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/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 +5,12 @@
 package sync_test
 
 import (
+	"bytes"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
 	. "sync"
 	"sync/atomic"
 	"testing"
@@ -110,6 +116,38 @@ func TestWaitGroupGo(t *testing.T) {
 	}
 }
 
+// This test ensures that an unhandled panic in a Go goroutine terminates
+// the process without causing Wait to unblock; previously there was a race.
+func TestIssue76126(t *testing.T) {
+	testenv.MustHaveExec(t)
+	// Call child in a child process
+	// and inspect its failure message.
+	cmd := exec.Command(os.Args[0], "-test.run=^TestIssue76126Child$")
+	cmd.Env = append(os.Environ(), "SYNC_TEST_CHILD=1")
+	buf := new(bytes.Buffer)
+	cmd.Stderr = buf
+	cmd.Run() // ignore error
+
+	got := buf.String()
+	if strings.Contains(got, "panic: test") {
+		// ok
+	} else {
+		t.Errorf("missing panic: test\n%s", got)
+	}
+}
+
+func TestIssue76126Child(t *testing.T) {
+	if os.Getenv("SYNC_TEST_CHILD") != "1" {
+		t.Skip("not child")
+	}
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		panic("test")
+	})
+	wg.Wait()              // process should terminate here
+	panic("Wait returned") // must not be reached
+}
+
 func BenchmarkWaitGroupUncontended(b *testing.B) {
 	type PaddedWaitGroup struct {
 		WaitGroup