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>

1 files changed, 5 insertions, 3 deletions

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
 }