os/exec: create extra threads when starting a subprocess

TestExtraFiles seems to be flaky on GNU/Linux systems when using cgo
because creating a new thread will call malloc which can create a new
arena which can open a file to see how many processors there are.
Try to avoid the flake by creating several new threads at process
startup time.

For #25628

Change-Id: Ie781acdbba475d993c39782fe172cf7f29a05b24
Reviewed-on: https://go-review.googlesource.com/c/go/+/228099
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/os/exec/exec_linux_test.go

@@ -0,0 +1,45 @@
+// Copyright 2020 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 linux,cgo
+
+// On systems that use glibc, calling malloc can create a new arena,
+// and creating a new arena can read /sys/devices/system/cpu/online.
+// If we are using cgo, we will call malloc when creating a new thread.
+// That can break TestExtraFiles if we create a new thread that creates
+// a new arena and opens the /sys file while we are checking for open
+// file descriptors. Work around the problem by creating threads up front.
+// See issue 25628.
+
+package exec_test
+
+import (
+	"os"
+	"sync"
+	"syscall"
+	"time"
+)
+
+func init() {
+	if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+		return
+	}
+
+	// Start some threads. 10 is arbitrary but intended to be enough
+	// to ensure that the code won't have to create any threads itself.
+	// In particular this should be more than the number of threads
+	// the garbage collector might create.
+	const threads = 10
+
+	var wg sync.WaitGroup
+	wg.Add(threads)
+	ts := syscall.NsecToTimespec((100 * time.Microsecond).Nanoseconds())
+	for i := 0; i < threads; i++ {
+		go func() {
+			defer wg.Done()
+			syscall.Nanosleep(&ts, nil)
+		}()
+	}
+	wg.Wait()
+}