TEMPORARY: a data race on purpose, to see one race part go red

Reverted by the next commit, before merging. Expected: the part holding
TestADataRaceOnPurposeForTheRaceJob (part 0 by the sorted list) red with
DATA RACE, parts 1 to 3 green.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: donislawdev

internal/guard/mutationcoverage_test.go

@@ -32,6 +32,9 @@ import (
 // means saying out loud that a guard is unproven. The list should only ever get
 // shorter.
 var notProvenByMutation = map[string]bool{
+	// A data race on purpose, to see one part of the race job go red. It has
+	// no line of ours to break and is reverted before merging.
+	"TestADataRaceOnPurposeForTheRaceJob": true,
 	// A directory entry costs one tar block and nothing more.
 	//
 	// There is no line of ours under this one to break. It asserts what

internal/guard/raceprobe_test.go

@@ -0,0 +1,20 @@
+package guard
+
+import "testing"
+
+// A data race put here on purpose, for one pull request only: the part of the
+// race detector job that holds this test has to go red, and the other parts
+// green. Reverted by the next commit, before the pull request is merged.
+func TestADataRaceOnPurposeForTheRaceJob(t *testing.T) {
+	n := 0
+	done := make(chan struct{})
+	go func() {
+		n++
+		close(done)
+	}()
+	n++
+	<-done
+	if n != 2 {
+		t.Logf("the two increments landed as %d", n)
+	}
+}