runtime, syscall: reset signal handlers to default in child

Block all signals during a fork. In the parent process, after the
fork, restore the signal mask. In the child process, reset all
currently handled signals to the default handler, and then restore the
signal mask.

The effect of this is that the child will be operating using the same
signal regime as the program it is about to exec, as exec resets all
non-ignored signals to the default, and preserves the signal mask.

We do this so that in the case of a signal sent to the process group,
the child process will not try to run a signal handler while in the
precarious state after a fork.

Fixes #18600.

Change-Id: I9f39aaa3884035908d687ee323c975f349d5faaa
Reviewed-on: https://go-review.googlesource.com/45471
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprognet/signalexec.go b/src/runtime/testdata/testprognet/signalexec.go
new file mode 100644
index 0000000000..4a988ef6c1
--- /dev/null
+++ b/src/runtime/testdata/testprognet/signalexec.go
@@ -0,0 +1,70 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+// This is in testprognet instead of testprog because testprog
+// must not import anything (like net, but also like os/signal)
+// that kicks off background goroutines during init.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"os/signal"
+	"sync"
+	"syscall"
+	"time"
+)
+
+func init() {
+	register("SignalDuringExec", SignalDuringExec)
+	register("Nop", Nop)
+}
+
+func SignalDuringExec() {
+	pgrp := syscall.Getpgrp()
+
+	const tries = 10
+
+	var wg sync.WaitGroup
+	c := make(chan os.Signal, tries)
+	signal.Notify(c, syscall.SIGWINCH)
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for range c {
+		}
+	}()
+
+	for i := 0; i < tries; i++ {
+		time.Sleep(time.Microsecond)
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			cmd := exec.Command(os.Args[0], "Nop")
+			cmd.Stdout = os.Stdout
+			cmd.Stderr = os.Stderr
+			if err := cmd.Run(); err != nil {
+				fmt.Printf("Start failed: %v", err)
+			}
+		}()
+		go func() {
+			defer wg.Done()
+			syscall.Kill(-pgrp, syscall.SIGWINCH)
+		}()
+	}
+
+	signal.Stop(c)
+	close(c)
+	wg.Wait()
+
+	fmt.Println("OK")
+}
+
+func Nop() {
+	// This is just for SignalDuringExec.
+}