Re: 2.29.0.rc0.windows.1: Duplicate commit id error message when fetching

[Jeff King <peff@peff.net>]

On Wed, Oct 07, 2020 at 10:28:36PM +0200, Thomas Braun wrote:

> $ git --version
> git version 2.29.0.rc0.windows.1
> 
> Since I upgraded to that version (thanks to dscho for providing these so
> early) I'm seeing occasionally
> 
> $ git fetch origin +refs/head/abcd:refs/remotes/origin/abcd
> fatal: unexpected duplicate commit id
> 31a13139875bc5f49ddcbd42b4b4d3dc18c16576

That message comes from the commit-graph code:

  $ git grep unexpected.duplicate.commit.id '*.c'
  commit-graph.c:                 die(_("unexpected duplicate commit id %s"),

So presumably it's related to the fetch.writeCommitGraph feature, though
I thought it was not on by default (for a while it was tied to
feature.experimental, but I think even that is not true in 2.29). Do you
have that option set?

The message is in sort_and_scan_merged_commits(), which is trying to
join multiple incremental commit-graph files together. Presumably you
have two such files with the same commit appearing in both. I think we
try to avoid that (by omitting commits from new incrementals that
already appear in another one), but I wonder if there is a race or other
condition that can cause it. In which case this code ought to be more
lenient, and just quietly ignore the duplicate.

Is it possible to share the contents of your .git directory? If not, can
you look in .git/objects/info/ and see if there are multiple
commit-graph files (and if so, possibly share those; they don't contain
any identifying info).

-Peff