[PATCH v1] config: Add hashtable for config parsing & retrival

[Tanay Abhra <tanayabh@gmail.com>]

Add a hash table to cache all key-value pairs read from config files
(repo specific .git/config, user wide ~/.gitconfig and the global
/etc/gitconfig). Add two external functions `git_config_get_string` and
`git_config_get_string_multi` for querying in a non-callback manner from the
hash table.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
---
 Documentation/technical/api-config.txt |  18 +++++
 cache.h                                |   2 +
 config.c                               | 122 +++++++++++++++++++++++++++++++++
 3 files changed, 142 insertions(+)

diff --git a/Documentation/technical/api-config.txt b/Documentation/technical/api-config.txt
index 230b3a0..5b6e376 100644
--- a/Documentation/technical/api-config.txt
+++ b/Documentation/technical/api-config.txt
@@ -77,6 +77,24 @@ To read a specific file in git-config format, use
 `git_config_from_file`. This takes the same callback and data parameters
 as `git_config`.
 
+Querying For Specific Variables
+-------------------------------
+
+For programs wanting to query for specific variables in a non-callback
+manner, the config API provides two functions `git_config_get_string`
+and `git_config_get_string_multi`. They both take a single parameter,
+
+- a key string in canonical flat form for which the corresponding values
+  will be retrieved and returned.
+
+They both read values from an internal cache generated previously from
+reading the config files. `git_config_get_string` returns the value with
+the highest priority(i.e. for the same variable value in the repo config
+will be preferred over value in user wide config).
+
+`git_config_get_string_multi` returns a `string_list` containing all the
+values for that particular key, sorted in order of increasing priority.
+
 Value Parsing Helpers
 ---------------------
 
diff --git a/cache.h b/cache.h
index 107ac61..2038150 100644
--- a/cache.h
+++ b/cache.h
@@ -1271,6 +1271,8 @@ extern int check_repository_format_version(const char *var, const char *value, v
 extern int git_env_bool(const char *, int);
 extern int git_config_system(void);
 extern int config_error_nonbool(const char *);
+extern const char *git_config_get_string(const char *);
+extern const struct string_list *git_config_get_string_multi(const char *);
 #if defined(__GNUC__) && ! defined(__clang__)
 #define config_error_nonbool(s) (config_error_nonbool(s), -1)
 #endif
diff --git a/config.c b/config.c
index a30cb5c..6f6b04e 100644
--- a/config.c
+++ b/config.c
@@ -9,6 +9,8 @@
 #include "exec_cmd.h"
 #include "strbuf.h"
 #include "quote.h"
+#include "hashmap.h"
+#include "string-list.h"
 
 struct config_source {
 	struct config_source *prev;
@@ -37,6 +39,120 @@ static struct config_source *cf;
 
 static int zlib_compression_seen;
 
+struct config_cache_entry {
+	struct hashmap_entry ent;
+	char *key;
+	struct string_list *value_list;
+};
+
+static int hashmap_is_init;
+
+static int config_cache_set_value(const char *key, const char *value);
+
+static int config_cache_entry_cmp_case(const struct config_cache_entry *e1,
+				 const struct config_cache_entry *e2, const void *unused)
+{
+	return strcmp(e1->key, e2->key);
+}
+
+static void config_cache_init(struct hashmap *config_cache)
+{
+	hashmap_init(config_cache, (hashmap_cmp_fn) config_cache_entry_cmp_case, 0);
+}
+
+static int config_cache_callback(const char *key, const char *value, void *unused)
+{
+	config_cache_set_value(key, value);
+	return 0;
+}
+
+static struct hashmap *get_config_cache(void)
+{
+	static struct hashmap config_cache;
+	if (!hashmap_is_init) {
+		config_cache_init(&config_cache);
+		hashmap_is_init = 1;
+		git_config(config_cache_callback, NULL);
+		return &config_cache;
+	}
+	return &config_cache;
+}
+
+static void config_cache_free(void)
+{
+	struct hashmap *config_cache;
+	struct config_cache_entry *entry;
+	struct hashmap_iter iter;
+	config_cache = get_config_cache();
+	hashmap_iter_init(config_cache, &iter);
+	while ((entry = hashmap_iter_next(&iter))) {
+		free(entry->key);
+		string_list_clear(entry->value_list, 0);
+	}
+	hashmap_free(config_cache, 1);
+	hashmap_is_init = 0;
+}
+
+static struct config_cache_entry *config_cache_find_entry(const char *key)
+{
+	struct hashmap *config_cache;
+	struct config_cache_entry k;
+	char *normalized_key;
+	int baselength = 0, ret;
+	config_cache = get_config_cache();
+	ret = git_config_parse_key(key, &normalized_key, &baselength);
+
+	if (ret)
+		return NULL;
+
+	hashmap_entry_init(&k, strhash(normalized_key));
+	k.key = (char*) key;
+	return hashmap_get(config_cache, &k, NULL);
+}
+
+static struct string_list *config_cache_get_value(const char *key)
+{
+	struct config_cache_entry *e = config_cache_find_entry(key);
+	return e ? e->value_list : NULL;
+}
+
+
+static int config_cache_set_value(const char *key, const char *value)
+{
+	struct hashmap *config_cache;
+	struct config_cache_entry *e;
+
+	config_cache = get_config_cache();
+	e = config_cache_find_entry(key);
+	if (!e) {
+		e = xmalloc(sizeof(*e));
+		hashmap_entry_init(e, strhash(key));
+		e->key = xstrdup(key);
+		e->value_list = xcalloc(sizeof(struct string_list), 1);
+		e->value_list->strdup_strings = 1;
+		string_list_append(e->value_list, value);
+		hashmap_add(config_cache, e);
+	} else {
+		string_list_append(e->value_list, value);
+	}
+	return 0;
+}
+
+extern const char *git_config_get_string(const char *key)
+{
+	struct string_list *values;
+	values = config_cache_get_value(key);
+	if (!values)
+		return NULL;
+	assert(values->nr > 0);
+	return values->items[values->nr - 1].string;
+}
+
+extern const struct string_list *git_config_get_string_multi(const char *key)
+{
+	return config_cache_get_value(key);
+}
+
 static int config_file_fgetc(struct config_source *conf)
 {
 	return fgetc(conf->u.file);
@@ -1700,6 +1816,12 @@ int git_config_set_multivar_in_file(const char *config_filename,
 	lock = NULL;
 	ret = 0;
 
+	/* contents of config file has changed, so invalidate the
+	 * config cache used by non-callback based query functions.
+	 */
+	if (hashmap_is_init)
+		config_cache_free();
+
 out_free:
 	if (lock)
 		rollback_lock_file(lock);
-- 
1.9.0.GIT