runtime: simplify trace buffer management around footer

Writing out the trace footer currently manages trace buffers
differently from the rest of trace code. Rearrange it so it looks like
the rest of the code. In particular, we now write the frequency event
out to the trace buffer rather than returning it in a special byte
slice, and (*traceStackTable).dump threads a traceBufPtr like most
other functions that write to the trace buffers.

Change-Id: I3d0e108e56df884e7bd19823310dfbc0e21af9a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/422974
Reviewed-by: Michael Knyszek <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/runtime/trace.go

@@ -459,6 +459,9 @@ func ReadTrace() []byte {
 		goparkunlock(&trace.lock, waitReasonTraceReaderBlocked, traceEvGoBlock, 2)
 		lock(&trace.lock)
 	}
+
+newFull:
+	assertLockHeld(&trace.lock)
 	// Write a buffer.
 	if trace.fullHead != 0 {
 		buf := traceFullDequeue()
@@ -478,13 +481,22 @@ func ReadTrace() []byte {
 		}
 		trace.lockOwner = nil
 		unlock(&trace.lock)
-		var data []byte
-		data = append(data, traceEvFrequency|0<<traceArgCountShift)
-		data = traceAppend(data, uint64(freq))
+
+		// Write frequency event.
+		bufp := traceFlush(0, 0)
+		buf := bufp.ptr()
+		buf.byte(traceEvFrequency | 0<<traceArgCountShift)
+		buf.varint(uint64(freq))
+
+		// Dump stack table.
 		// This will emit a bunch of full buffers, we will pick them up
 		// on the next iteration.
-		trace.stackTab.dump()
-		return data
+		bufp = trace.stackTab.dump(bufp)
+
+		// Flush final buffer.
+		lock(&trace.lock)
+		traceFullQueue(bufp)
+		goto newFull // trace.lock should be held at newFull
 	}
 	// Done.
 	if trace.shutdown {
@@ -914,15 +926,6 @@ func traceString(bufp *traceBufPtr, pid int32, s string) (uint64, *traceBufPtr)
 	return id, bufp
 }
 
-// traceAppend appends v to buf in little-endian-base-128 encoding.
-func traceAppend(buf []byte, v uint64) []byte {
-	for ; v >= 0x80; v >>= 7 {
-		buf = append(buf, 0x80|byte(v))
-	}
-	buf = append(buf, byte(v))
-	return buf
-}
-
 // varint appends v to buf in little-endian-base-128 encoding.
 func (buf *traceBuf) varint(v uint64) {
 	pos := buf.pos
@@ -1058,8 +1061,7 @@ func traceFrames(bufp traceBufPtr, pcs []uintptr) ([]traceFrame, traceBufPtr) {
 
 // dump writes all previously cached stacks to trace buffers,
 // releases all memory and resets state.
-func (tab *traceStackTable) dump() {
-	bufp := traceFlush(0, 0)
+func (tab *traceStackTable) dump(bufp traceBufPtr) traceBufPtr {
 	for i, _ := range tab.tab {
 		stk := tab.tab[i].ptr()
 		for ; stk != nil; stk = stk.link.ptr() {
@@ -1097,13 +1099,11 @@ func (tab *traceStackTable) dump() {
 		}
 	}
 
-	lock(&trace.lock)
-	traceFullQueue(bufp)
-	unlock(&trace.lock)
-
 	tab.mem.drop()
 	*tab = traceStackTable{}
 	lockInit(&((*tab).lock), lockRankTraceStackTab)
+
+	return bufp
 }
 
 type traceFrame struct {