runtime: no SIGWINCH to pgrp while GDB is running

When run with stdin == /dev/null and stdout/stderr == pipe (i.e., as
os/exec.Command.CombinedOutput), GDB suffers from a bug
(https://sourceware.org/bugzilla/show_bug.cgi?id=26056) that causes
SIGSEGV when sent a SIGWINCH signal.

Package runtime tests TestEINTR and TestSignalDuringExec both send
SIGWINCH signals to the entire process group, thus including GDB if one
of the GDB tests is running in parallel.

TestEINTR only intends its signals for the current process, so it is
changed to do so. TestSignalDuringExec, really does want its signals to
go to children. However, it does not call t.Parallel(), so it won't run
at the same time as GDB tests.

This is a simple fix, but GDB is vulnerable, so we must be careful not
to add new parallel tests that send SIGWINCH to the entire process
group.

Fixes #39021

Change-Id: I803606fb000f08c65c1b10ec554d4ef6819e5dd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/235557
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/runtime/runtime-gdb_test.go

@@ -19,6 +19,12 @@ import (
 	"testing"
 )
 
+// NOTE: In some configurations, GDB will segfault when sent a SIGWINCH signal.
+// Some runtime tests send SIGWINCH to the entire process group, so those tests
+// must never run in parallel with GDB tests.
+//
+// See issue 39021 and https://sourceware.org/bugzilla/show_bug.cgi?id=26056.
+
 func checkGdbEnvironment(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 	switch runtime.GOOS {

src/runtime/testdata/testprogcgo/eintr.go

@@ -89,8 +89,9 @@ func spin() (float64, []byte) {
 func winch() {
 	ticker := time.NewTicker(100 * time.Microsecond)
 	defer ticker.Stop()
+	pid := syscall.Getpid()
 	for n := 10; n > 0; n-- {
-		syscall.Kill(0, syscall.SIGWINCH)
+		syscall.Kill(pid, syscall.SIGWINCH)
 		<-ticker.C
 	}
 }