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>
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