os: make openFdAt act like openFileNolog

- add EINTR loop on Darwin
- return PathError on error
- call newFile rather than NewFile

This tries to minimize the possibility of any future changes.
It would be nice to put openFdAt in the same file as openFileNolog,
but build tags forbid.

Updates golang#29983

Change-Id: I866002416d6473fbfd80ff6ef09b2bc4607f2934
Reviewed-on: https://go-review.googlesource.com/c/160181
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

src/os/file_unix.go

@@ -186,6 +186,7 @@ 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.
 func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 	setSticky := false
 	if !supportsCreateWithStickyBit && flag&O_CREATE != 0 && perm&ModeSticky != 0 {

src/os/removeall_at.go

@@ -9,6 +9,7 @@ package os
 import (
 	"internal/syscall/unix"
 	"io"
+	"runtime"
 	"syscall"
 )
 
@@ -128,11 +129,31 @@ func removeAllFrom(parent *File, path string) error {
 	return unlinkError
 }
 
-func openFdAt(fd int, path string) (*File, error) {
-	fd, err := unix.Openat(fd, path, O_RDONLY, 0)
-	if err != nil {
-		return nil, err
+// openFdAt opens path relative to the directory in fd.
+// Other than that this should act like openFileNolog.
+// This acts like openFileNolog rather than OpenFile because
+// we are going to (try to) remove the file.
+// The contents of this file are not relevant for test caching.
+func openFdAt(dirfd int, name string) (*File, error) {
+	var r int
+	for {
+		var e error
+		r, e = unix.Openat(dirfd, name, O_RDONLY, 0)
+		if e == nil {
+			break
+		}
+
+		// See comment in openFileNolog.
+		if runtime.GOOS == "darwin" && e == syscall.EINTR {
+			continue
+		}
+
+		return nil, &PathError{"openat", name, e}
+	}
+
+	if !supportsCloseOnExec {
+		syscall.CloseOnExec(r)
 	}
 
-	return NewFile(uintptr(fd), path), nil
+	return newFile(uintptr(r), name, kindOpenFile), nil
 }