midx: enable core.multiPackIndex by default

The core.multiPackIndex setting has been around since c4d25228eb
(config: create core.multiPackIndex setting, 2018-07-12), but has been
disabled by default. If a user wishes to use the multi-pack-index
feature, then they must enable this config and run 'git multi-pack-index
write'.

The multi-pack-index feature is relatively stable now, so make the
config option true by default. For users that do not use a
multi-pack-index, the only extra cost will be a file lookup to see if a
multi-pack-index file exists (once per process, per object directory).

Also, this config option will be referenced by an upcoming
"incremental-repack" task in the maintenance builtin, so move the config
option into the repository settings struct. Note that if
GIT_TEST_MULTI_PACK_INDEX=1, then we want to ignore the config option
and treat core.multiPackIndex as enabled.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/core.txt

@@ -606,8 +606,8 @@ core.useReplaceRefs::
 
 core.multiPackIndex::
 	Use the multi-pack-index file to track multiple packfiles using a
-	single index. See link:technical/multi-pack-index.html[the
-	multi-pack-index design document].
+	single index. See linkgit:git-multi-pack-index[1] for more
+	information. Defaults to true.
 
 core.sparseCheckout::
 	Enable "sparse checkout" feature. See linkgit:git-sparse-checkout[1]

midx.c

@@ -10,6 +10,7 @@
 #include "progress.h"
 #include "trace2.h"
 #include "run-command.h"
+#include "repository.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -384,15 +385,9 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 {
 	struct multi_pack_index *m;
 	struct multi_pack_index *m_search;
-	int config_value;
-	static int env_value = -1;
 
-	if (env_value < 0)
-		env_value = git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0);
-
-	if (!env_value &&
-	    (repo_config_get_bool(r, "core.multipackindex", &config_value) ||
-	    !config_value))
+	prepare_repo_settings(r);
+	if (!r->settings.core_multi_pack_index)
 		return 0;
 
 	for (m_search = r->objects->multi_pack_index; m_search; m_search = m_search->next)

repo-settings.c

@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "config.h"
 #include "repository.h"
+#include "midx.h"
 
 #define UPDATE_DEFAULT_BOOL(s,v) do { if (s == -1) { s = v; } } while(0)
 
@@ -47,6 +48,11 @@ void prepare_repo_settings(struct repository *r)
 		r->settings.pack_use_sparse = value;
 	UPDATE_DEFAULT_BOOL(r->settings.pack_use_sparse, 1);
 
+	value = git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0);
+	if (value || !repo_config_get_bool(r, "core.multipackindex", &value))
+		r->settings.core_multi_pack_index = value;
+	UPDATE_DEFAULT_BOOL(r->settings.core_multi_pack_index, 1);
+
 	if (!repo_config_get_bool(r, "feature.manyfiles", &value) && value) {
 		UPDATE_DEFAULT_BOOL(r->settings.index_version, 4);
 		UPDATE_DEFAULT_BOOL(r->settings.core_untracked_cache, UNTRACKED_CACHE_WRITE);

repository.h

@@ -37,6 +37,8 @@ struct repo_settings {
 
 	int pack_use_sparse;
 	enum fetch_negotiation_setting fetch_negotiation_algorithm;
+
+	int core_multi_pack_index;
 };
 
 struct repository {