runtime: skip TestLockOSThreadAvoidsStatePropagation if one can't unshare

This change splits a testprog out of TestLockOSThreadExit and makes it
its own test. Then, this change makes the testprog exit prematurely with
a special message if unshare fails with EPERM because not all of the
builders allow the user to call the unshare syscall.

Also, do some minor cleanup on the TestLockOSThread* tests.

Fixes #29366.

Change-Id: Id8a9f6c4b16e26af92ed2916b90b0249ba226dbe
Reviewed-on: https://go-review.googlesource.com/c/155437
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/proc_test.go

@@ -885,23 +885,28 @@ func TestLockOSThreadNesting(t *testing.T) {
 
 func TestLockOSThreadExit(t *testing.T) {
 	testLockOSThreadExit(t, "testprog")
-
-	want := "OK\n"
-	output := runTestProg(t, "testprog", "LockOSThreadAvoidsStatePropagation", "GOMAXPROCS=1")
-	if output != want {
-		t.Errorf("want %s, got %s\n", want, output)
-	}
 }
 
 func testLockOSThreadExit(t *testing.T, prog string) {
 	output := runTestProg(t, prog, "LockOSThreadMain", "GOMAXPROCS=1")
 	want := "OK\n"
 	if output != want {
-		t.Errorf("want %s, got %s\n", want, output)
+		t.Errorf("want %q, got %q", want, output)
 	}
 
 	output = runTestProg(t, prog, "LockOSThreadAlt")
 	if output != want {
-		t.Errorf("want %s, got %s\n", want, output)
+		t.Errorf("want %q, got %q", want, output)
+	}
+}
+
+func TestLockOSThreadAvoidsStatePropagation(t *testing.T) {
+	want := "OK\n"
+	skip := "unshare not permitted\n"
+	output := runTestProg(t, "testprog", "LockOSThreadAvoidsStatePropagation", "GOMAXPROCS=1")
+	if output == skip {
+		t.Skip("unshare syscall not permitted on this system")
+	} else if output != want {
+		t.Errorf("want %q, got %q", want, output)
 	}
 }

src/runtime/testdata/testprog/lockosthread.go

@@ -144,6 +144,10 @@ func LockOSThreadAvoidsStatePropagation() {
 		// the rest of the process on this thread.
 		// On systems other than Linux, this is a no-op.
 		if err := unshareFs(); err != nil {
+			if err == errNotPermitted {
+				println("unshare not permitted")
+				os.Exit(0)
+			}
 			println("failed to unshare fs:", err.Error())
 			os.Exit(1)
 		}

src/runtime/testdata/testprog/syscalls.go

@@ -2,53 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux
-
 package main
 
 import (
-	"bytes"
-	"fmt"
-	"io/ioutil"
-	"os"
-	"syscall"
+	"errors"
 )
 
-func gettid() int {
-	return syscall.Gettid()
-}
-
-func tidExists(tid int) (exists, supported bool) {
-	stat, err := ioutil.ReadFile(fmt.Sprintf("/proc/self/task/%d/stat", tid))
-	if os.IsNotExist(err) {
-		return false, true
-	}
-	// Check if it's a zombie thread.
-	state := bytes.Fields(stat)[2]
-	return !(len(state) == 1 && state[0] == 'Z'), true
-}
-
-func getcwd() (string, error) {
-	if !syscall.ImplementsGetwd {
-		return "", nil
-	}
-	// Use the syscall to get the current working directory.
-	// This is imperative for checking for OS thread state
-	// after an unshare since os.Getwd might just check the
-	// environment, or use some other mechanism.
-	var buf [4096]byte
-	n, err := syscall.Getcwd(buf[:])
-	if err != nil {
-		return "", err
-	}
-	// Subtract one for null terminator.
-	return string(buf[:n-1]), nil
-}
-
-func unshareFs() error {
-	return syscall.Unshare(syscall.CLONE_FS)
-}
-
-func chdir(path string) error {
-	return syscall.Chdir(path)
-}
+var errNotPermitted = errors.New("operation not permitted")

src/runtime/testdata/testprog/syscalls_linux.go

+// 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.
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"syscall"
+)
+
+func gettid() int {
+	return syscall.Gettid()
+}
+
+func tidExists(tid int) (exists, supported bool) {
+	stat, err := ioutil.ReadFile(fmt.Sprintf("/proc/self/task/%d/stat", tid))
+	if os.IsNotExist(err) {
+		return false, true
+	}
+	// Check if it's a zombie thread.
+	state := bytes.Fields(stat)[2]
+	return !(len(state) == 1 && state[0] == 'Z'), true
+}
+
+func getcwd() (string, error) {
+	if !syscall.ImplementsGetwd {
+		return "", nil
+	}
+	// Use the syscall to get the current working directory.
+	// This is imperative for checking for OS thread state
+	// after an unshare since os.Getwd might just check the
+	// environment, or use some other mechanism.
+	var buf [4096]byte
+	n, err := syscall.Getcwd(buf[:])
+	if err != nil {
+		return "", err
+	}
+	// Subtract one for null terminator.
+	return string(buf[:n-1]), nil
+}
+
+func unshareFs() error {
+	err := syscall.Unshare(syscall.CLONE_FS)
+	if err != nil {
+		errno, ok := err.(syscall.Errno)
+		if ok && errno == syscall.EPERM {
+			return errNotPermitted
+		}
+	}
+	return err
+}
+
+func chdir(path string) error {
+	return syscall.Chdir(path)
+}