runtime/race: doesn't support sync.Pool

[pascaldekloe]

The sync.Pool is disabled when the race detector is active with issue #7203. Accidental reuse of data after pool submissions is not unheard of.

What version of Go are you using (go version)?

go version devel +03a1dc3 Wed Sep 28 23:13:53 2016 +0000 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/pdekloe/repo/colfer"
GORACE=""
GOROOT="/Users/pdekloe/repo/go"
GOTOOLDIR="/Users/pdekloe/repo/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sz/9wzvxsb57p73r4099d6_5zgr39574r/T/go-build242286039=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

Cause a race condition by leaking data that was submitted into sync.Pool for reuse.

What did you expect to see?

A race detection.

What did you see instead?

My bug.

[ianlancetaylor]

CC @dvyukov

[quentinmit]

This may be hard to fix; this isn't a race in the traditional sense. (It's only a race if the object is then pulled out of the pool again before the next GC.) I don't know if the race detector has enough options to handle this.

[dvyukov]

@pascaldekloe do you have any real bug? If it's in open-source code, please point to it.

[dvyukov]

@quentinmit race detector has acquire, release annotations and ignores. It should be enough.

The problem with straightforward enabling of Pool under race detector is that it will introduce lots of parasitic synchronization (esp the Mutex) that can hide lots of unrelated races.

We could annotate Pool to synchronize on the objects cached (i.e. if goroutine A takes object X put by goroutine B, then A synchronizes only with B). But values are not necessary pointers.
We could also enable cache probablistically -- randomly decide to Put or to drop.

[pascaldekloe]

The uncommited code was ment for an opensource project. I've chosen to use a buffered channel as a pool instead.

Here's a playground example: https://play.golang.org/p/vwn35eBL8-

[gopherbot]

CL https://golang.org/cl/31589 mentions this issue.