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

[Copilot]

🟡 Changes recommended

Argument handling is incorrect and the package is incompatible with its declared minimum Go version.

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

Pull request overview

Adds test helpers for re-executing the current test binary as a controlled child process.

Changes:

• Adds child dispatch and command helpers.
• Adds Linux parent-death signaling.
• Adds subprocess behavior tests.

File summaries

File	Description
reexec/reexectest/reexectest.go	Implements the public helpers.
reexec/reexectest/reexectest_test.go	Tests subprocess behavior.
reexec/reexectest/reexectest_linux.go	Configures Linux Pdeathsig.
reexec/reexectest/reexectest_other.go	Provides non-Linux behavior.

Review details

Suppressed comments (1)

reexec/reexectest/reexectest.go:107

• User arguments that begin with - are parsed by the test binary before the selected test runs. Unknown flags abort the child, while recognized -test.* flags can alter or override its execution, so this command cannot reliably pass CLI flags to f. Add a -- separator before user arguments and update Run to scrub that separator together with the injected -test.run argument.

	cmd.Args = append([]string{argv0, "-test.run=" + pattern}, args...)

• Files reviewed: 4/4 changed files
• Comments generated: 3
• Review effort level: Balanced

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[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.