On 6/11/2021 3:01 PM, Taylor Blau wrote: > On Fri, Jun 11, 2021 at 01:47:28PM -0400, Derrick Stolee wrote: >> On 6/10/2021 8:50 PM, Taylor Blau wrote: >>> On Fri, Jun 11, 2021 at 01:56:31AM +0200, Ævar Arnfjörð Bjarmason wrote: >>>> So yeah, maybe we can just unlink() them right away, or another way to >>>> handle the race is that load_commit_graph_chain() could just try again >>>> from the beginning in such a case, and presumably picking up the fresh >>>> just-rewritten chain. >>> >>> I'd probably be in favor of the latter. >> >> I want to point out that on Windows we cannot successfully unlink() >> a layer that is currently being read by another Git process. That >> will not affect server scenarios (to the best of my knowledge) but >> is important to many end users. > > Right, but isn't this already a problem today? Since the expiration > window is zero we are already effectively trying to unlink all merged > layers immediately: > > - Marking merged commit-graph layers as expired via > mark_commit_graphs() by setting their mtime to "now", and then > > - Immediately removing all layers which have mtime older than an > instant later in expire_commit_graphs(). The commit-graph management we built in Scalar and VFS for Git uses a non-zero expire time to avoid this problem. However, since we will clean up the ones that fail to unlink() in a future cleanup, we did not do a similar thing in Git's background maintenance and have not had any issues. > (I almost suggested that a race already exists between multiple writers > that merge multiple layers of the commit-graph, but that race doesn't > exist because the commit-graph chain is written before other layers are > marked and expired.) And the maintenance buitin solves this by using a maintenance.lock file to avoid concurrent processes operating on the same repo. > In any case, it seems like the return value from unlink() is > deliberately ignored in case another process is holding an expired layer > open when we try to unlink it. So we'll eventually clean up all layers > that don't belong to the commit-graph-chain, but at the granularity of > new writes. Correct. It also requires new data for the write, or the commit-graph write will leave early and skip the cleanup IIRC. > (FWIW, I had to re-read 8d84097f96 (commit-graph: expire commit-graph > files, 2019-06-18) which mentions that a configuration variable would be > introduced to change the expiration window, but we don't have any such > configuration option. It also doesn't make any mention of handling this > problem on Windows, which made me think that the unlink() calls weren't > checking their return values by accident when in fact it was probably on > purpose.) That config option never appeared, probably because ignoring the unlink() return was sufficient to get around this problem. Thanks for digging in and making sure I remembered this correctly. Thanks, -Stolee
