reftable/stack: add env to disable autocompaction

In future tests it will be neccesary to create repositories with a set
number of tables. To make this easier, introduce the
`GIT_TEST_REFTABLE_AUTOCOMPACTION` environment variable that, when set
to false, disables autocompaction of reftables.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jltobler

refs/reftable-backend.c

@@ -18,6 +18,7 @@
 #include "../reftable/reftable-merged.h"
 #include "../setup.h"
 #include "../strmap.h"
+#include "parse.h"
 #include "refs-internal.h"
 
 /*
@@ -247,6 +248,8 @@ static struct ref_store *reftable_be_init(struct repository *repo,
 	refs->write_options.block_size = 4096;
 	refs->write_options.hash_id = repo->hash_algo->format_id;
 	refs->write_options.default_permissions = calc_shared_perm(0666 & ~mask);
+	refs->write_options.disable_auto_compact =
+		!git_env_bool("GIT_TEST_REFTABLE_AUTOCOMPACTION", 1);
 
 	/*
 	 * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.

t/t0610-reftable-basics.sh

@@ -299,6 +299,27 @@ test_expect_success 'ref transaction: writes cause auto-compaction' '
 	test_line_count = 1 repo/.git/reftable/tables.list
 '
 
+test_expect_success 'ref transaction: env var disables compaction' '
+	test_when_finished "rm -rf repo" &&
+
+	git init repo &&
+	test_commit -C repo A &&
+
+	start=$(wc -l <repo/.git/reftable/tables.list) &&
+	iterations=5 &&
+	expected=$((start + iterations)) &&
+
+	for i in $(test_seq $iterations)
+	do
+		GIT_TEST_REFTABLE_AUTOCOMPACTION=false \
+		git -C repo update-ref branch-$i HEAD || return 1
+	done &&
+	test_line_count = $expected repo/.git/reftable/tables.list &&
+
+	git -C repo update-ref foo HEAD &&
+	test_line_count -lt $expected repo/.git/reftable/tables.list
+'
+
 check_fsync_events () {
 	local trace="$1" &&
 	shift &&