syscall: add test for Flock_t roundtrip

See CL 9962 for the rationale.

Change-Id: I73c714fce258430eea1e61d3835f5c8e9014ca1f
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/9925
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/syscall_unix_test.go

@@ -60,7 +60,19 @@ func _() {
 
 // TestFcntlFlock tests whether the file locking structure matches
 // the calling convention of each kernel.
+// On some Linux systems, glibc uses another set of values for the
+// commands and translates them to the correct value that the kernel
+// expects just before the actual fcntl syscall. As Go uses raw
+// syscalls directly, it must use the real value, not the glibc value.
+// Thus this test also verifies that the Flock_t structure can be
+// roundtripped with F_SETLK and F_GETLK.
 func TestFcntlFlock(t *testing.T) {
+	flock := syscall.Flock_t{
+		Type:  syscall.F_WRLCK,
+		Start: 31415, Len: 271828, Whence: 1,
+	}
+	if os.Getenv("GO_WANT_HELPER_PROCESS") == "" {
+		// parent
 		name := filepath.Join(os.TempDir(), "TestFcntlFlock")
 		fd, err := syscall.Open(name, syscall.O_CREAT|syscall.O_RDWR|syscall.O_CLOEXEC, 0)
 		if err != nil {
@@ -68,12 +80,35 @@ func TestFcntlFlock(t *testing.T) {
 		}
 		defer syscall.Unlink(name)
 		defer syscall.Close(fd)
-	flock := syscall.Flock_t{
+		if err := syscall.Ftruncate(fd, 1<<20); err != nil {
-		Type:  syscall.F_RDLCK,
+			t.Fatalf("Ftruncate(1<<20) failed: %v", err)
-		Start: 0, Len: 0, Whence: 1,
 		}
-	if err := syscall.FcntlFlock(uintptr(fd), syscall.F_GETLK, &flock); err != nil {
+		if err := syscall.FcntlFlock(uintptr(fd), syscall.F_SETLK, &flock); err != nil {
-		t.Fatalf("FcntlFlock failed: %v", err)
+			t.Fatalf("FcntlFlock(F_SETLK) failed: %v", err)
+		}
+		cmd := exec.Command(os.Args[0], "-test.run=^TestFcntlFlock$")
+		cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
+		cmd.ExtraFiles = []*os.File{os.NewFile(uintptr(fd), name)}
+		out, err := cmd.CombinedOutput()
+		if len(out) > 0 || err != nil {
+			t.Fatalf("child process: %q, %v", out, err)
+		}
+	} else {
+		// child
+		got := flock
+		// make sure the child lock is conflicting with the parent lock
+		got.Start--
+		got.Len++
+		if err := syscall.FcntlFlock(3, syscall.F_GETLK, &got); err != nil {
+			t.Fatalf("FcntlFlock(F_GETLK) failed: %v", err)
+		}
+		flock.Pid = int32(syscall.Getppid())
+		// Linux kernel always set Whence to 0
+		flock.Whence = 0
+		if got.Type == flock.Type && got.Start == flock.Start && got.Len == flock.Len && got.Pid == flock.Pid && got.Whence == flock.Whence {
+			os.Exit(0)
+		}
+		t.Fatalf("FcntlFlock got %v, want %v", got, flock)
 	}
 }
 
@@ -121,7 +156,7 @@ func TestPassFD(t *testing.T) {
 	defer readFile.Close()
 
 	cmd := exec.Command(os.Args[0], "-test.run=^TestPassFD$", "--", tempDir)
-	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
+	cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
 	cmd.ExtraFiles = []*os.File{writeFile}
 
 	out, err := cmd.CombinedOutput()