read-cache: regenerate shared index if necessary

When writing a new split-index and there is a big number of cache
entries in the split-index compared to the shared index, it is a
good idea to regenerate the shared index.

By default when the ratio reaches 20%, we will push back all
the entries from the split-index into a new shared index file
instead of just creating a new split-index file.

The threshold can be configured using the
"splitIndex.maxPercentChange" config variable.

We need to adjust the existing tests in t1700 by setting
"splitIndex.maxPercentChange" to 100 at the beginning of t1700,
as the existing tests are assuming that the shared index is
regenerated only when `git update-index --split-index` is used.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -2220,6 +2220,36 @@ static int write_shared_index(struct index_state *istate,
 	return ret;
 }
 
+static const int default_max_percent_split_change = 20;
+
+static int too_many_not_shared_entries(struct index_state *istate)
+{
+	int i, not_shared = 0;
+	int max_split = git_config_get_max_percent_split_change();
+
+	switch (max_split) {
+	case -1:
+		/* not or badly configured: use the default value */
+		max_split = default_max_percent_split_change;
+		break;
+	case 0:
+		return 1; /* 0% means always write a new shared index */
+	case 100:
+		return 0; /* 100% means never write a new shared index */
+	default:
+		break; /* just use the configured value */
+	}
+
+	/* Count not shared entries */
+	for (i = 0; i < istate->cache_nr; i++) {
+		struct cache_entry *ce = istate->cache[i];
+		if (!ce->index)
+			not_shared++;
+	}
+
+	return (int64_t)istate->cache_nr * max_split < (int64_t)not_shared * 100;
+}
+
 int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		       unsigned flags)
 {
@@ -2237,6 +2267,8 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		if ((v & 15) < 6)
 			istate->cache_changed |= SPLIT_INDEX_ORDERED;
 	}
+	if (too_many_not_shared_entries(istate))
+		istate->cache_changed |= SPLIT_INDEX_ORDERED;
 	if (istate->cache_changed & SPLIT_INDEX_ORDERED) {
 		int ret = write_shared_index(istate, lock, flags);
 		if (ret)

t/t1700-split-index.sh

@@ -8,6 +8,7 @@ test_description='split index mode tests'
 sane_unset GIT_TEST_SPLIT_INDEX
 
 test_expect_success 'enable split index' '
+	git config splitIndex.maxPercentChange 100 &&
 	git update-index --split-index &&
 	test-dump-split-index .git/index >actual &&
 	indexversion=$(test-index-version <.git/index) &&