[PATCH 14/14] refs: reuse iterators when determining refname availability

[Patrick Steinhardt <ps@pks.im>]

When verifying whether refnames are available we have to verify whether
any reference exists that is nested under the current reference. E.g.
given a reference "refs/heads/foo", we must make sure that there is no
other reference "refs/heads/foo/*".

This check is performed using a ref iterator with the prefix set to the
nested reference namespace. Until now it used to not be possible to
reseek iterators, so we always had to reallocate the iterator for every
single reference we're about to check. This keeps us from reusing state
that the iterator may have and that may make it work more efficiently.

Refactor the logic to reseek iterators. This leads to a speedup with the
reftable backend, which is the only backend that knows to batch refname
availability checks:

    Benchmark 1: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~)
      Time (mean ± σ):      39.8 ms ±   0.9 ms    [User: 29.7 ms, System: 9.8 ms]
      Range (min … max):    38.4 ms …  42.0 ms    62 runs

    Benchmark 2: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD)
      Time (mean ± σ):      31.9 ms ±   1.1 ms    [User: 27.0 ms, System: 4.5 ms]
      Range (min … max):    29.8 ms …  34.3 ms    74 runs

    Summary
      update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD) ran
        1.25 ± 0.05 times faster than update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~)

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/refs.c b/refs.c
index 8eff60a2186..6cbb9decdb0 100644
--- a/refs.c
+++ b/refs.c
@@ -2555,8 +2555,13 @@ int refs_verify_refnames_available(struct ref_store *refs,
 		if (!initial_transaction) {
 			int ok;
 
-			iter = refs_ref_iterator_begin(refs, dirname.buf, NULL, 0,
-						       DO_FOR_EACH_INCLUDE_BROKEN);
+			if (!iter) {
+				iter = refs_ref_iterator_begin(refs, dirname.buf, NULL, 0,
+							       DO_FOR_EACH_INCLUDE_BROKEN);
+			} else if (ref_iterator_seek(iter, dirname.buf) < 0) {
+				goto cleanup;
+			}
+
 			while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
 				if (skip &&
 				    string_list_has_string(skip, iter->refname))
@@ -2569,9 +2574,6 @@ int refs_verify_refnames_available(struct ref_store *refs,
 
 			if (ok != ITER_DONE)
 				BUG("error while iterating over references");
-
-			ref_iterator_free(iter);
-			iter = NULL;
 		}
 
 		extra_refname = find_descendant_ref(dirname.buf, extras, skip);

-- 
2.48.1.666.gff9fcf71b7.dirty