no magic

internal/profiling/profiling.go

@@ -223,8 +223,10 @@ func InitStats(streamStatsSize uint32) error {
 }
 
 const (
-	// StreamStatMetadataSize is the size of the metadata field StreamStats have
-	// in bytes. A 12-byte slice that is a big-endian concatenation of a stream's
-	// connection ID (first eight bytes) and stream ID (next four bytes) is used.
-	StreamStatMetadataSize = 12
+	// StreamStatMetadataConnectionIDSize is the number of bytes reserved for the
+	// connection ID in a stream stat's metadata field.
+	StreamStatMetadataConnectionIDSize = 8
+	// StreamStatMetadataStreamIDSize is the number of bytes reserved for the
+	// stream ID in a stream stat's metadata field.
+	StreamStatMetadataStreamIDSize = 4
 )

internal/transport/http2_client.go

@@ -572,10 +572,8 @@ func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Strea
 
 	if profiling.IsEnabled() {
 		s.stat = profiling.NewStat("client")
-		s.stat.Metadata = make([]byte, profiling.StreamStatMetadataSize)
-		// See comment above StreamStatMetadataSize definition for more information
-		// on this encoding.
-		binary.BigEndian.PutUint64(s.stat.Metadata[0:8], t.connectionID)
+		s.stat.Metadata = make([]byte, profiling.StreamStatMetadataConnectionIDSize + profiling.StreamStatMetadataStreamIDSize)
+		binary.BigEndian.PutUint64(s.stat.Metadata[0:profiling.StreamStatMetadataConnectionIDSize], t.connectionID)
 		// Stream ID will be set when loopy writer actually establishes the stream
 		// and obtains a stream ID
 		profiling.StreamStats.Push(s.stat)
@@ -617,7 +615,9 @@ func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Strea
 			}
 			t.activeStreams[id] = s
 			if s.stat != nil {
-				binary.BigEndian.PutUint32(s.stat.Metadata[8:12], id)
+				from := profiling.StreamStatMetadataConnectionIDSize
+				to := from + profiling.StreamStatMetadataStreamIDSize
+				binary.BigEndian.PutUint32(s.stat.Metadata[from:to], id)
 			}
 			if channelz.IsOn() {
 				atomic.AddInt64(&t.czData.streamsStarted, 1)

internal/transport/http2_server.go

@@ -338,9 +338,11 @@ func (t *http2Server) operateHeaders(frame *http2.MetaHeadersFrame, handle func(
 		s.stat = profiling.NewStat("server")
 		// See comment above StreamStatMetadataSize definition for more information
 		// on this encoding.
-		s.stat.Metadata = make([]byte, profiling.StreamStatMetadataSize)
-		binary.BigEndian.PutUint64(s.stat.Metadata[0:8], t.connectionID)
-		binary.BigEndian.PutUint32(s.stat.Metadata[8:12], streamID)
+		s.stat.Metadata = make([]byte, profiling.StreamStatMetadataConnectionIDSize + profiling.StreamStatMetadataStreamIDSize)
+		written := 0
+		binary.BigEndian.PutUint64(s.stat.Metadata[written:written+profiling.StreamStatMetadataConnectionIDSize], t.connectionID)
+		written += profiling.StreamStatMetadataConnectionIDSize
+		binary.BigEndian.PutUint32(s.stat.Metadata[written:written+profiling.StreamStatMetadataStreamIDSize], streamID)
 		profiling.StreamStats.Push(s.stat)
 		defer s.stat.NewTimer("/http2/recv/header").Egress()
 	}

profiling/cmd/catapult.go

@@ -28,6 +28,7 @@ import (
 
 	"google.golang.org/grpc/grpclog"
 	ppb "google.golang.org/grpc/profiling/proto"
+	"google.golang.org/grpc/internal/profiling"
 )
 
 type jsonNode struct {
@@ -126,8 +127,10 @@ func streamStatsCatapultJSONSingle(stat *ppb.Stat, baseSec int64, baseNsec int32
 
 	// See comment above StreamStatMetadataSize definition for more information
 	// on this encoding.
-	connectionCounter := binary.BigEndian.Uint64(stat.Metadata[0:8])
-	streamID := binary.BigEndian.Uint32(stat.Metadata[8:12])
+	read := 0
+	connectionCounter := binary.BigEndian.Uint64(stat.Metadata[read:read+profiling.StreamStatMetadataConnectionIDSize])
+	read += profiling.StreamStatMetadataConnectionIDSize
+	streamID := binary.BigEndian.Uint32(stat.Metadata[read:read+profiling.StreamStatMetadataStreamIDSize])
 	opid := fmt.Sprintf("/%s/%d/%d", stat.Tags, connectionCounter, streamID)
 
 	var loopyReaderGoID, loopyWriterGoID int64