runtime/pprof: emit end-of-log marker at end of CPU profile.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6489065

src/pkg/runtime/cpuprof.c

@@ -99,6 +99,7 @@ struct Profile {
 	uint32 wtoggle;
 	bool wholding;	// holding & need to release a log half
 	bool flushing;	// flushing hash table - profile is over
+	bool eod_sent;  // special end-of-data record sent; => flushing
 };
 
 static Lock lk;
@@ -109,6 +110,8 @@ static void add(Profile*, uintptr*, int32);
 static bool evict(Profile*, Entry*);
 static bool flushlog(Profile*);
 
+static uintptr eod[3] = {0, 1, 0};
+
 // LostProfileData is a no-op function used in profiles
 // to mark the number of profiling stack traces that were
 // discarded due to slow data writers.
@@ -163,6 +166,7 @@ runtime·SetCPUProfileRate(int32 hz)
 		prof->wholding = false;
 		prof->wtoggle = 0;
 		prof->flushing = false;
+		prof->eod_sent = false;
 		runtime·noteclear(&prof->wait);
 
 		runtime·setcpuprofilerate(tick, hz);
@@ -409,6 +413,16 @@ breakflush:
 	}
 
 	// Made it through the table without finding anything to log.
+	if(!p->eod_sent) {
+		// We may not have space to append this to the partial log buf,
+		// so we always return a new slice for the end-of-data marker.
+		p->eod_sent = true;
+		ret.array = (byte*)eod;
+		ret.len = sizeof eod;
+		ret.cap = ret.len;
+		return ret;
+	}
+
 	// Finally done.  Clean up and return nil.
 	p->flushing = false;
 	if(!runtime·cas(&p->handoff, p->handoff, 0))

src/pkg/runtime/pprof/pprof_test.go

@@ -49,19 +49,25 @@ func TestCPUProfile(t *testing.T) {
 
 	// Convert []byte to []uintptr.
 	bytes := prof.Bytes()
+	l := len(bytes) / int(unsafe.Sizeof(uintptr(0)))
 	val := *(*[]uintptr)(unsafe.Pointer(&bytes))
-	val = val[:len(bytes)/int(unsafe.Sizeof(uintptr(0)))]
+	val = val[:l]
 
-	if len(val) < 10 {
+	if l < 13 {
 		t.Fatalf("profile too short: %#x", val)
 	}
-	if val[0] != 0 || val[1] != 3 || val[2] != 0 || val[3] != 1e6/100 || val[4] != 0 {
-		t.Fatalf("unexpected header %#x", val[:5])
+
+	hd, val, tl := val[:5], val[5:l-3], val[l-3:]
+	if hd[0] != 0 || hd[1] != 3 || hd[2] != 0 || hd[3] != 1e6/100 || hd[4] != 0 {
+		t.Fatalf("unexpected header %#x", hd)
+	}
+
+	if tl[0] != 0 || tl[1] != 1 || tl[2] != 0 {
+		t.Fatalf("malformed end-of-data marker %#x", tl)
 	}
 
 	// Check that profile is well formed and contains ChecksumIEEE.
 	found := false
-	val = val[5:]
 	for len(val) > 0 {
 		if len(val) < 2 || val[0] < 1 || val[1] < 1 || uintptr(len(val)) < 2+val[1] {
 			t.Fatalf("malformed profile.  leftover: %#x", val)