files-backend: don't rewrite the `packed-refs` file unnecessarily

Even when we are deleting references, we needn't overwrite the
`packed-refs` file if the references that we are deleting only exist
as loose references. Implement this optimization as follows:

* Add a function `is_packed_transaction_needed()`, which checks
  whether a given packed-refs transaction actually needs to be carried
  out (i.e., it returns false if the transaction obviously wouldn't
  have any effect). This function must be called while holding the
  `packed-refs` lock to avoid races.

* Change `files_transaction_prepare()` to check whether the
  packed-refs transaction is actually needed. If not, squelch it, but
  continue holding the `packed-refs` lock until the end of the
  transaction to avoid races.

This fixes a mild regression caused by dc39e09942 (files_ref_store:
use a transaction to update packed refs, 2017-09-08). Before that
commit, unnecessary rewrites of `packed-refs` were suppressed by
`repack_without_refs()`. But the transaction-based writing introduced
by that commit didn't perform that optimization.

Note that the pre-dc39e09942 code still had to *read* the whole
`packed-refs` file to determine that the rewrite could be skipped, so
the performance for the cases that the write could be elided was
`O(N)` in the number of packed references both before and after
dc39e09942. But after that commit the constant factor increased.

This commit reimplements the optimization of eliding unnecessary
`packed-refs` rewrites. That, plus the fact that since
cfa2e29c34 (packed_ref_store: get rid of the `ref_cache` entirely,
2017-03-17) we don't necessarily have to read the whole `packed-refs`
file at all, means that deletes of one or a few loose references can
now be done with `O(n lg N)` effort, where `n` is the number of loose
references being deleted and `N` is the total number of packed
references.

This commit fixes two tests in t1409.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/t/t1409-avoid-packing-refs.sh b/t/t1409-avoid-packing-refs.sh
index a2397c7b71..e5cb8a252d 100755
--- a/t/t1409-avoid-packing-refs.sh
+++ b/t/t1409-avoid-packing-refs.sh
@@ -26,7 +26,7 @@ test_expect_success 'setup' '
 	C=$(git rev-parse HEAD)
 '
 
-test_expect_failure 'do not create packed-refs file gratuitously' '
+test_expect_success 'do not create packed-refs file gratuitously' '
 	test_must_fail test -f .git/packed-refs &&
 	git update-ref refs/heads/foo $A &&
 	test_must_fail test -f .git/packed-refs &&
@@ -107,7 +107,7 @@ test_expect_success 'leave packed-refs untouched on verify of loose' '
 	check_packed_refs_marked
 '
 
-test_expect_failure 'leave packed-refs untouched on delete of loose' '
+test_expect_success 'leave packed-refs untouched on delete of loose' '
 	git pack-refs --all &&
 	git update-ref refs/heads/loose-delete $A &&
 	mark_packed_refs &&