Stopping the server waits for report file writes under way (closes #122)

The report buffer's stop now waits, through a sync.WaitGroup, for
flushLoop and for the write Append starts at 10 MiB before its final
flush. It used to return at once, so the process exiting cut such a
write off and its reports were lost.

A write under way that fails puts its reports back in the buffer, so
the final flush writes them; the stop fails only if that flush fails
too.

Tests hold the periodic write and the one at 10 MiB open in a synctest
bubble and find the stop waiting until each is released; a third makes
the held write fail and finds its report written by the final flush.

Model: opus-5-5

internal/reportbuf/reportbuf.go

@@ -96,6 +96,10 @@ type Buffer struct {
 	// refuses reports, and the reports of that write wait in buf for
 	// the next flush.
 	writeErr error
+	// writers are the goroutines that write report files: flushLoop,
+	// and each one Append starts at flushSizeThreshold. OnStop waits
+	// for them before the final flush.
+	writers sync.WaitGroup
 }
 
 // New creates a Buffer and registers lifecycle hooks to
@@ -147,7 +151,7 @@ func New(
 
 			b.mu.Unlock()
 
-			go b.flushLoop()
+			b.writers.Go(b.flushLoop)
 
 			return nil
 		},
@@ -158,7 +162,14 @@ func New(
 			var err error
 
 			b.stopOnce.Do(func() {
+				// Closing done ends flushLoop after any flush it is
+				// running. Wait for it and for the writes Append
+				// started, which the process exiting would cut off;
+				// one that fails puts its reports back in buf, for the
+				// final flush to write.
 				close(b.done)
+				b.writers.Wait()
+
 				err = b.flushLocked()
 			})
 
@@ -211,12 +222,12 @@ func (b *Buffer) Append(v any) error {
 		data := b.drainBuf()
 		b.mu.Unlock()
 
-		go func() {
+		b.writers.Go(func() {
 			writeErr := b.writeFile(data)
 			if writeErr != nil {
 				b.log.Error("flush reports failed", "error", writeErr)
 			}
-		}()
+		})
 
 		return nil
 	}