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.

Extract out the heuristics logic from 'reftable_stack_auto_compact()'
into an internal function 'update_segment_if_compaction_required()'.
Then use this to 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>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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

@@ -1647,19 +1647,51 @@ static int stack_segments_for_compaction(struct reftable_stack *st,
 	return 0;
 }
 
-int reftable_stack_auto_compact(struct reftable_stack *st)
+static int update_segment_if_compaction_required(struct reftable_stack *st,
+						 struct segment *seg,
+						 bool use_geometric,
+						 bool *required)
 {
-	struct segment seg;
 	int err;
 
-	if (st->merged->tables_len < 2)
+	if (st->merged->tables_len < 2) {
+		*required = false;
 		return 0;
+	}
 
-	err = stack_segments_for_compaction(st, &seg);
+	if (!use_geometric) {
+		*required = true;
+		return 0;
+	}
+
+	err = stack_segments_for_compaction(st, seg);
 	if (err)
 		return err;
 
-	if (segment_size(&seg) > 0)
+	*required = segment_size(seg) > 0;
+	return 0;
+}
+
+int reftable_stack_compaction_required(struct reftable_stack *st,
+				       bool use_heuristics,
+				       bool *required)
+{
+	struct segment seg;
+	return update_segment_if_compaction_required(st, &seg, use_heuristics,
+						     required);
+}
+
+int reftable_stack_auto_compact(struct reftable_stack *st)
+{
+	struct segment seg;
+	bool required;
+	int err;
+
+	err = update_segment_if_compaction_required(st, &seg, true, &required);
+	if (err)
+		return err;
+
+	if (required)
 		return stack_compact_range(st, seg.start, seg.end - 1,
 					   NULL, STACK_COMPACT_RANGE_BEST_EFFORT);
 

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);