[pacman-dev] Improving locking - Was: [PATCH] Use file locks (flock) to stop multiple pacman instances from running concurrently. This improves the current model by automatically removing locks held by programs has failed to remove the lock, for example at a crash, forced kill or power outage.

[Jan Alexander Steffens]

On Mon, Aug 19, 2013 at 10:58 AM, Martin Panter <vadmium+patch at gmail.com> wrote:
> On 19 August 2013 08:48, Martin Panter <vadmium+patch at gmail.com> wrote:
>> Pardon me if I don’t know enough details of Pacman’s locking, but this
>> sounds like a race condition, defeating the purpose of a lock. Even on
>> a single machine, what happens if two instances A and B have both
>> decided the old process does not exist, then instance A removes the
>> old lock, and creates a new valid lock for itself, just before
>> instance B also tries to remove the old lock? How do you stop A from
>> illegally removing B’s valid lock? Maybe you could have a two stage
>> thing or something but it’s getting a bit complex a.k.a. silly.
>
> Sorry I got my processes mixed up at the end. Here’s the steps:
>
> * Lock file exists with process X which no longer exists
> * Pacman process A starts and determines X does not exist
> * Pacman process  B starts and determines X does not exist
> * Process A removes the old lock file which referenced process X
> * Process A creates a new lock file for itself (referencing process A now)
> * How do you make sure B does not delete the new lock file A just created?

How about this?

* Open lock file with O_RDWR | O_APPEND | O_CREAT and read.
-> If file is empty, we can run.
-> If file contains a non-existing PID and the current hostname on its
last line, we can run.
* If we can run, write new line containing current PID and hostname.
* Rewind file and read again. Verify content is as expected.
* Do database operation.
* Remove lock file.

I'm not sure how well it works over a network. Maybe one or more
fsyncs can be introduced to ensure the appending and reading is
synchronized with the server?