Re: Auto packing the repository - foreground or background in Windows?

From: Jeff Hostetler <git@jeffhostetler.com>
Date: Dec 6, 2022, 22:41 UTC
Message-ID: <091329e0-9af3-b853-0ed3-edd10d2a2b72@jeffhostetler.com>
In-Reply-To: <4dcad1f5-9ebd-d15a-b663-a3513ae1bcb8@github.com>

On 12/6/22 1:03 PM, Derrick Stolee wrote:

Show 9 quoted lines

> On 12/1/2022 7:25 AM, Tao Klerks wrote:
>> But my *question* is: Does anyone know where I could/should look to
>> understand why the GC was happening in the foreground, even though the
>> message says it will run in the background?
> 
> On Windows, Git's foreground process cannot complete without also
> killing the background process. I'm not sure on the concrete details,
> but the lack of a background "git gc --auto" here is deliberate for
> that platform.

Here the GC code uses `daemonize()`. On Posix this is a wrapper around `fork()` where the parent exits and the child continues the computation (without stdin/out).

However, `daemonize()` just returns an ENOSYS on Windows, since Windows doesn't have `fork()`. The net result is that the foreground process falls thru and does the actual work.

[...]

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> Instead, the modern recommendation for repositories where "git gc --auto"
> would be slow is to run "git maintenance start" which will schedule
> background maintenance jobs with the Windows scheduler. Those processes
> are built to do updates that are non-invasive to concurrent foreground
> processes. It also sets config to avoid "git gc --auto" commands at the
> end of foreground Git processes.
> 
> See [1] for more details.
> 
> [1] https://git-scm.com/docs/git-maintenance

It is possible to do the GC in the background on Windows (using other techniques), but I don't think it is worth the bother now that we have `git maintenance` to handle it.

And yes, AEvar's suggested fix for printing the correct error message looks helpful.

Jeff