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>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/os/file.go b/src/os/file.go
index c0c972bbd7..fae7bf1039 100644
--- a/src/os/file.go
+++ b/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