runtime/pprof: fix data race between Profile.Add and Profile.WriteTo

p.m is accessed in WriteTo without holding p.mu.
Move the access inside the critical section.

The race detector catches this bug using this program:


package main

import (
	"os"
	"runtime/pprof"
	"time"
)

func main() {
	p := pprof.NewProfile("ABC")
	go func() {
		p.WriteTo(os.Stdout, 1)
		time.Sleep(time.Second)
	}()
	p.Add("abc", 0)
	time.Sleep(time.Second)
}


$ go run -race x.go 
==================
WARNING: DATA RACE
Write at 0x00c42007c240 by main goroutine:
  runtime.mapassign()
      /Users/josh/go/tip/src/runtime/hashmap.go:485 +0x0
  runtime/pprof.(*Profile).Add()
      /Users/josh/go/tip/src/runtime/pprof/pprof.go:281 +0x255
  main.main()
      /Users/josh/go/tip/src/p.go:15 +0x9d

Previous read at 0x00c42007c240 by goroutine 6:
  runtime/pprof.(*Profile).WriteTo()
      /Users/josh/go/tip/src/runtime/pprof/pprof.go:314 +0xc5
  main.main.func1()
      /Users/josh/go/tip/src/x.go:12 +0x69

Goroutine 6 (running) created at:
  main.main()
      /Users/josh/go/tip/src/x.go:11 +0x6e
==================
ABC profile: total 1
1 @ 0x110ccb4 0x111aeee 0x1055053 0x107f031

Found 1 data race(s)
exit status 66


(Exit status 66?)

Change-Id: I49d884dc3af9cce2209057a3448fe6bf50653523
Reviewed-on: https://go-review.googlesource.com/37730
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/pprof/pprof.go

@@ -311,8 +311,8 @@ func (p *Profile) WriteTo(w io.Writer, debug int) error {
 	}
 
 	// Obtain consistent snapshot under lock; then process without lock.
-	all := make([][]uintptr, 0, len(p.m))
 	p.mu.Lock()
+	all := make([][]uintptr, 0, len(p.m))
 	for _, stk := range p.m {
 		all = append(all, stk)
 	}