reexec: add reexectest package

[thaJeztah]

This package allows using the reexec functionality to execute child processes as part of a test.

[vvoland]

Hmm does it only work against test binaries created by go test -c, or does it also work with direct go test?

[thaJeztah]

Yup, should still work; adding a quick patch to show the output;

diff --git a/reexec/reexectest/reexectest_test.go b/reexec/reexectest/reexectest_test.go
index c8dd5cd..1b57bf3 100644
--- a/reexec/reexectest/reexectest_test.go
+++ b/reexec/reexectest/reexectest_test.go
@@ -33,6 +33,8 @@ func TestRun(t *testing.T) {
                }
                if got := strings.TrimSpace(strings.TrimSuffix(string(out), "PASS\n")); got != expected {
                        t.Errorf("env-and-output output: got %q, want %q", got, expected)
+               } else {
+                       t.Logf("env-and-output output: got %q", out)
                }
        })

Then run;

go test -v -run TestRun
=== RUN   TestRun
=== RUN   TestRun/env-and-output
    reexectest_test.go:37: env-and-output output: got "child-env-and-output-ok\nPASS\n"
=== RUN   TestRun/exit-code
=== RUN   TestRun/args-passthrough
=== RUN   TestRun/context
--- PASS: TestRun (0.05s)
    --- PASS: TestRun/env-and-output (0.02s)
    --- PASS: TestRun/exit-code (0.01s)
    --- PASS: TestRun/args-passthrough (0.01s)
    --- PASS: TestRun/context (0.01s)
=== RUN   TestRunNonSubtest
--- PASS: TestRunNonSubtest (0.01s)
PASS
ok  	github.com/moby/sys/reexec/reexectest	0.401s

[vvoland]

AI review:

[High] Anchor every -test.run path component

reexec/reexectest/reexectest.go:103

Go’s test runner splits -test.run expressions at unbracketed /. For a subtest named TestTarget/child, the generated expression:

^TestTarget/child$

is treated as separate ^TestTarget and child$ expressions. The first also matches TestTargetExtra, so TestTargetExtra/child runs in the re-exec child. Its Run call rejects the
child because argv[0] differs, after which its parent branch can execute and start further subprocesses.

I reproduced this deterministically: targeting TestTarget/child produced output from both the intended test and TestTargetExtra/child.

Construct an exact expression for each slash-delimited test-name component rather than anchoring the complete name only.

[High] Preserve the module’s Go 1.20 compatibility

reexec/reexectest/reexectest.go:73

testing.T.Context was added in Go 1.24, while reexec/go.mod declares Go 1.20. Consequently, consumers cannot compile the new package with Go 1.20–1.23.

A Go 1.20 container reported:

reexectest/reexectest.go:73:29: t.Context undefined

The green CI matrix does not catch this because its Go 1.18 job skips the reexec module and oldstable is already new enough. This is also raised in an existing review thread.

[High] Separate test-runner flags from child arguments

reexec/reexectest/reexectest.go:107

The test binary parses child arguments beginning with - before the selected test runs. For example:

reexectest.Command(t, "worker", "-user-flag")

exits with status 2:

flag provided but not defined: -user-flag

Recognized -test.* arguments can instead alter the child test runner, including overriding test selection. Add a -- terminator before user arguments and have Run remove both the
injected test flag and separator. This was noted in the existing Copilot review summary.

[Medium] Do not corrupt the saved os.Args

reexec/reexectest/reexectest.go:59

origArgs := os.Args preserves only the slice header. The subsequent append reuses the same backing array because the result is shorter, overwriting the arguments that origArgs is meant
to restore. With child arguments, deferred cleanup restores a list with the test flag removed and the final argument duplicated.

Allocate a separate scrubbed slice before assigning it to os.Args. This is also covered by an existing review thread.

[Copilot]

🟡 Changes recommended

The run-pattern coverage is ineffective, and the new package’s tests are excluded from repository CI.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (1)

reexec/reexectest/reexectest.go:100

• As with Command, the helper marker in commandContext does not hide this exported wrapper, so fatal errors are attributed to package internals instead of the caller. Mark this wrapper with t.Helper() before delegating.

func CommandContext(t *testing.T, ctx context.Context, name string, args ...string) *exec.Cmd {
	return commandContext(t, ctx, name, args...)

• Files reviewed: 5/5 changed files
• Comments generated: 3
• Review effort level: Balanced

[Copilot]

🟢 Approval recommended

Only minor cleanup remains for unused platform shims.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[thaJeztah]

Yay! Looks like I pleased CoPilot now.

@vvoland PTAL 😅