Merge branch 'dt/commit-preserve-base-index-upon-opportunistic-cache-tree-update'

When re-priming the cache-tree opportunistically while committing
the in-core index as-is, we mistakenly invalidated the in-core
index too aggressively, causing the experimental split-index code
to unnecessarily rewrite the on-disk index file(s).

* dt/commit-preserve-base-index-upon-opportunistic-cache-tree-update:
  commit: don't rewrite shared index unnecessarily

3 files changed, 15 insertions, 5 deletions

diff --git a/builtin/commit.c b/builtin/commit.c
index b37cb6c8b..63772d016 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -405,10 +405,8 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
 		hold_locked_index(&index_lock, 1);
 		refresh_cache_or_die(refresh_flags);
 		if (active_cache_changed
-		    || !cache_tree_fully_valid(active_cache_tree)) {
+		    || !cache_tree_fully_valid(active_cache_tree))
 			update_main_cache_tree(WRITE_TREE_SILENT);
-			active_cache_changed = 1;
-		}
 		if (active_cache_changed) {
 			if (write_locked_index(&the_index, &index_lock,
 					       COMMIT_LOCK))
diff --git a/t/t0090-cache-tree.sh b/t/t0090-cache-tree.sh
index 055cc1900..adfd4f0b5 100755
--- a/t/t0090-cache-tree.sh
+++ b/t/t0090-cache-tree.sh
@@ -242,4 +242,14 @@ test_expect_success 'no phantom error when switching trees' '
 	! test -s errors
 '
 
+test_expect_success 'switching trees does not invalidate shared index' '
+	git update-index --split-index &&
+	>split &&
+	git add split &&
+	test-dump-split-index .git/index | grep -v ^own >before &&
+	git commit -m "as-is" &&
+	test-dump-split-index .git/index | grep -v ^own >after &&
+	test_cmp before after
+'
+
 test_done
diff --git a/test-dump-split-index.c b/test-dump-split-index.c
index 9cf3112c9..861d28c9b 100644
--- a/test-dump-split-index.c
+++ b/test-dump-split-index.c
@@ -26,9 +26,11 @@ int main(int ac, char **av)
 		       sha1_to_hex(ce->sha1), ce_stage(ce), ce->name);
 	}
 	printf("replacements:");
-	ewah_each_bit(si->replace_bitmap, show_bit, NULL);
+	if (si->replace_bitmap)
+		ewah_each_bit(si->replace_bitmap, show_bit, NULL);
 	printf("\ndeletions:");
-	ewah_each_bit(si->delete_bitmap, show_bit, NULL);
+	if (si->delete_bitmap)
+		ewah_each_bit(si->delete_bitmap, show_bit, NULL);
 	printf("\n");
 	return 0;
 }