runtime/race: race detector misses when copying into an interface

[jbardin]

The following is missed by the race detector

var (
    a = "hi"
    c = make(chan struct{})
)

func f() {
    a = "bye"
    close(c)
}

func main() {
    go f()
    fmt.Println(a)
    // fmt.Printf(a)
    <-c
}

But changing the fmt call to fmt.Printf where the a value is copied into the format string triggers a race

[dvyukov]

Just tested on tip:

diff --git a/src/runtime/race/testdata/mop_test.go b/src/runtime/race/testdata/mop_test.go
index d7cbc98..af12c96 100644
--- a/src/runtime/race/testdata/mop_test.go
+++ b/src/runtime/race/testdata/mop_test.go
@@ -297,6 +297,17 @@ func TestRaceCaseTypeIssue5890(t *testing.T) {
        <-c
 }

+func TestRaceIssue12664(t *testing.T) {
+       a := "hi"
+       c := make(chan struct{})
+       go func() {
+               a = "bye"
+               close(c)
+       }()
+       fmt.Println(a)
+       <-c
+}
+
 func TestNoRaceRange(t *testing.T) {
        ch := make(chan int, 3)
        a := [...]int{1, 2, 3}

produces:

==================
WARNING: DATA RACE
Write by goroutine 7:
  command-line-arguments_test.TestRaceIssue12664.func1()
      src/runtime/race/testdata/mop_test.go:304 +0x2e

Previous read by goroutine 6:
  command-line-arguments_test.TestRaceIssue12664()
      src/runtime/race/testdata/mop_test.go:307 +0xc1
  testing.tRunner()
      src/testing/testing.go:456 +0xdc

Goroutine 7 (running) created at:
  command-line-arguments_test.TestRaceIssue12664()
      src/runtime/race/testdata/mop_test.go:306 +0xb3
  testing.tRunner()
      src/testing/testing.go:456 +0xdc

Goroutine 6 (running) created at:
  testing.RunTests()
      src/testing/testing.go:561 +0xaaf
  testing.(*M).Run()
      src/testing/testing.go:494 +0xe4
  main.main()
      command-line-arguments/_test/_testmain.go:312 +0x210
==================

go version devel +d5fe165 Fri Sep 18 07:15:52 2015 +0000 linux/amd64

[dvyukov]

But it in fact reproduces on the provided program. It seems to matter whether a is a global or not.

[gopherbot]

CL https://golang.org/cl/14752 mentions this issue.