Re: [PATCH] qemu-nbd: Document benefit of --pid-file

[Vladimir Sementsov-Ogievskiy]

08.10.2019 12:24, Daniel P. Berrangé wrote:
> On Mon, Oct 07, 2019 at 02:48:40PM -0500, Eric Blake wrote:
>> One benefit of --pid-file is that it is easier to probe the file
>> system to see if a pid file has been created than it is to probe if a
>> socket is available for connection. Document that this is an
>> intentional feature.
> 
> I'm not seeing how checking the pid file is better than checking
> the socket directly ? I think it is probably actually worse.
> 
> The main problem with the socket is that while we unlink on clean
> shutdown, it may still exist in disk if the process has exitted
> abnormally.
> 
> With the pidfile though we don't ever unlink it, even on clean
> shutdown, as we don't use the pid files existance as a mutual
> exclusion check. We instead acquire fcntl locks on it.
> 
> IOW the pidfile could exist already when qemu-nbd starts up and
> will still exist when it quits.

Good point.

I was just a bit confused, because pid file is something unrelated to
socket, and why use one thing to check the existence of another, if we
can directly try to connect.

> 
>> Signed-off-by: Eric Blake <address@hidden>
>> ---
>>   qemu-nbd.texi | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/qemu-nbd.texi b/qemu-nbd.texi
>> index 7f55657722bd..d495bbe8a0ed 100644
>> --- a/qemu-nbd.texi
>> +++ b/qemu-nbd.texi
>> @@ -118,7 +118,8 @@ in list mode.
>>   @item --fork
>>   Fork off the server process and exit the parent once the server is running.
>>   @item --pid-file=PATH
>> -Store the server's process ID in the given file.
>> +Store the server's process ID in the given file.  The pid file is not
>> +created until after the server socket is open.
>>   @item --tls-authz=ID
>>   Specify the ID of a qauthz object previously created with the
>>   --object option. This will be used to authorize connecting users
>> -- 
>> 2.21.0
>>
>>
> 
> Regards,
> Daniel
> 


-- 
Best regards,
Vladimir