From: Derrick Stolee <stolee@gmail.com>
Date: Fri, 23 Feb 2018 17:59:00 GMT
Subject: Re: [PATCH v4 08/13] commit-graph: implement --delete-expired
Message-ID: <67871288-ea70-69d4-9c3d-5cc946d8dbe2@gmail.com>
In-Reply-To: <xmqqfu5sx3md.fsf@gitster-ct.c.googlers.com>

On 2/22/2018 1:48 PM, Junio C Hamano wrote:
> Derrick Stolee <stolee@gmail.com> writes:
>
>> Teach git-commit-graph to delete the .graph files that are siblings of a
>> newly-written graph file, except for the file referenced by 'graph-latest'
>> at the beginning of the process and the newly-written file. If we fail to
>> delete a graph file, only report a warning because another git process may
>> be using that file. In a multi-process environment, we expect the previoius
>> graph file to be used by a concurrent process, so we do not delete it to
>> avoid race conditions.
> I do not understand the later part of the above.  On some operating
> systems, you actually can remove a file that is open by another
> process without any ill effect.  There are systems that do not allow
> removing a file that is in use, and an attempt to unlink it may
> fail.  The need to handle such a failure gracefully is not limited
> to the case of removing a commit graph file---we need to deal with
> it when removing file of _any_ type.

My thought is that we should _warn_ when we fail to delete a .graph file 
that we think should be safe to delete. However, if we are warning for a 
file that is currently being accessed (as is the case on Windows, at 
least), then we will add a lot of noise. This is especially true when 
using IDEs that run 'status' or 'fetch' in the background, frequently.

> Especially the last sentence "we do not delete it to avoid race
> conditions" I find problematic.  If a system does not allow removing
> a file in use and we detect a failure after an attempt to do so, it
> is not "we do not delete it" --- even if you do, you won't succeed
> anyway, so there is no point saying that.  And on systems that do
> allow safe removal of a file in use (i.e. they allow an open file to
> be used by processes that have open filehandles to it after its
> removal), there is no point refraining to delete it "to avoid race
> conditions", either---in fact it is unlikely that you would even know
> somebody else had it open and was using it.

The (unlikely, but possible) race condition involves two processes (P1 
and P2):

1. P1 reads from graph-latest to see commit graph file F1.
2. P2 updates graph-latest to point to F2 and deletes F1.
3. P1 tries to read F1 and fails.

I could explicitly mention this condition in the message, or we can just 
let P2 fail by deleting all files other than the one referenced by 
'graph-latest'. Thoughts?

> In any case, I do not think '--delete-expired' option that can be
> given only when you are writing makes much sense as an API.  An
> 'expire' command, just like 'set-latest' command, that is a separate
> command from 'write',  may make sense, though.

In another message, I proposed dropping the argument and assuming 
expires happen on every write.

Thanks,
-Stolee