runtime/pprof: add tracing support

runtime/pprof part of tracing functionality:
https://docs.google.com/document/u/1/d/1FP5apqzBgr7ahCCgFO-yoVhk4YZrNIDNf9RybngBc14/pub
Full change:
https://codereview.appspot.com/146920043

Change-Id: I3143a569cbd33576f19ca47308d1ff5200d8c955
Reviewed-on: https://go-review.googlesource.com/1452Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/pprof/pprof.go

@@ -615,6 +615,33 @@ func StopCPUProfile() {
 	<-cpu.done
 }
 
+// TODO(rsc): Decide if StartTrace belongs in this package.
+// See golang.org/issue/9710.
+// StartTrace enables tracing for the current process.
+// While tracing, the trace will be buffered and written to w.
+// StartTrace returns an error if profiling is tracing enabled.
+func StartTrace(w io.Writer) error {
+	if err := runtime.StartTrace(); err != nil {
+		return err
+	}
+	go func() {
+		for {
+			data := runtime.ReadTrace()
+			if data == nil {
+				break
+			}
+			w.Write(data)
+		}
+	}()
+	return nil
+}
+
+// StopTrace stops the current tracing, if any.
+// StopTrace only returns after all the writes for the trace have completed.
+func StopTrace() {
+	runtime.StopTrace()
+}
+
 type byCycles []runtime.BlockProfileRecord
 
 func (x byCycles) Len() int           { return len(x) }