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>

4 files changed, 18 insertions, 3 deletions

diff --git a/src/os/dir_darwin.go b/src/os/dir_darwin.go
index 2f9ba78d68..a274dd1268 100644
--- a/src/os/dir_darwin.go
+++ b/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()
diff --git a/src/os/dir_unix.go b/src/os/dir_unix.go
index e0c4989756..2856a2dc0f 100644
--- a/src/os/dir_unix.go
+++ b/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 {
diff --git a/src/os/file_unix.go b/src/os/file_unix.go
index 31c43eb61e..6945937fd6 100644
--- a/src/os/file_unix.go
+++ b/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
diff --git a/src/syscall/fs_js.go b/src/syscall/fs_js.go
index f7079e9d09..16d9f58b8c 100644
--- a/src/syscall/fs_js.go
+++ b/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
 }