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From: | Eric Blake |
Subject: | Re: [PATCH] qemu-nbd: Document benefit of --pid-file |
Date: | Tue, 8 Oct 2019 08:28:16 -0500 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 |
On 10/8/19 4:40 AM, Vladimir Sementsov-Ogievskiy wrote:
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.
Consider the case of writing a testsuite that involves an nbd client, where you want to fire up qemu-nbd as the server. Checking for a pid file in shell is easy, and can be done prior to the point of spawning a client. Checking for a successful connect is harder - the shell is not the point that would actually connect, so checking if connect works involves the shell actually spawning off the child process that attempts the connect. If the client itself has a retry builtin, then you don't need to do anything in shell - just spawn the client with retry (at which point, the client retrying on the connection is smarter than the client retrying on the pid file). But pid files are immensely useful when you have a client that does not have builtin retry, and when writing a testing framework where you use shell to learn whether it is safe to spawn the client: rather than having to modify the client or write a shell loop that respawns child attempts, you merely need a shell loop probing for the pid file to exist.
-- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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