rewrite git_config() to use the config-set API

Of all the functions in `git_config*()` family, `git_config()` has the
most invocations in the whole code base. Each `git_config()` invocation
causes config file rereads which can be avoided using the config-set API.

Use the config-set API to rewrite `git_config()` to use the config caching
layer to avoid config file rereads on each invocation during a git process
lifetime. First invocation constructs the cache, and after that for each
successive invocation, `git_config()` feeds values from the config cache
instead of rereading the configuration files.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -8,6 +8,7 @@
 #include "gettext.h"
 #include "convert.h"
 #include "trace.h"
+#include "string-list.h"
 
 #include SHA1_HEADER
 #ifndef git_SHA_CTX
@@ -1351,9 +1352,32 @@ extern int parse_config_key(const char *var,
 			    const char **subsection, int *subsection_len,
 			    const char **key);
 
+struct config_set_element {
+	struct hashmap_entry ent;
+	char *key;
+	struct string_list value_list;
+};
+
+struct configset_list_item {
+	struct config_set_element *e;
+	int value_index;
+};
+
+/*
+ * the contents of the list are ordered according to their
+ * position in the config files and order of parsing the files.
+ * (i.e. key-value pair at the last position of .git/config will
+ * be at the last item of the list)
+ */
+struct configset_list {
+	struct configset_list_item *items;
+	unsigned int nr, alloc;
+};
+
 struct config_set {
 	struct hashmap config_hash;
 	int hash_initialized;
+	struct configset_list list;
 };
 
 extern void git_configset_init(struct config_set *cs);

config.c

@@ -35,12 +35,6 @@ struct config_source {
 	long (*do_ftell)(struct config_source *c);
 };
 
-struct config_set_element {
-	struct hashmap_entry ent;
-	char *key;
-	struct string_list value_list;
-};
-
 static struct config_source *cf;
 
 static int zlib_compression_seen;
@@ -1232,7 +1226,7 @@ int git_config_with_options(config_fn_t fn, void *data,
 	return ret;
 }
 
-void git_config(config_fn_t fn, void *data)
+static void git_config_raw(config_fn_t fn, void *data)
 {
 	if (git_config_with_options(fn, data, NULL, 1) < 0)
 		/*
@@ -1249,6 +1243,33 @@ void git_config(config_fn_t fn, void *data)
 		die(_("unknown error occured while reading the configuration files"));
 }
 
+static void configset_iter(struct config_set *cs, config_fn_t fn, void *data)
+{
+	int i, value_index;
+	struct string_list *values;
+	struct config_set_element *entry;
+	struct configset_list *list = &cs->list;
+	struct key_value_info *kv_info;
+
+	for (i = 0; i < list->nr; i++) {
+		entry = list->items[i].e;
+		value_index = list->items[i].value_index;
+		values = &entry->value_list;
+		if (fn(entry->key, values->items[value_index].string, data) < 0) {
+			kv_info = values->items[value_index].util;
+			git_die_config_linenr(entry->key, kv_info->filename, kv_info->linenr);
+		}
+	}
+}
+
+static void git_config_check_init(void);
+
+void git_config(config_fn_t fn, void *data)
+{
+	git_config_check_init();
+	configset_iter(&the_config_set, fn, data);
+}
+
 static struct config_set_element *configset_find_element(struct config_set *cs, const char *key)
 {
 	struct config_set_element k;
@@ -1275,6 +1296,7 @@ static int configset_add_value(struct config_set *cs, const char *key, const cha
 {
 	struct config_set_element *e;
 	struct string_list_item *si;
+	struct configset_list_item *l_item;
 	struct key_value_info *kv_info = xmalloc(sizeof(*kv_info));
 
 	e = configset_find_element(cs, key);
@@ -1290,6 +1312,12 @@ static int configset_add_value(struct config_set *cs, const char *key, const cha
 		hashmap_add(&cs->config_hash, e);
 	}
 	si = string_list_append_nodup(&e->value_list, value ? xstrdup(value) : NULL);
+
+	ALLOC_GROW(cs->list.items, cs->list.nr + 1, cs->list.alloc);
+	l_item = &cs->list.items[cs->list.nr++];
+	l_item->e = e;
+	l_item->value_index = e->value_list.nr - 1;
+
 	if (cf) {
 		kv_info->filename = strintern(cf->name);
 		kv_info->linenr = cf->linenr;
@@ -1313,6 +1341,9 @@ void git_configset_init(struct config_set *cs)
 {
 	hashmap_init(&cs->config_hash, (hashmap_cmp_fn)config_set_element_cmp, 0);
 	cs->hash_initialized = 1;
+	cs->list.nr = 0;
+	cs->list.alloc = 0;
+	cs->list.items = NULL;
 }
 
 void git_configset_clear(struct config_set *cs)
@@ -1329,6 +1360,10 @@ void git_configset_clear(struct config_set *cs)
 	}
 	hashmap_free(&cs->config_hash, 1);
 	cs->hash_initialized = 0;
+	free(cs->list.items);
+	cs->list.nr = 0;
+	cs->list.alloc = 0;
+	cs->list.items = NULL;
 }
 
 static int config_set_callback(const char *key, const char *value, void *cb)
@@ -1447,7 +1482,7 @@ static void git_config_check_init(void)
 	if (the_config_set.hash_initialized)
 		return;
 	git_configset_init(&the_config_set);
-	git_config(config_set_callback, &the_config_set);
+	git_config_raw(config_set_callback, &the_config_set);
 }
 
 void git_config_clear(void)

t/t4055-diff-context.sh

@@ -79,7 +79,7 @@ test_expect_success 'non-integer config parsing' '
 test_expect_success 'negative integer config parsing' '
 	git config diff.context -1 &&
 	test_must_fail git diff 2>output &&
-	test_i18ngrep "bad config file" output
+	test_i18ngrep "bad config variable" output
 '
 
 test_expect_success '-U0 is valid, so is diff.context=0' '