sync: repanic when f() panics for WaitGroup.Go

This is a copy-paste of
#76126 (comment)
by adonovan.

Fixes #76126
Fixes #74702

Author: callthingsoff

src/sync/testdata/issue76126/issue76126_test.go

@@ -0,0 +1,14 @@
+package issue76126
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestIssue76126(t *testing.T) {
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		panic("test")
+	})
+	wg.Wait()
+}

src/sync/waitgroup.go

@@ -236,7 +236,18 @@ func (wg *WaitGroup) Wait() {
 func (wg *WaitGroup) Go(f func()) {
 	wg.Add(1)
 	go func() {
-		defer wg.Done()
+		defer func() {
+			if x := recover(); x != nil {
+				// Don't call Done as it may cause Wait to unblock,
+				// so that the main goroutine races with the runtime.fatal
+				// resulting from unhandled panic.
+				panic(x)
+			}
+
+			// f completed normally, or abruptly using goexit.
+			// Either way, decrement the semaphore.
+			wg.Done()
+		}()
 		f()
 	}()
 }

src/sync/waitgroup_test.go

@@ -5,6 +5,9 @@
 package sync_test
 
 import (
+	"internal/testenv"
+	"os/exec"
+	"path/filepath"
 	. "sync"
 	"sync/atomic"
 	"testing"
@@ -110,6 +113,22 @@ func TestWaitGroupGo(t *testing.T) {
 	}
 }
 
+func TestIssue76126(t *testing.T) {
+	testenv.MustHaveGoRun(t)
+
+	cmd := exec.Command(testenv.GoToolPath(t), "test", "-race")
+	cmd.Dir = filepath.Join("testdata", "issue76126")
+	b, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("go test -race failed: %v\n%s", err, b)
+	}
+	if code := cmd.ProcessState.ExitCode(); code == 0 {
+		t.Fatalf("expected non-zero exit code, got %d", code)
+	}
+
+	// Should fail with panic.
+}
+
 func BenchmarkWaitGroupUncontended(b *testing.B) {
 	type PaddedWaitGroup struct {
 		WaitGroup