t-reftable-stack: add test for non-default compaction factor

In a recent codebase update (commit ae8e378, merge branch
'ps/reftable-write-options', 2024/05/13) the geometric factor used
in auto-compaction of reftable tables was made configurable. Add
a test to verify the functionality introduced by this update.

Mentored-by: Patrick Steinhardt <[email protected]>
Mentored-by: Christian Couder <[email protected]>
Signed-off-by: Chandra Pratap <[email protected]>

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

@@ -837,12 +837,12 @@ static void t_empty_add(void)
 	reftable_stack_destroy(st2);
 }
 
-static int fastlog2(uint64_t sz)
+static int fastlogN(uint64_t sz, uint64_t N)
 {
 	int l = 0;
 	if (sz == 0)
 		return 0;
-	for (; sz; sz /= 2)
+	for (; sz; sz /= N)
 		l++;
 	return l - 1;
 }
@@ -875,11 +875,43 @@ static void t_reftable_stack_auto_compaction(void)
 
 		err = reftable_stack_auto_compact(st);
 		check(!err);
-		check(i < 3 || st->merged->stack_len < 2 * fastlog2(i));
+		check(i < 2 || st->merged->stack_len < 2 * fastlogN(i, 2));
 	}
 
 	check_int(reftable_stack_compaction_stats(st)->entries_written, <,
-	       (uint64_t)(N * fastlog2(N)));
+	       (uint64_t)(N * fastlogN(N, 2)));
+
+	reftable_stack_destroy(st);
+	clear_dir(dir);
+}
+
+static void t_reftable_stack_auto_compaction_factor(void)
+{
+	struct reftable_write_options opts = {
+		.auto_compaction_factor = 5,
+	};
+	struct reftable_stack *st = NULL;
+	char *dir = get_tmp_dir(__LINE__);
+	int err;
+	size_t N = 100;
+
+	err = reftable_new_stack(&st, dir, &opts);
+	check(!err);
+
+	for (size_t i = 0; i < N; i++) {
+		char name[20];
+		struct reftable_ref_record ref = {
+			.refname = name,
+			.update_index = reftable_stack_next_update_index(st),
+			.value_type = REFTABLE_REF_VAL1,
+		};
+		xsnprintf(name, sizeof(name), "branch%04"PRIuMAX, (uintmax_t)i);
+
+		err = reftable_stack_add(st, &write_test_ref, &ref);
+		check(!err);
+
+		check(i < 5 || st->merged->stack_len < 5 * fastlogN(i, 5));
+	}
 
 	reftable_stack_destroy(st);
 	clear_dir(dir);
@@ -1086,6 +1118,7 @@ int cmd_main(int argc, const char *argv[])
 	TEST(t_reftable_stack_add_one(), "add a single ref record to stack");
 	TEST(t_reftable_stack_add_performs_auto_compaction(), "addition to stack triggers auto-compaction");
 	TEST(t_reftable_stack_auto_compaction(), "stack must form geometric sequence after compaction");
+	TEST(t_reftable_stack_auto_compaction_factor(), "auto-compaction with non-default geometric factor");
 	TEST(t_reftable_stack_auto_compaction_fails_gracefully(), "failure on auto-compaction");
 	TEST(t_reftable_stack_auto_compaction_with_locked_tables(), "auto compaction with locked tables");
 	TEST(t_reftable_stack_compaction_concurrent(), "compaction with concurrent stack");