testing: race detected in goroutine can report an error in an unnamed test

[ianlancetaylor]

Test case:

package foo_test

import (
	"fmt"
	"testing"
)

func makeRace() {
	var x int
	c := make(chan bool)
	go func() {
		x = 1
		c <- true
	}()
	fmt.Println(x)
	<-c
}

func TestFoo(t *testing.T) {
	makeRace()
}

Running this test with go test -race -test.v prints

=== RUN   TestFoo
0
==================
WARNING: DATA RACE
Write at 0x00c000014288 by goroutine 8:
  command-line-arguments_test.makeRace.func1()
      /home/iant/foo_test.go:12 +0x33

Previous read at 0x00c000014288 by goroutine 7:
  command-line-arguments_test.makeRace()
      /home/iant/foo_test.go:15 +0x10c
  command-line-arguments_test.TestFoo()
      /home/iant/foo_test.go:20 +0x1c
  testing.tRunner()
      /home/iant/go/src/testing/testing.go:1595 +0x238
  ??()
      -:0 +0x5243c4

Goroutine 8 (running) created at:
  command-line-arguments_test.makeRace()
      /home/iant/foo_test.go:11 +0xfc
  command-line-arguments_test.TestFoo()
      /home/iant/foo_test.go:20 +0x1c
  testing.tRunner()
      /home/iant/go/src/testing/testing.go:1595 +0x238
  ??()
      -:0 +0x5243c4

Goroutine 7 (running) created at:
  testing.(*T).Run()
      /home/iant/go/src/testing/testing.go:1648 +0x835
  testing.runTests.func1()
      /home/iant/go/src/testing/testing.go:2054 +0x84
  testing.tRunner()
      /home/iant/go/src/testing/testing.go:1595 +0x238
  testing.runTests()
      /home/iant/go/src/testing/testing.go:2052 +0x896
  testing.(*M).Run()
      /home/iant/go/src/testing/testing.go:1925 +0xb57
  main.main()
      _testmain.go:47 +0x2bd
==================
    testing.go:1465: race detected during execution of test
--- FAIL: TestFoo (0.00s)
=== NAME  
    testing.go:1465: race detected during execution of test
FAIL
FAIL	command-line-arguments	0.007s
FAIL

Note the === NAME near the end. A === NAME line should be followed by the name of the test. Omitting the name is confusing and can break code that is parsing the test output.

[cherrymui]

cc @bcmills

[bcmills]

I think this is plausibly correct? The empty NAME line could mean “this line is independent of the individual tests”, which is true for race reports in general (albeit perhaps not this specific case).

[ianlancetaylor]

I tend to think that an empty NAME line is confusing and unhelpful. If we want to say that the report is independent of individual tests, let's say that.

[bcmills]

I believe it is currently necessary in order for cmd/test2json to correctly interpret the output, but perhaps it could be omitted (or changed) when -test.v is not set to test2json.

[ianlancetaylor]

Can you expand on that? test2json currently doesn't seem to count race detector reports at all.

[cip999]

Hi! While we're at it, let me point out a related "weird" pattern in race error messages for tests. Unlike other errors, these get propagated to all the ancestors of the subtest that generated the race, and the same hardcoder error message (testing.go:1465: race detected during execution of test) is printed for all of them. [Since the ancestors include the "root" test that encapsulates the whole suite, and which has an empty name, this causes the === NAME line to be printed.]

Just wanted to mention this before I forgot. It's a different problem (if we want to call it that) from the subject of this issue, in the sense that fixing any of them won't solve the other. Maybe it's worth filing another bug for this?

[ianlancetaylor]

@cip999 Thanks, I was just looking at that.

[cip999]

The reason for that is how race errors are counted:

• Total number of race errors (race.Errors()) is logged right before running the (sub)test.
• (Sub)test is run.
• Total number of race errors is logged again, and if greater than the previous one the race detection error is thrown.

(By the way, for some reason this is done by storing the negative of the first number, adding the second number, and checking whether the result is greater than 0. I'm confused.)

It shouldn't be difficult to fix it.

[ianlancetaylor]

Using the negative count and then adding to it is because the number of race detector errors is monotonically increasing. We want to do the right thing when we run multiple tests, and some have race errors.

[cip999]

Yes, my confusion was about why not storing the first error count (not its negative) and comparing it to the second one. But I think I figured it out, it's to handle the counting for parallel tests. Anyway, I'll stop diverging from the main subject. :)

[gopherbot]

Change https://go.dev/cl/494057 mentions this issue: testing: only report subtest races once

[cip999]

Following up on this comment thread, I investigated the way race errors are handled and reported in parallel tests.

There are a couple notable points.

• Parallel tests are run after the parent test has finished. Most importantly, the order of the events is the following: parent test function returns -> race errors for parent test are counted (and possibly reported) -> parallel subtests are released.
• This means that direct parents of parallel tests aren't affected by this issue: if a race happens inside a parallel test, by that time its parent will have already logged the number of races. However, the race will be reported for the parent's parent (unless the parent is also run in parallel) and all previous ancestors.
• As a concrete example, if we call t.Parallel() inside TestFoo in the proof of concept above, we no longer get the === NAME line and the race is reported only once.

[Incidentally, the semantics of T.Parallel is a bit counterintuitive; but let's not go there.]

Now, what happens with the proposed change? What I said in the comments isn't exactly true. Parallel tests will still be "deferred" to when race errors for the parent have been already reported, so the parent won't be affected at all. However, the parent's parent (and all previous ancestors) will be: if a race happens during parallel execution of two or more sub-sub-tests, the race count will be decremented by 2 or more (instead of 1). Then, if another race happens outside the subtests, it won't be detected. (Disclaimer: I haven't verified this, I'm too lazy to patch the CL...)

Admittedly, sub-sub-tests are seldom a thing (if ever). Still, something that works "by chance" is not super solid.

Edit: Just to clarify, a minimal working example of the scenario I describe should be the following:

func makeRace() {
  var x int
  c := make(chan bool)
  go func() {
    x = 1
    c <- true
  }()
  fmt.Println(x)
  <-c
}

func TestFoo(t *testing.T) {
  t.Run("subtest", func(t *testing.T) {
    c := make(chan bool)

    t.Run("first subsubtest", func(t *testing.T) {
      t.Parallel()
      <-c
      makeRace() // All the signaling is to make sure that this happens inside both subsubtests.
      c <- true
    })

    t.Run("second subsubtest", func(t *testing.T) {
      t.Parallel()
      c <- true
      <-c
    })
  })
}