os, syscall: revert Yosemite readdir workaround

Reverts https://golang.org/cl/119530044 (OS X 10.10 Yosemite beta
14A299l workaround), since it was fixed in the final Yosemite release.

I verified that the C program http://swtch.com/~rsc/readdirbug.c
passes on Yosemite.

Adds a new test to the os package too, to verify that reading a
regular file as a directory fails.

Fixes #9789 (ReadDir: no error if dirname is a file)

Change-Id: I75286cef88fbb2ebccf045b479e33c810749dcbc
Reviewed-on: https://go-review.googlesource.com/4164Reviewed-by: Dave Cheney <dave@cheney.net>

src/os/os_test.go

@@ -491,6 +491,30 @@ func TestReaddirStatFailures(t *testing.T) {
 	}
 }
 
+// Readdir on a regular file should fail.
+func TestReaddirOfFile(t *testing.T) {
+	f, err := ioutil.TempFile("", "_Go_ReaddirOfFile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Remove(f.Name())
+	f.Write([]byte("foo"))
+	f.Close()
+	reg, err := Open(f.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer reg.Close()
+
+	names, err := reg.Readdirnames(-1)
+	if err == nil {
+		t.Error("Readdirnames succeeded; want non-nil error")
+	}
+	if len(names) > 0 {
+		t.Errorf("unexpected dir names in regular file: %q", names)
+	}
+}
+
 func TestHardLink(t *testing.T) {
 	// Hardlinks are not supported under windows or Plan 9.
 	if runtime.GOOS == "plan9" {

src/syscall/syscall_bsd.go

@@ -68,40 +68,7 @@ func ReadDirent(fd int, buf []byte) (n int, err error) {
 	// actual system call is getdirentries64, 64 is a good guess.
 	// TODO(rsc): Can we use a single global basep for all calls?
 	var base = (*uintptr)(unsafe.Pointer(new(uint64)))
-	n, err = Getdirentries(fd, buf, base)
-
-	// On OS X 10.10 Yosemite, if you have a directory that can be returned
-	// in a single getdirentries64 call (for example, a directory with one file),
-	// and you read from the directory at EOF twice, you get EOF both times:
-	//	fd = open("dir")
-	//	getdirentries64(fd) // returns data
-	//	getdirentries64(fd) // returns 0 (EOF)
-	//	getdirentries64(fd) // returns 0 (EOF)
-	//
-	// But if you remove the file in the middle between the two calls, the
-	// second call returns an error instead.
-	//	fd = open("dir")
-	//	getdirentries64(fd) // returns data
-	//	getdirentries64(fd) // returns 0 (EOF)
-	//	remove("dir/file")
-	//	getdirentries64(fd) // returns ENOENT/EINVAL
-	//
-	// Whether you get ENOENT or EINVAL depends on exactly what was
-	// in the directory. It is deterministic, just data-dependent.
-	//
-	// This only happens in small directories. A directory containing more data
-	// than fits in a 4k getdirentries64 call will return EOF correctly.
-	// (It's not clear if the criteria is that the directory be split across multiple
-	// getdirentries64 calls or that it be split across multiple file system blocks.)
-	//
-	// We could change package os to avoid the second read at EOF,
-	// and maybe we should, but that's a bit involved.
-	// For now, treat the EINVAL/ENOENT as EOF.
-	if runtime.GOOS == "darwin" && (err == EINVAL || err == ENOENT) {
-		err = nil
-	}
-
-	return
+	return Getdirentries(fd, buf, base)
 }
 
 // Wait status is 7 bits at bottom, either 0 (exited),