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Date: Fri, 21 Nov 2025 19:00:09 +0900
To: cygwin AT cygwin DOT com
Subject: Re: flock/open random error
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Fri, 14 Nov 2025 11:27:04 -0800
Nahor wrote:
> If `flock()` was used on the same file descriptor, then this might
> have been a valid point. However, each thread has its own file
> descriptor in this case, so this would be very surprising if it wasn't
> thread-safe.

IIUC, flock() locks file itself, but not file descriptor. Usually,
flock() is used for inter-process file protection, isn't it?

> Moreover, it's not just `flock()` failing, it's also (and mostly!)
> `open()` that fails. And it's the `open()` for a completely different
> file than the one being locked. So that would suggest that `open()` is
> not also not MT-safe. And not safe when using different files. And not
> safe across multiple different functions (flock+open).

Indeed, this is really weird. I looked into this, and found 'upath' in
path.cc is destroyed after 'NtCreateFile()' call at the following line.

I added assertion as follows:

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index 710775e38..562100161 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -3189,6 +3189,8 @@ restart:
 	 symlink (which would spoil the task of this method quite a bit).
 	 Fortunately it's ignored on most other file systems so we don't have
 	 to special case NFS too much. */
+      wchar_t c;
+      c = upath.Buffer[0];
       status = NtCreateFile (&h,
 			     READ_CONTROL | FILE_READ_ATTRIBUTES | FILE_READ_EA,
 			     &attr, &io, NULL, 0, FILE_SHARE_VALID_FLAGS,
@@ -3196,6 +3198,7 @@ restart:
 			     FILE_OPEN_REPARSE_POINT
 			     | FILE_OPEN_FOR_BACKUP_INTENT,
 			     eabuf, easize);
+      assert (upath.Buffer[0] == c);
       debug_printf ("%y = NtCreateFile (%S)", status, &upath);
       /* No right to access EAs or EAs not supported? */
       if (!NT_SUCCESS (status)

then, the assertion fails for your test case like:
tmp_dir: /tmp/flockAQ4Hbb
assertion "upath.Buffer[0] == c" failed: file "../../.././winsup/cygwin/path.cc", line 3201, function: int symlink_info::check(char*, const suffix_info*, fs_info&, path_conv_handle&)
                      Abort

Does another thread destroy the puthbuf? But pathbuf is thread local, IIUC.
Corinna, have you noticed anything?

> And more generally, I've never heard of file operations not being
> thread safe. Atomicity and ordering are common problems, but not
> thread safety.

The linux man page for flock() does not mention about MT-safe
https://man7.org/linux/man-pages/man2/flock.2.html

while it mentions about MT-safe for
https://man7.org/linux/man-pages/man3/lockf.3.html

So I doubt flock() should really be MT-safe.

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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