maintner: fix netsource Update offset errors.

When resuming from a point, the reclog reading code would double check
the record headers offsets but because netsource MutationSource would
seek on the *os.File, what reclog saw and expected didn't match what
it read.

Also move temporary 5 second sleep to 2 seconds and adjust some
logging.

Updates golang/go#19866

Change-Id: I66d1f9df8bb36cf028b715ddd284cb10bc74b45b
Reviewed-on: https://go-review.googlesource.com/42184
Reviewed-by: Brad Fitzpatrick <[email protected]>

cmd/gopherbot/gopherbot.go

@@ -71,10 +71,13 @@ func main() {
 
 	ctx := context.Background()
 	for {
+		t0 := time.Now()
 		err := bot.doTasks(ctx)
 		if err != nil {
 			log.Print(err)
 		}
+		botDur := time.Since(t0)
+		log.Printf("gopherbot ran in %v", botDur)
 		if !*daemon {
 			if err != nil {
 				os.Exit(1)

maintner/maintnerd/gcslog.go

@@ -396,7 +396,7 @@ func (gl *gcsLog) GetMutations(ctx context.Context) <-chan maintner.MutationStre
 	ch := make(chan maintner.MutationStreamEvent, 50) // buffered: overlap gunzip/unmarshal with loading
 	go func() {
 		err := gl.foreachSegmentReader(ctx, func(r io.Reader) error {
-			return reclog.ForeachRecord(r, func(off int64, hdr, rec []byte) error {
+			return reclog.ForeachRecord(r, 0, func(off int64, hdr, rec []byte) error {
 				m := new(maintpb.Mutation)
 				if err := proto.Unmarshal(rec, m); err != nil {
 					return err

maintner/netsource.go

@@ -74,21 +74,20 @@ func (ns *netMutSource) waitForServerSegmentUpdate(ctx context.Context) error {
 		return fn(ctx)
 	}
 
-	// TODO: 5 second sleep is dumb. make it
+	// TODO: few second sleep is dumb. make it
 	// subscribe to pubsubhelper? maybe the
 	// server's response header should reference
 	// its pubsubhelper server URL. but then we
 	// can't assume activity means it'll be picked
 	// up right away. so maybe wait for activity,
 	// and then poll every second for 10 seconds
 	// or so, or until there's changes, and then
-	// go back to every 5 second polling or
+	// go back to every 2 second polling or
 	// something. or maybe the maintnerd server should
 	// have its own long poll functionality.
-	// for now, just 5 second polling:
-	log.Printf("sleeping for 5s...")
+	// for now, just 2 second polling:
 	select {
-	case <-time.After(5 * time.Second):
+	case <-time.After(2 * time.Second):
 		return nil
 	case <-ctx.Done():
 		return ctx.Err()
@@ -247,7 +246,7 @@ func (ns *netMutSource) sendMutations(ctx context.Context, ch chan<- MutationStr
 				return err
 			}
 		}
-		return reclog.ForeachRecord(io.LimitReader(f, seg.size), func(off int64, hdr, rec []byte) error {
+		return reclog.ForeachRecord(io.LimitReader(f, seg.size-seg.skip), seg.skip, func(off int64, hdr, rec []byte) error {
 			m := new(maintpb.Mutation)
 			if err := proto.Unmarshal(rec, m); err != nil {
 				return err

maintner/reclog/reclog.go

@@ -55,7 +55,7 @@ func ForeachFileRecord(path string, fn RecordCallback) error {
 		return err
 	}
 	defer f.Close()
-	if err := ForeachRecord(f, fn); err != nil {
+	if err := ForeachRecord(f, 0, fn); err != nil {
 		return fmt.Errorf("error in %s: %v", path, err)
 	}
 	return nil
@@ -64,8 +64,9 @@ func ForeachFileRecord(path string, fn RecordCallback) error {
 // ForeachRecord calls fn for each record in r.
 // Calls to fn are made serially.
 // If fn returns an error, iteration ends and that error is returned.
-func ForeachRecord(r io.Reader, fn RecordCallback) error {
-	var off int64
+// The startOffset be 0 if reading from the beginning of a file.
+func ForeachRecord(r io.Reader, startOffset int64, fn RecordCallback) error {
+	off := startOffset
 	br := bufio.NewReader(r)
 	var buf bytes.Buffer
 	var hdrBuf bytes.Buffer