[PATCH v2] refs: return conflict error when checking packed refs

From: Ivan Tse <ivan.tse1@gmail.com>

The TRANSACTION_NAME_CONFLICT error code refers to a failure to create a ref due to a name conflict with another ref. An example of this is a directory/file conflict such as ref names A/B and A.

"git fetch" uses this error code to more accurately describe the error by recommending to the user that they try running "git remote prune" to remove any old refs that are deleted by the remote which would clear up any directory/file conflicts.

This helpful error message is not displayed when the conflicted ref is stored in packed refs. This change fixes this by ensuring error return code consistency in `lock_raw_ref`.

Signed-off-by: Ivan Tse <ivan.tse1@gmail.com>
---
    refs: return conflict error when checking packed refs
    
    Changes against v1:
    
     * added test for the error message during git fetch
    
    Thanks for reviewing! I've gone ahead and attempted to add tests for
    this behavior. It tests that the error message is shown for both cases
    when the ref is stored as loose vs packed-refs. How does this test look?
    Also, should this test have a REFFILES prerequisite?

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1716%2Fivantsepp%2Freturn_name_conflict_error_for_packed_refs-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1716/ivantsepp/return_name_conflict_error_for_packed_refs-v2
Pull-Request: https://github.com/git/git/pull/1716

Range-diff vs v1:

 1:  a87ba267e44 ! 1:  58b2cda5c18 refs: return conflict error when checking packed refs
     @@ refs/files-backend.c: static int lock_raw_ref(struct files_ref_store *refs,
       	}
       
       	ret = 0;
     +
     + ## t/t5510-fetch.sh ##
     +@@ t/t5510-fetch.sh: test_expect_success 'branchname D/F conflict resolved by --prune' '
     + 	test_cmp expect actual
     + '
     + 
     ++test_expect_success 'branchname D/F conflict rejected with targeted error message' '
     ++	git clone . df-conflict-error &&
     ++	git branch dir_conflict &&
     ++	(
     ++		cd df-conflict-error &&
     ++		git update-ref refs/remotes/origin/dir_conflict/file HEAD &&
     ++		test_must_fail git fetch 2>../err &&
     ++		git pack-refs --all &&
     ++		test_must_fail git fetch 2>../err-packed
     ++	) &&
     ++	test_grep "error: some local refs could not be updated; try running" err &&
     ++	test_grep " '\''git remote prune origin'\'' to remove any old, conflicting branches" err &&
     ++	test_grep "error: some local refs could not be updated; try running" err-packed &&
     ++	test_grep " '\''git remote prune origin'\'' to remove any old, conflicting branches" err-packed
     ++'
     ++
     + test_expect_success 'fetching a one-level ref works' '
     + 	test_commit extra &&
     + 	git reset --hard HEAD^ &&

 refs/files-backend.c |  4 +++-
 t/t5510-fetch.sh     | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)