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([2a0a:ef40:7d8:fa01:60c8:18fb:2acc:d4f]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43fe4cb1365sm21821047f8f.7.2026.04.19.07.04.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 19 Apr 2026 07:04:49 -0700 (PDT) Message-ID: Date: Sun, 19 Apr 2026 15:04:47 +0100 Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: phillip.wood@dunelm.org.uk Subject: Re: [PATCH v2 2/3] builtin/log: prefetch necessary blobs for `git cherry` To: Elijah Newren via GitGitGadget , git@vger.kernel.org Cc: Elijah Newren References: Content-Language: en-US From: Phillip Wood In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi Elijah On 18/04/2026 01:32, Elijah Newren via GitGitGadget wrote: > From: Elijah Newren > > In partial clones, `git cherry` fetches necessary blobs on-demand one > at a time, which can be very slow. We would like to prefetch all > necessary blobs upfront. To do so, we need to be able to first figure > out which blobs are needed. "git rebase" without "--reapply-cherry-picks" suffers from this problem as well as it does the equivalent of "git log --cherry-pick". Is there any way to share prefetch_cherry_blobs() with the cherry-pick detection in revision.c? Thanks Phillip > `git cherry` does its work in a two-phase approach: first computing > header-only IDs (based on file paths and modes), then falling back to > full content-based IDs only when header-only IDs collide -- or, more > accurately, whenever the oidhash() of the header-only object_ids > collide. > > patch-ids.c handles this by creating an ids->patches hashmap that has > all the data we need, but the problem is that any attempt to query the > hashmap will invoke the patch_id_neq() function on any colliding objects, > which causes the on-demand fetching. > > Insert a new prefetch_cherry_blobs() function before checking for > collisions. Use a temporary replacement on the ids->patches.cmpfn > in order to enumerate the blobs that would be needed without yet > fetching them, and then fetch them all at once, then restore the old > ids->patches.cmpfn. > > Signed-off-by: Elijah Newren > --- > builtin/log.c | 125 ++++++++++++++++++++++++++++++++++++++++++++++ > t/t3500-cherry.sh | 18 +++++++ > 2 files changed, 143 insertions(+) > > diff --git a/builtin/log.c b/builtin/log.c > index 8c0939dd42..df19876be6 100644 > --- a/builtin/log.c > +++ b/builtin/log.c > @@ -21,10 +21,12 @@ > #include "color.h" > #include "commit.h" > #include "diff.h" > +#include "diffcore.h" > #include "diff-merges.h" > #include "revision.h" > #include "log-tree.h" > #include "oid-array.h" > +#include "oidset.h" > #include "tag.h" > #include "reflog-walk.h" > #include "patch-ids.h" > @@ -43,9 +45,11 @@ > #include "utf8.h" > > #include "commit-reach.h" > +#include "promisor-remote.h" > #include "range-diff.h" > #include "tmp-objdir.h" > #include "tree.h" > +#include "userdiff.h" > #include "write-or-die.h" > > #define MAIL_DEFAULT_WRAP 72 > @@ -2602,6 +2606,125 @@ static void print_commit(char sign, struct commit *commit, int verbose, > } > } > > +/* > + * Enumerate blob OIDs from a single commit's diff, inserting them into blobs. > + * Skips files whose userdiff driver explicitly declares binary status > + * (drv->binary > 0), since patch-ID uses oid_to_hex() for those and > + * never reads blob content. Use userdiff_find_by_path() since > + * diff_filespec_load_driver() is static in diff.c. > + * > + * Clean up with diff_queue_clear() (from diffcore.h). > + */ > +static void collect_diff_blob_oids(struct commit *commit, > + struct diff_options *opts, > + struct oidset *blobs) > +{ > + struct diff_queue_struct *q; > + > + /* > + * Merge commits are filtered out by patch_id_defined() in patch-ids.c, > + * so we'll never be called with one. > + */ > + assert(!commit->parents || !commit->parents->next); > + > + if (commit->parents) > + diff_tree_oid(&commit->parents->item->object.oid, > + &commit->object.oid, "", opts); > + else > + diff_root_tree_oid(&commit->object.oid, "", opts); > + diffcore_std(opts); > + > + q = &diff_queued_diff; > + for (int i = 0; i < q->nr; i++) { > + struct diff_filepair *p = q->queue[i]; > + struct userdiff_driver *drv; > + > + /* Skip binary files */ > + drv = userdiff_find_by_path(opts->repo->index, p->one->path); > + if (drv && drv->binary > 0) > + continue; > + > + if (DIFF_FILE_VALID(p->one)) > + oidset_insert(blobs, &p->one->oid); > + if (DIFF_FILE_VALID(p->two)) > + oidset_insert(blobs, &p->two->oid); > + } > + diff_queue_clear(q); > +} > + > +static int always_match(const void *cmp_data UNUSED, > + const struct hashmap_entry *entry1 UNUSED, > + const struct hashmap_entry *entry2 UNUSED, > + const void *keydata UNUSED) > +{ > + return 0; > +} > + > +/* > + * Prefetch blobs for git cherry in partial clones. > + * > + * Called between the revision walk (which builds the head-side > + * commit list) and the has_commit_patch_id() comparison loop. > + * > + * Uses a cmpfn-swap trick to avoid reading blobs: temporarily > + * replaces the hashmap's comparison function with a trivial > + * always-match function, so hashmap_get()/hashmap_get_next() match > + * any entry with the same oidhash bucket. These are the set of oids > + * that would trigger patch_id_neq() during normal lookup and cause > + * blobs to be read on demand, and we want to prefetch them all at > + * once instead. > + */ > +static void prefetch_cherry_blobs(struct repository *repo, > + struct commit_list *list, > + struct patch_ids *ids) > +{ > + struct oidset blobs = OIDSET_INIT; > + hashmap_cmp_fn original_cmpfn; > + > + /* Exit if we're not in a partial clone */ > + if (!repo_has_promisor_remote(repo)) > + return; > + > + /* Save original cmpfn, replace with always_match */ > + original_cmpfn = ids->patches.cmpfn; > + ids->patches.cmpfn = always_match; > + > + /* Find header-only collisions, gather blobs from those commits */ > + for (struct commit_list *l = list; l; l = l->next) { > + struct commit *c = l->item; > + bool match_found = false; > + for (struct patch_id *cur = patch_id_iter_first(c, ids); > + cur; > + cur = patch_id_iter_next(cur, ids)) { > + match_found = true; > + collect_diff_blob_oids(cur->commit, &ids->diffopts, > + &blobs); > + } > + if (match_found) > + collect_diff_blob_oids(c, &ids->diffopts, &blobs); > + } > + > + /* Restore original cmpfn */ > + ids->patches.cmpfn = original_cmpfn; > + > + /* If we have any blobs to fetch, fetch them */ > + if (oidset_size(&blobs)) { > + struct oid_array to_fetch = OID_ARRAY_INIT; > + struct oidset_iter iter; > + const struct object_id *oid; > + > + oidset_iter_init(&blobs, &iter); > + while ((oid = oidset_iter_next(&iter))) > + oid_array_append(&to_fetch, oid); > + > + promisor_remote_get_direct(repo, to_fetch.oid, to_fetch.nr); > + > + oid_array_clear(&to_fetch); > + } > + > + oidset_clear(&blobs); > +} > + > int cmd_cherry(int argc, > const char **argv, > const char *prefix, > @@ -2673,6 +2796,8 @@ int cmd_cherry(int argc, > commit_list_insert(commit, &list); > } > > + prefetch_cherry_blobs(the_repository, list, &ids); > + > for (struct commit_list *l = list; l; l = l->next) { > char sign = '+'; > > diff --git a/t/t3500-cherry.sh b/t/t3500-cherry.sh > index 78c3eac54b..17507d9a28 100755 > --- a/t/t3500-cherry.sh > +++ b/t/t3500-cherry.sh > @@ -78,4 +78,22 @@ test_expect_success 'cherry ignores whitespace' ' > test_cmp expect actual > ' > > +# Reuse the expect file from the previous test, in a partial clone > +test_expect_success 'cherry in partial clone does bulk prefetch' ' > + test_config uploadpack.allowfilter 1 && > + test_config uploadpack.allowanysha1inwant 1 && > + test_when_finished "rm -rf copy" && > + > + git clone --bare --filter=blob:none file://"$(pwd)" copy && > + ( > + cd copy && > + GIT_TRACE2_EVENT="$(pwd)/trace.output" git cherry upstream-with-space feature-without-space >actual && > + test_cmp ../expect actual && > + > + grep "child_start.*fetch.negotiationAlgorithm" trace.output >fetches && > + test_line_count = 1 fetches && > + test_trace2_data promisor fetch_count 4 + ) > +' > + > test_done