Fix panics setting TestResult asynchronously (#36)

Panic handling is deferred before the defer that marks tests complete.
This could lead to a test being marked complete, returning a result but
then mutating that result after returning.

This happened infrequently but could lead to a flaky result where a
panicked test isn't marked as `Failed()`. Added test runs many
iterations so we are more likely to trigger a failure on flakes.

fake_tb.go

@@ -55,8 +55,9 @@ func RunTest(testFn func(t testing.TB)) TestResult {
 	tb := newFakeTB()
 
 	go func() {
+		defer close(tb.completed)
 		defer tb.checkPanic()
-		defer tb.postTest()
+		defer tb.runCleanups()
 
 		testFn(tb)
 	}()
@@ -102,12 +103,6 @@ func (tb *fakeTB) checkPanic() {
 	}
 }
 
-func (tb *fakeTB) postTest() {
-	defer close(tb.completed)
-
-	tb.runCleanups()
-}
-
 func (tb *fakeTB) runCleanups() {
 	// Set cleanupRoot so log callers can use cleanup's callers.
 	if self := getCaller(withSelf); self != 0 {

faket_test.go

@@ -3,6 +3,7 @@ package faket
 import (
 	"testing"
 
+	"github.com/prashantv/faket/internal/syncutil"
 	"github.com/prashantv/faket/internal/want"
 )
 
@@ -41,3 +42,15 @@ func TestFakeT_Helpers(t *testing.T) {
 func testHelper1(t testing.TB) { t.Helper() }
 func testHelper2(t testing.TB) {}
 func testHelper3(t testing.TB) { t.Helper() }
+
+func TestFakeT_PanicRace(t *testing.T) {
+	syncutil.RunN(100, func(int) {
+		for i := 0; i < 100; i++ {
+			res := RunTest(func(t testing.TB) {
+				panic("panicked")
+			})
+			want.Equal(t, "Failed", res.Failed(), true)
+			want.Equal(t, "Panicked", res.Panicked(), true)
+		}
+	})
+}

internal/syncutil/syncutil.go

@@ -0,0 +1,19 @@
+// Package syncutil has helpers for running concurrent code.
+package syncutil
+
+import "sync"
+
+// RunN runs n items in parallel, with no concurrency limiting.
+func RunN(n int, fn func(i int)) {
+	var wg sync.WaitGroup
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+
+		go func(i int) {
+			defer wg.Done()
+
+			fn(i)
+		}(i)
+	}
+	wg.Wait()
+}

internal/syncutil/syncutil_test.go

@@ -0,0 +1,21 @@
+package syncutil
+
+import (
+	"sync/atomic"
+	"testing"
+
+	"github.com/prashantv/faket/internal/want"
+)
+
+func TestRunN(t *testing.T) {
+	const n = 10
+	var counter atomic.Int32
+	done := make(chan struct{})
+	RunN(n, func(i int) {
+		if counter.Add(1) == n {
+			close(done)
+		}
+		<-done
+	})
+	want.Equal(t, "counter", counter.Load(), 10)
+}