fetch: make `--atomic` flag cover pruning of refs

When fetching with the `--prune` flag we will delete any local
references matching the fetch refspec which have disappeared on the
remote. This step is not currently covered by the `--atomic` flag: we
delete branches even though updating of local references has failed,
which means that the fetch is not an all-or-nothing operation.

Fix this bug by passing in the global transaction into `prune_refs()`:
if one is given, then we'll only queue up deletions and not commit them
right away.

This change also improves performance when pruning many branches in a
repository with a big packed-refs file: every references is pruned in
its own transaction, which means that we potentially have to rewrite
the packed-refs files for every single reference we're about to prune.

The following benchmark demonstrates this: it performs a pruning fetch
from a repository with a single reference into a repository with 100k
references, which causes us to prune all but one reference. This is of
course a very artificial setup, but serves to demonstrate the impact of
only having to write the packed-refs file once:

    Benchmark 1: git fetch --prune --atomic +refs/*:refs/* (HEAD~)
      Time (mean ± σ):      2.366 s ±  0.021 s    [User: 0.858 s, System: 1.508 s]
      Range (min … max):    2.328 s …  2.407 s    10 runs

    Benchmark 2: git fetch --prune --atomic +refs/*:refs/* (HEAD)
      Time (mean ± σ):      1.369 s ±  0.017 s    [User: 0.715 s, System: 0.641 s]
      Range (min … max):    1.346 s …  1.400 s    10 runs

    Summary
      'git fetch --prune --atomic +refs/*:refs/* (HEAD)' ran
        1.73 ± 0.03 times faster than 'git fetch --prune --atomic +refs/*:refs/* (HEAD~)'

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 70d51f343b..48e14e2dab 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -354,17 +354,13 @@ test_expect_success 'fetch --atomic --prune executes a single reference transact
 	head_oid=$(git rev-parse HEAD) &&
 
 	# Fetching with the `--atomic` flag should update all references in a
-	# single transaction. It is currently missing coverage of pruned
-	# references though, and as a result those may be committed to disk
-	# even if updating references fails later.
+	# single transaction.
 	cat >expected <<-EOF &&
 		prepared
 		$ZERO_OID $ZERO_OID refs/remotes/origin/scheduled-for-deletion
-		committed
-		$ZERO_OID $ZERO_OID refs/remotes/origin/scheduled-for-deletion
-		prepared
 		$ZERO_OID $head_oid refs/remotes/origin/new-branch
 		committed
+		$ZERO_OID $ZERO_OID refs/remotes/origin/scheduled-for-deletion
 		$ZERO_OID $head_oid refs/remotes/origin/new-branch
 	EOF