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 new test flushes pairs
until one falls within one millisecond; the 1 ms pauses earlier tests
used to dodge the collision are gone.

Model: opus-5-5

backend/internal/reportbuf/export_test.go

@@ -1,15 +1,7 @@
 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

@@ -58,9 +58,6 @@ 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
@@ -87,7 +84,6 @@ func New(
 		done:     make(chan struct{}),
 		log:      params.Logger.Get(),
 		maxBytes: params.Config.DataDirMaxBytes,
-		now:      time.Now,
 	}
 
 	lc.Append(fx.Hook{
@@ -222,7 +218,7 @@ 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 := b.now().UTC().Format("2006-01-02T15-04-05.000Z")
+	ts := time.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

@@ -314,23 +314,40 @@ 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) {
-	const flushes = 2
+	// A pair of flushes may straddle a millisecond, which proves
+	// nothing, so pairs are flushed until one falls within one.
+	const maxPairs = 1000
 
 	dir := t.TempDir()
 	t.Setenv("DATA_DIR", dir)
 
 	buf := startBuffer(t)
-	buf.StopClock(time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC))
+	flushes := 0
 
-	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)
+	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)
+			}
 		}
 
-		err = buf.Flush()
-		if err != nil {
-			t.Fatalf("flush %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)
 		}
 	}