os: reset dirinfo when seeking on Darwin

The first Readdirnames calls opendir and caches the result.
The behavior of that cached opendir result isn't specified on a seek
of the underlying fd. Free the opendir result on a seek so that
we'll allocate a new one the next time around.

Also fix wasm behavior in this regard, so that a seek to the
file start resets the Readdirnames position, regardless of platform.

p.s. I hate the Readdirnames API.

Fixes #35767.

Change-Id: Ieffb61b3c5cdd42591f69ab13f932003966f2297
Reviewed-on: https://go-review.googlesource.com/c/go/+/209961
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/dir_darwin.go

@@ -24,6 +24,16 @@ func (d *dirInfo) close() {
 	d.dir = 0
 }
 
+func (f *File) seekInvalidate() {
+	if f.dirinfo == nil {
+		return
+	}
+	// Free cached dirinfo, so we allocate a new one if we
+	// access this file as a directory again. See #35767.
+	f.dirinfo.close()
+	f.dirinfo = nil
+}
+
 func (f *File) readdirnames(n int) (names []string, err error) {
 	if f.dirinfo == nil {
 		dir, call, errno := f.pfd.OpenDir()

src/os/dir_unix.go

@@ -26,6 +26,8 @@ const (
 
 func (d *dirInfo) close() {}
 
+func (f *File) seekInvalidate() {}
+
 func (f *File) readdirnames(n int) (names []string, err error) {
 	// If this file has no dirinfo, create one.
 	if f.dirinfo == nil {

src/os/file_unix.go

@@ -295,6 +295,7 @@ func (f *File) pwrite(b []byte, off int64) (n int, err error) {
 // relative to the current offset, and 2 means relative to the end.
 // It returns the new offset and an error, if any.
 func (f *File) seek(offset int64, whence int) (ret int64, err error) {
+	f.seekInvalidate()
 	ret, err = f.pfd.Seek(offset, whence)
 	runtime.KeepAlive(f)
 	return ret, err

src/syscall/fs_js.go

@@ -34,6 +34,7 @@ var (
 type jsFile struct {
 	path    string
 	entries []string
+	dirIdx  int // entries[:dirIdx] have already been returned in ReadDirent
 	pos     int64
 	seeked  bool
 }
@@ -141,8 +142,8 @@ func ReadDirent(fd int, buf []byte) (int, error) {
 	}
 
 	n := 0
-	for len(f.entries) > 0 {
-		entry := f.entries[0]
+	for f.dirIdx < len(f.entries) {
+		entry := f.entries[f.dirIdx]
 		l := 2 + len(entry)
 		if l > len(buf) {
 			break
@@ -152,7 +153,7 @@ func ReadDirent(fd int, buf []byte) (int, error) {
 		copy(buf[2:], entry)
 		buf = buf[l:]
 		n += l
-		f.entries = f.entries[1:]
+		f.dirIdx++
 	}
 
 	return n, nil
@@ -470,6 +471,7 @@ func Seek(fd int, offset int64, whence int) (int64, error) {
 	}
 
 	f.seeked = true
+	f.dirIdx = 0 // Reset directory read position. See issue 35767.
 	f.pos = newPos
 	return newPos, nil
 }

test/fixedbugs/issue35767.go

@@ -0,0 +1,51 @@
+// run
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"log"
+	"os"
+)
+
+func main() {
+	wd, err := os.Getwd()
+	if err != nil {
+		log.Fatal(err)
+	}
+	f, err := os.Open(wd)
+	if err != nil {
+		log.Fatal(err)
+	}
+	dirnames1, err := f.Readdirnames(0)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	ret, err := f.Seek(0, 0)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if ret != 0 {
+		log.Fatalf("seek result not zero: %d", ret)
+	}
+
+	dirnames2, err := f.Readdirnames(0)
+	if err != nil {
+		log.Fatal(err)
+		return
+	}
+
+	if len(dirnames1) != len(dirnames2) {
+		log.Fatalf("listings have different lengths: %d and %d\n", len(dirnames1), len(dirnames2))
+	}
+	for i, n1 := range dirnames1 {
+		n2 := dirnames2[i]
+		if n1 != n2 {
+			log.Fatalf("different name i=%d n1=%s n2=%s\n", i, n1, n2)
+		}
+	}
+}