os: don't try to make the directory FD non-blocking in os.ReadDir

This will fail because epoll_ctl() fails on directory FDs, so we
end up issuing unnecessary syscalls. My test program that calls
filepath.WalkDir on a large directory tree runs 1.23 ± 0.04 times
faster than with the original implementation.

Change-Id: Ie33d798c48057a7b2d0bacac80fcdde5b5a8bb1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/570877
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/os/dir.go

@@ -115,7 +115,7 @@ var testingForceReadDirLstat bool
 // ReadDir returns the entries it was able to read before the error,
 // along with the error.
 func ReadDir(name string) ([]DirEntry, error) {
-	f, err := Open(name)
+	f, err := openDir(name)
 	if err != nil {
 		return nil, err
 	}

src/os/file.go

@@ -380,6 +380,14 @@ func OpenFile(name string, flag int, perm FileMode) (*File, error) {
 	return f, nil
 }
 
+// openDir opens a file which is assumed to be a directory. As such, it skips
+// the syscalls that make the file descriptor non-blocking as these take time
+// and will fail on file descriptors for directories.
+func openDir(name string) (*File, error) {
+	testlog.Open(name)
+	return openDirNolog(name)
+}
+
 // lstat is overridden in tests.
 var lstat = Lstat
 

src/os/file_plan9.go

@@ -139,6 +139,10 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	return NewFile(uintptr(fd), name), nil
 }
 
+func openDirNolog(name string) (*File, error) {
+	return openFileNolog(name, O_RDONLY, 0)
+}
+
 // Close closes the File, rendering it unusable for I/O.
 // On files that support SetDeadline, any pending I/O operations will
 // be canceled and return immediately with an ErrClosed error.

src/os/file_unix.go

@@ -157,7 +157,7 @@ const (
 	kindNonBlock
 	// kindNoPoll means that we should not put the descriptor into
 	// non-blocking mode, because we know it is not a pipe or FIFO.
-	// Used by openFdAt for directories.
+	// Used by openFdAt and openDirNolog for directories.
 	kindNoPoll
 )
 
@@ -256,7 +256,7 @@ func epipecheck(file *File, e error) {
 const DevNull = "/dev/null"
 
 // openFileNolog is the Unix implementation of OpenFile.
-// Changes here should be reflected in openFdAt, if relevant.
+// Changes here should be reflected in openFdAt and openDirNolog, if relevant.
 func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	setSticky := false
 	if !supportsCreateWithStickyBit && flag&O_CREATE != 0 && perm&ModeSticky != 0 {
@@ -303,6 +303,32 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	return f, nil
 }
 
+func openDirNolog(name string) (*File, error) {
+	var r int
+	var s poll.SysFile
+	for {
+		var e error
+		r, s, e = open(name, O_RDONLY|syscall.O_CLOEXEC, 0)
+		if e == nil {
+			break
+		}
+
+		if e == syscall.EINTR {
+			continue
+		}
+
+		return nil, &PathError{Op: "open", Path: name, Err: e}
+	}
+
+	if !supportsCloseOnExec {
+		syscall.CloseOnExec(r)
+	}
+
+	f := newFile(r, name, kindNoPoll)
+	f.pfd.SysFile = s
+	return f, nil
+}
+
 func (file *file) close() error {
 	if file == nil {
 		return syscall.EINVAL

src/os/file_windows.go

@@ -119,6 +119,10 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	return newFile(r, name, "file"), nil
 }
 
+func openDirNolog(name string) (*File, error) {
+	return openFileNolog(name, O_RDONLY, 0)
+}
+
 func (file *file) close() error {
 	if file == nil {
 		return syscall.EINVAL