Merge branch 'mh/tidy-ref-update-flags'
[git/git.git] / refs / packed-backend.c
index 72164a1..023243f 100644 (file)
@@ -1261,6 +1261,100 @@ error:
        return -1;
 }
 
+int is_packed_transaction_needed(struct ref_store *ref_store,
+                                struct ref_transaction *transaction)
+{
+       struct packed_ref_store *refs = packed_downcast(
+                       ref_store,
+                       REF_STORE_READ,
+                       "is_packed_transaction_needed");
+       struct strbuf referent = STRBUF_INIT;
+       size_t i;
+       int ret;
+
+       if (!is_lock_file_locked(&refs->lock))
+               BUG("is_packed_transaction_needed() called while unlocked");
+
+       /*
+        * We're only going to bother returning false for the common,
+        * trivial case that references are only being deleted, their
+        * old values are not being checked, and the old `packed-refs`
+        * file doesn't contain any of those reference(s). This gives
+        * false positives for some other cases that could
+        * theoretically be optimized away:
+        *
+        * 1. It could be that the old value is being verified without
+        *    setting a new value. In this case, we could verify the
+        *    old value here and skip the update if it agrees. If it
+        *    disagrees, we could either let the update go through
+        *    (the actual commit would re-detect and report the
+        *    problem), or come up with a way of reporting such an
+        *    error to *our* caller.
+        *
+        * 2. It could be that a new value is being set, but that it
+        *    is identical to the current packed value of the
+        *    reference.
+        *
+        * Neither of these cases will come up in the current code,
+        * because the only caller of this function passes to it a
+        * transaction that only includes `delete` updates with no
+        * `old_id`. Even if that ever changes, false positives only
+        * cause an optimization to be missed; they do not affect
+        * correctness.
+        */
+
+       /*
+        * Start with the cheap checks that don't require old
+        * reference values to be read:
+        */
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
+
+               if (update->flags & REF_HAVE_OLD)
+                       /* Have to check the old value -> needed. */
+                       return 1;
+
+               if ((update->flags & REF_HAVE_NEW) && !is_null_oid(&update->new_oid))
+                       /* Have to set a new value -> needed. */
+                       return 1;
+       }
+
+       /*
+        * The transaction isn't checking any old values nor is it
+        * setting any nonzero new values, so it still might be able
+        * to be skipped. Now do the more expensive check: the update
+        * is needed if any of the updates is a delete, and the old
+        * `packed-refs` file contains a value for that reference.
+        */
+       ret = 0;
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
+               unsigned int type;
+               struct object_id oid;
+
+               if (!(update->flags & REF_HAVE_NEW))
+                       /*
+                        * This reference isn't being deleted -> not
+                        * needed.
+                        */
+                       continue;
+
+               if (!refs_read_raw_ref(ref_store, update->refname,
+                                      &oid, &referent, &type) ||
+                   errno != ENOENT) {
+                       /*
+                        * We have to actually delete that reference
+                        * -> this transaction is needed.
+                        */
+                       ret = 1;
+                       break;
+               }
+       }
+
+       strbuf_release(&referent);
+       return ret;
+}
+
 struct packed_transaction_backend_data {
        /* True iff the transaction owns the packed-refs lock. */
        int own_lock;