os: handle EINTR from open(2) on darwin.

The man page for sigaction(2) doesn't guarantee that SA_RESTART will
work for open(2) on regular files:

    The affected system calls include open(2), read(2), write(2),
    sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a
    communications channel or a slow device (such as a terminal, but not
    a regular file) and during a wait(2) or ioctl(2).

I've never observed EINTR from open(2) for a traditional file system
such as HFS+, but it's easy to observe with a fuse file system that is
slightly slow (cf. https://goo.gl/UxsVgB). After this change, the
problem can no longer be reproduced when calling os.OpenFile.

Fixes golang#11180.

Change-Id: I967247430e20a7d29a285b3d76bf3498dc4773db

Author: jacobsa

src/os/file_unix.go

@@ -90,8 +90,16 @@ func OpenFile(name string, flag int, perm FileMode) (*File, error) {
 		}
 	}
 
+retry:
 	r, e := syscall.Open(name, flag|syscall.O_CLOEXEC, syscallMode(perm))
-	if e != nil {
+	switch {
+	// On OS X, sigaction(2) doesn't guarantee that SA_RESTART will cause open(2)
+	// to be restarted for regular files. This is easy to reproduce on fuse file
+	// systems (see http://golang.org/issue/11180).
+	case runtime.GOOS == "darwin" && e == syscall.EINTR:
+		goto retry
+
+	case e != nil:
 		return nil, &PathError{"open", name, e}
 	}