reftable/stack: add function to check if optimization is required

The reftable backend performs auto-compaction as part of its regular
flow, which is required to keep the number of tables part of a stack at
bay. This allows it to stay optimized.

Compaction can also be triggered voluntarily by the user via the 'git
pack-refs' or the 'git refs optimize' command. However, currently there
is no way for the user to check if optimization is required without
actually performing it.

Add and expose `reftable_stack_compaction_required()` which will allow
users to check if the reftable backend can be optimized.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: KarthikNayak

reftable/reftable-stack.h

@@ -123,6 +123,17 @@ struct reftable_log_expiry_config {
 int reftable_stack_compact_all(struct reftable_stack *st,
 			       struct reftable_log_expiry_config *config);
 
+/*
+ * Check if compaction is required.
+ *
+ * When `use_heuristics` is false, check if all tables can be compacted to a
+ * single table. If true, use heuristics to determine if the tables need to be
+ * compacted to maintain geometric progression.
+ */
+int reftable_stack_compaction_required(struct reftable_stack *st,
+				       bool use_heuristics,
+				       bool *required);
+
 /* heuristically compact unbalanced table stack. */
 int reftable_stack_auto_compact(struct reftable_stack *st);
 

reftable/stack.c

@@ -1647,6 +1647,31 @@ static int stack_segments_for_compaction(struct reftable_stack *st,
 	return 0;
 }
 
+int reftable_stack_compaction_required(struct reftable_stack *st,
+				       bool use_heuristics,
+				       bool *required)
+{
+	struct segment seg;
+	int err = 0;
+
+	if (st->merged->tables_len < 2) {
+		*required = false;
+		return 0;
+	}
+
+	if (!use_heuristics) {
+		*required = true;
+		return 0;
+	}
+
+	err = stack_segments_for_compaction(st, &seg);
+	if (err)
+		return err;
+
+	*required = segment_size(&seg) > 0;
+	return 0;
+}
+
 int reftable_stack_auto_compact(struct reftable_stack *st)
 {
 	struct segment seg;

t/unit-tests/u-reftable-stack.c

@@ -1067,6 +1067,7 @@ void test_reftable_stack__add_performs_auto_compaction(void)
 			.value_type = REFTABLE_REF_SYMREF,
 			.value.symref = (char *) "master",
 		};
+		bool required = false;
 		char buf[128];
 
 		/*
@@ -1087,10 +1088,17 @@ void test_reftable_stack__add_performs_auto_compaction(void)
 		 * auto compaction is disabled. When enabled, we should merge
 		 * all tables in the stack.
 		 */
-		if (i != n)
+		cl_assert_equal_i(reftable_stack_compaction_required(st, true, &required), 0);
+		if (i != n) {
 			cl_assert_equal_i(st->merged->tables_len, i + 1);
-		else
+			if (i < 1)
+				cl_assert_equal_b(required, false);
+			else
+				cl_assert_equal_b(required, true);
+		} else {
 			cl_assert_equal_i(st->merged->tables_len, 1);
+			cl_assert_equal_b(required, false);
+		}
 	}
 
 	reftable_stack_destroy(st);