threadentry_test.go RawHistoryFolder

1// SPDX-License-Identifier: Apache-2.0
2// SPDX-FileCopyrightText: 2026 The Ebitengine Authors
3
4// 386/arm are excluded: the standalone fakecgo test binary resolves pthread
5// from libpthread.so.0, which fails the dynamic symbol lookup on 32-bit linux.
6//go:build !cgo && (linux || darwin || freebsd || netbsd) && !386 && !arm
7
8package fakecgo
9
10import (
11	"runtime"
12	"sync"
13	"testing"
14)
15
16// TestThreadEntryReturn exercises the fakecgo threadentry -> runtime.mstart
17// return path, i.e. that an M created by fakecgo can exit cleanly.
18//
19// When iscgo is forced true by fakecgo, the runtime creates every new OS
20// thread through _cgo_thread_start -> threadentry_trampoline -> threadentry,
21// which calls runtime.mstart. For a fakecgo thread the M's stack is
22// system-allocated, so when the M exits, mexit(osStack=true) returns from
23// mstart back into threadentry. This return path has been the source of
24// crashes (a frame-pointer LEAVE fault on amd64, a bad indirect call and race
25// instrumentation on other platforms), so it is worth exercising everywhere.
26//
27// Locking an OS thread and then letting the goroutine exit (without calling
28// UnlockOSThread) forces the locked fakecgo M to exit, driving mstart to return
29// into threadentry. A busy loop that never lets the goroutine exit does NOT
30// reproduce the crash, because the M never exits and mstart never returns.
31func TestThreadEntryReturn(t *testing.T) {
32	// The M exits on the first goroutine return, so a modest amount of churn is
33	// enough to catch a broken teardown while staying quick under emulation.
34	const rounds = 10
35	const workers = 16
36	for range rounds {
37		var wg sync.WaitGroup
38		wg.Add(workers)
39		for range workers {
40			go func() {
41				defer wg.Done()
42				runtime.LockOSThread()
43				// Intentionally do NOT call UnlockOSThread: returning here exits
44				// the goroutine while its OS thread is locked, forcing the M to
45				// exit and mstart to return into fakecgo.threadentry.
46			}()
47		}
48		wg.Wait()
49	}
50}