bug#72641: 31.0.50; "Unlocking file: Invalid argument" when deleting lock file on network file system

[Michal Nazarewicz <mina86@mina86.com>]

>> From: Michal Nazarewicz <mina86@mina86.com>
>> Date: Thu, 15 Aug 2024 17:41:03 +0200
>> 
>> I’m getting the same warning on Linux with Emacs 31.0.50 and it’s not
>> caused by a crash.
>> 
>>     emacs> M-x find-file RET /o/foo RET blah RET
>>     cli> $ ls -l /o/.#foo 
>>     cli> -rw------- 1 mpn mpn 0 2024-08-15 16:30 /o/.#foo
>>     emacs> M-x save-buffer RET
>>     emacs> ⛔ Warning (unlock-file): Unlocking file: Invalid argument, /o/foo, ignored
>>     cli> $ $ ls -l /o/.#foo /o/foo
>>     cli> -rw------- 1 mpn mpn 5 2024-08-15 16:31 /o/foo
>>     cli> -rw------- 1 mpn mpn 0 2024-08-15 16:30 /o/.#foo
>> 
>> The problem appears to be that /o is a network file-system which does
>> not support symbolic links:
>> 
>>     $ mount |grep /o
>>     //192.168.x.x/data on /o type cifs (rw,relatime,vers=3.0,cache=strict,username=mpn,uid=1000,forceuid,gid=1000,forcegid,addr=192.168.x.x,file_mode=0600,dir_mode=0700,soft,nounix,serverino,mapposix,reparse=nfs,rsize=4194304,wsize=4194304,bsize=1048576,retrans=1,echo_interval=60,actimeo=1,closetimeo=1,_netdev)
>>     $ ln -s /o/foo /o/f
>>     ln: failed to create symbolic link '/o/f': Input/output error
>> 
>> I guess Emacs notices that when it tries to create a lock file and falls
>> back to creating a file, but then it assumes it’s a symlink when trying
>> to remove it.

On Thu, Aug 15 2024, Eli Zaretskii wrote:
> I'm not sure we need to do anything here, either, but maybe Paul
> (CC'ed) has other suggestions or ideas.

The way I see it, Emacs has created those files so Emacs should clean
after itself.  Though I’ve looked at it a bit more closely and it’s
weirder than my original guess.

symlink(2) creates the file (as in it creates a regular file rather than
a symbolic link) and returns EIO.  create_lock_file reports an error and
does not go into its fallback.  This is why the lock file is empty.  In
the end, the error comes from current_lock_owner as it tries to parse
the empty file.

The *Warnings* buffer constantly popping up is annoying so I’d love this
to be addressed though to be honest I’m not sure what would be a good
solution.  Though I guess you can also argue this is CIFS bug.

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»