fix(backend): give each report file a name of its own (closes #61)

Report files were named by a millisecond timestamp and created with
O_EXCL, so two flushes in the same millisecond, such as a flush for
size and the final flush at shutdown, got the same name and the second
failed, losing its reports. Each name now carries a number after the
timestamp that counts the files written since the server started, so
names still sort by time and never repeat. The buffer reads the time
through a clock the new test stops, so its two flushes share one
timestamp on every run; the 1 ms pauses earlier tests used to dodge
the collision are gone.

Model: opus-5-5

backend/internal/reportbuf/export_test.go

@@ -1,7 +1,15 @@
 package reportbuf
 
+import "time"
+
 // Flush writes the buffered reports to a file now, as the periodic
 // flush does, so tests need not wait a minute for it.
 func (b *Buffer) Flush() error {
 	return b.flushLocked()
 }
+
+// StopClock makes every report file the buffer writes from now on
+// carry the timestamp at, as if all were written in one millisecond.
+func (b *Buffer) StopClock(at time.Time) {
+	b.now = func() time.Time { return at }
+}

backend/internal/reportbuf/reportbuf.go

@@ -58,6 +58,9 @@ type Buffer struct {
 	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
 	// seq numbers the report files, so that two named in the same
 	// millisecond still get different names.
 	seq      atomic.Uint64
@@ -84,6 +87,7 @@ func New(
 		done:     make(chan struct{}),
 		log:      params.Logger.Get(),
 		maxBytes: params.Config.DataDirMaxBytes,
+		now:      time.Now,
 	}
 
 	lc.Append(fx.Hook{
@@ -218,7 +222,7 @@ func (b *Buffer) drainBuf() []byte {
 func (b *Buffer) writeFile(data []byte) error {
 	// The timestamp comes first, so the names sort by time; the number
 	// after it tells apart files named in the same millisecond.
-	ts := time.Now().UTC().Format("2006-01-02T15-04-05.000Z")
+	ts := b.now().UTC().Format("2006-01-02T15-04-05.000Z")
 	name := fmt.Sprintf("%s%s-%d%s", filePrefix, ts, b.seq.Add(1), fileSuffix)
 	path := filepath.Join(b.dataDir, name)
 

backend/internal/reportbuf/reportbuf_test.go

@@ -314,40 +314,23 @@ func TestConcurrentAppendsStopAtCap(t *testing.T) {
 // as a flush for size and the final flush at shutdown can: each flush
 // must write a file of its own, and the files must hold every report.
 func TestTwoFlushesInOneMillisecond(t *testing.T) {
-	// A pair of flushes may straddle a millisecond, which proves
-	// nothing, so pairs are flushed until one falls within one.
-	const maxPairs = 1000
+	const flushes = 2
 
 	dir := t.TempDir()
 	t.Setenv("DATA_DIR", dir)
 
 	buf := startBuffer(t)
-	flushes := 0
+	buf.StopClock(time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC))
 
-	for pair := 1; ; pair++ {
-		start := time.Now().Truncate(time.Millisecond)
-
-		for range 2 {
-			flushes++
-
-			err := buf.Append(map[string]int{"id": flushes})
-			if err != nil {
-				t.Fatalf("append report %d: %v", flushes, err)
-			}
-
-			err = buf.Flush()
-			if err != nil {
-				t.Fatalf("flush %d: %v", flushes, err)
-			}
+	for id := 1; id <= flushes; id++ {
+		err := buf.Append(map[string]int{"id": id})
+		if err != nil {
+			t.Fatalf("append report %d: %v", id, err)
 		}
 
-		if time.Now().Truncate(time.Millisecond).Equal(start) {
-			break
-		}
-
-		if pair == maxPairs {
-			t.Fatalf("no pair of flushes fell within one millisecond "+
-				"in %d tries", maxPairs)
+		err = buf.Flush()
+		if err != nil {
+			t.Fatalf("flush %d: %v", id, err)
 		}
 	}