fetch: skip prune walk without refspec destinations#2336
fetch: skip prune walk without refspec destinations#2336michaelm-openai wants to merge 1 commit into
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With `--prune` and explicit refspecs, get_stale_heads() compares
every local ref with every refspec. A refspec without a destination
cannot match a local ref, but get_stale_heads() still walks all local
refs.
In a repository with an `origin` remote, a user can trigger this scan
by fetching a branch without its blobs and then checking it out:
git fetch --filter=blob:none origin <branch>
git -c fetch.prune=true checkout FETCH_HEAD
The filtered fetch may omit blobs needed to check out FETCH_HEAD.
`git checkout` sends the missing blob OIDs to `git fetch --stdin` as
refspecs without destinations. For M missing blobs and N local refs,
get_stale_heads() performs O(MN) refspec checks and cannot prune any
ref. The checkout cannot update the worktree until the scan finishes.
Make get_stale_heads() return before it builds the fetched-ref list or
calls refs_for_each_ref() when no positive refspec has a destination.
Otherwise, build the list and walk the refs so pruning still handles
the local refs named by refspec destinations.
Add t5510 tests showing that OIDs supplied through `--stdin` keep
remote-tracking branches, while adding a refspec with a destination
still prunes mapped branches.
Add `t/perf/p5552-fetch-prune.sh` to measure elapsed time for
`git fetch --prune --stdin`. The test feeds 10,000 OIDs through
`--stdin`; each OID forms a refspec without a destination. It also
creates 20,000 packed remote-tracking refs. To isolate the local scan
from object transfer, the child repository already contains every
requested object.
Compare this commit with its parent using:
./t/perf/run HEAD^ HEAD -- p5552-fetch-prune.sh
The perf harness selects the best of three runs:
5552.3: fetch --prune with refspecs without destinations
HEAD^: 0.80(0.68+0.04)
HEAD: 0.14(0.06+0.04) -82.5%
Signed-off-by: Michael Mitchell <michaelm@openai.com>
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