file kept: the failed deletion " +
+			"counted as making room")
+	}
+
+	if !exists(t, newest) {
+		t.Fatal("newest report file deleted, though deleting one made room")
+	}
+}
+
+// TestFileDeletedByHandFreesRoom deletes the oldest report file by
+// hand after start. When room is needed, its room counts as freed, so
+// no other file is deleted.
+func TestFileDeletedByHandFreesRoom(t *testing.T) {
+	const fileBytes = 100
+
+	report := map[string]string{"id": "by-hand"}
+	dir := t.TempDir()
+	gone := reportFilePath(dir, 1)
+	kept := reportFilePath(dir, 2)
+
+	writeBytes(t, gone, fileBytes)
+	writeBytes(t, kept, fileBytes)
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(2*fileBytes))
+
+	buf := startBuffer(t)
+
+	err := os.Remove(gone)
+	if err != nil {
+		t.Fatalf("remove %s: %v", gone, err)
+	}
+
+	err = buf.Append(report)
+	if err != nil {
+		t.Fatalf("report past the cap: %v", err)
+	}
+
+	if !exists(t, kept) {
+		t.Fatal("report file deleted, though the one deleted by hand " +
+			"had made room")
+	}
+}
+
+// TestUncountedReportFileIsNeverDeleted: only the report files counted,
+// those in DATA_DIR at start and those the buffer has finished
+// writing, are deleted to make room. A file still being written is not
+// among them, so it is kept even when that means refusing a report. A
+// write cannot be paused here, so a report file put in DATA_DIR after
+// start, which is not counted either, stands in for one being written.
+func TestUncountedReportFileIsNeverDeleted(t *testing.T) {
+	report := map[string]string{"id": "uncounted"}
+	dir := t.TempDir()
+	uncounted := reportFilePath(dir, 1)
+
+	t.Setenv("DATA_DIR", dir)
+	t.Setenv("DATA_DIR_MAX_BYTES", strconv.Itoa(lineBytes(t, report)))
+
+	buf := startBuffer(t)
+
+	err := buf.Append(report)
+	if err != nil {
+		t.Fatalf("report that fills the cap exactly: %v", err)
+	}
+
+	writeBytes(t, uncounted, 100)
+
+	err = buf.Append(report)
+	if !errors.Is(err, reportbuf.ErrFull) {
+		t.Fatalf("report past the cap: error = %v, want ErrFull", err)
+	}
+
+	if !exists(t, uncounted) {
+		t.Fatal("report file deleted, though the buffer had not counted it")
+	}
 }
 
 // TestConcurrentAppendsStopAtCap appends from many goroutines at once
@@ -407,6 +602,14 @@ func readReportFiles(t *testing.T, dir string) []string {
 	return contents
 }
 
+// reportFilePath returns the path in dir of a report file named as
+// written on the given day of January 2026, so that a lower day sorts
+// as older.
+func reportFilePath(dir string, day int) string {
+	return filepath.Join(dir,
+		fmt.Sprintf("reports-2026-01-%02dT00-00-00.000Z-1.jsonl.zst", day))
+}
+
 func writeBytes(t *testing.T, path string, n int) {
 	t.Helper()
 
@@ -416,6 +619,21 @@ func writeBytes(t *testing.T, path string, n int) {
 	}
 }
 
+func exists(t *testing.T, path string) bool {
+	t.Helper()
+
+	_, err := os.Stat(path)
+	if errors.Is(err, fs.ErrNotExist) {
+		return false
+	}
+
+	if err != nil {
+		t.Fatalf("stat %s: %v", path, err)
+	}
+
+	return true
+}
+
 func hasReportFile(t *testing.T, dir string) bool {
 	t.Helper()
 

backend/internal/server/http.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"net"
 	"net/http"
+	"runtime"
 	"strconv"
 	"time"
 
@@ -53,7 +54,7 @@ func (s *Server) listenAndServe() {
 	s.log.Info("http begin listen",
 		"listenaddr", s.httpServer.Addr,
 		"version", s.params.Globals.Version,
-		"buildarch", s.params.Globals.Buildarch,
+		"arch", runtime.GOARCH,
 	)
 
 	err := s.httpServer.ListenAndServe()

backend/script/build

@@ -1,6 +1,6 @@
 #!/bin/sh
 # script/build: compile the static netwatch-server binary into the
-# backend project root, with its version and architecture stamped in.
+# backend project root, with its version stamped in.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -14,7 +14,7 @@ main() {
     version="${VERSION:-$(git describe --always --dirty 2>/dev/null || echo dev)}"
 
     CGO_ENABLED=0 go build -trimpath \
-        -ldflags "-s -w -X main.Version=$version -X main.Buildarch=$(uname -m)" \
+        -ldflags "-s -w -X main.Version=$version" \
         -o netwatch-server ./cmd/netwatch-server/
 }
 

script/cibuild

@@ -16,9 +16,8 @@ main() {
     "$SCRIPT_DIR/check"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. VERSION is computed here because .dockerignore
+    # empty constant. The VERSION build argument takes precedence over
-    # excludes .git, so `git describe` in a build stage yields an empty
+    # the version a build stage derives from the .git in the context.
-    # version without failing.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build --no-cache \

script/docker

@@ -12,9 +12,8 @@ main() {
     cd "$ROOT"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. VERSION is computed here because .dockerignore
+    # empty constant. The VERSION build argument takes precedence over
-    # excludes .git, so `git describe` in a build stage yields an empty
+    # the version a build stage derives from the .git in the context.
-    # version without failing.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build --no-cache \