Re: [PATCH v4 09/10] merge: check config before loading commits

> Derrick Stolee <dstolee@microsoft.com> writes:
>
>> Now that we use generation numbers from the commit-graph, we must
>> ensure that all commits that exist in the commit-graph are loaded
>> from that file instead of from the object database. Since the
>> commit-graph file is only checked if core.commitGraph is true, we
>> must check the default config before we load any commits.
>>
>> In the merge builtin, the config was checked after loading the HEAD
>> commit. This was due to the use of the global 'branch' when checking
>> merge-specific config settings.
>>
>> Move the config load to be between the initialization of 'branch' and
>> the commit lookup.
> Sidenote: I wonder why reading config was postponed to later in the
> command lifetime... I guess it was to avoid having to read config if
> HEAD was invalid.

>
>> Without this change, a fast-forward merge would hit a BUG("bad
>> generation skip") statement in commit.c during paint_down_to_common().
>> This is because the HEAD commit would be loaded with "infinite"
>> generation but then reached by commits with "finite" generation
>> numbers.
> I guess this is because we avoid re-parsing objects at all costs; we
> want to avoid re-reading commit graph too.
>
>> Add a test to t5318-commit-graph.sh that exercises this code path to
>> prevent a regression.
> I would prefer if this commit was put earlier in the series, to avoid
> having broken Git (and thus a possibility of problems when bisecting) in
> between those two commits.
>
>> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
>> ---
>>   builtin/merge.c         | 7 ++++---
>>   t/t5318-commit-graph.sh | 9 +++++++++
>>   2 files changed, 13 insertions(+), 3 deletions(-)
>>
>> diff --git a/builtin/merge.c b/builtin/merge.c
>> index 5e5e4497e3..b819756946 100644
>> --- a/builtin/merge.c
>> +++ b/builtin/merge.c
>> @@ -1148,14 +1148,15 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
>>   	branch = branch_to_free = resolve_refdup("HEAD", 0, &head_oid, NULL);
>>   	if (branch)
>>   		skip_prefix(branch, "refs/heads/", &branch);
>> +
>> +	init_diff_ui_defaults();
>> +	git_config(git_merge_config, NULL);
>> +
>>   	if (!branch || is_null_oid(&head_oid))
>>   		head_commit = NULL;
>>   	else
>>   		head_commit = lookup_commit_or_die(&head_oid, "HEAD");
>>   
>> -	init_diff_ui_defaults();
>> -	git_config(git_merge_config, NULL);
>> -
> Good.
>
>>   	if (branch_mergeoptions)
>>   		parse_branch_merge_options(branch_mergeoptions);
>>   	argc = parse_options(argc, argv, prefix, builtin_merge_options,
>> diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
>> index a380419b65..77d85aefe7 100755
>> --- a/t/t5318-commit-graph.sh
>> +++ b/t/t5318-commit-graph.sh
>> @@ -221,4 +221,13 @@ test_expect_success 'write graph in bare repo' '
>>   graph_git_behavior 'bare repo with graph, commit 8 vs merge 1' bare commits/8 merge/1
>>   graph_git_behavior 'bare repo with graph, commit 8 vs merge 2' bare commits/8 merge/2
>>   
>> +test_expect_success 'perform fast-forward merge in full repo' '
>> +	cd "$TRASH_DIRECTORY/full" &&
>> +	git checkout -b merge-5-to-8 commits/5 &&
>> +	git merge commits/8 &&
>> +	git show-ref -s merge-5-to-8 >output &&
>> +	git show-ref -s commits/8 >expect &&
>> +	test_cmp expect output
>> +'
> All right.  (though I wonder if this tests catches all problems where
> BUG("bad generation skip") could have been encountered.