config: fix leaks from git_config_get_string_const()

There are two functions to get a single config string:

  - git_config_get_string()

  - git_config_get_string_const()

One might naively think that the first one allocates a new string and
the second one just points us to the internal configset storage. But
in fact they both allocate a new copy; the second one exists only to
avoid having to cast when using it with a const global which we never
intend to free.

The documentation for the function explains that clearly, but it seems
I'm not alone in being surprised by this. Of 17 calls to the function,
13 of them leak the resulting value.

We could obviously fix these by adding the appropriate free(). But it
would be simpler still if we actually had a non-allocating way to get
the string. There's git_config_get_value() but that doesn't quite do
what we want. If the config key is present but is a boolean with no
value (e.g., "[foo]bar" in the file), then we'll get NULL (whereas the
string versions will print an error and die).

So let's introduce a new variant, git_config_get_string_tmp(), that
behaves as these callers expect. We need a new name because we have new
semantics but the same function signature (so even if we converted the
four remaining callers, topics in flight might be surprised). The "tmp"
is because this value should only be held onto for a short time. In
practice it's rare for us to clear and refresh the configset,
invalidating the pointer, but hopefully the "tmp" makes callers think
about the lifetime. In each of the converted cases here the value only
needs to last within the local function or its immediate caller.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -645,7 +645,7 @@ static void prepare_format_display(struct ref *ref_map)
 	struct ref *rm;
 	const char *format = "full";
 
-	git_config_get_string_const("fetch.output", &format);
+	git_config_get_string_tmp("fetch.output", &format);
 	if (!strcasecmp(format, "full"))
 		compact_format = 0;
 	else if (!strcasecmp(format, "compact"))

builtin/submodule--helper.c

@@ -1511,7 +1511,7 @@ static void determine_submodule_update_strategy(struct repository *r,
 		if (parse_submodule_update_strategy(update, out) < 0)
 			die(_("Invalid update mode '%s' for submodule path '%s'"),
 				update, path);
-	} else if (!repo_config_get_string_const(r, key, &val)) {
+	} else if (!repo_config_get_string_tmp(r, key, &val)) {
 		if (parse_submodule_update_strategy(val, out) < 0)
 			die(_("Invalid update mode '%s' configured for submodule path '%s'"),
 				val, path);
@@ -1667,7 +1667,7 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 	}
 
 	key = xstrfmt("submodule.%s.update", sub->name);
-	if (!repo_config_get_string_const(the_repository, key, &update_string)) {
+	if (!repo_config_get_string_tmp(the_repository, key, &update_string)) {
 		update_type = parse_submodule_update_type(update_string);
 	} else {
 		update_type = sub->update_strategy.type;
@@ -1690,7 +1690,7 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "submodule.%s.url", sub->name);
-	if (repo_config_get_string_const(the_repository, sb.buf, &url)) {
+	if (repo_config_get_string_tmp(the_repository, sb.buf, &url)) {
 		if (starts_with_dot_slash(sub->url) ||
 		    starts_with_dot_dot_slash(sub->url)) {
 			url = compute_submodule_clone_url(sub->url);
@@ -1976,7 +1976,7 @@ static const char *remote_submodule_branch(const char *path)
 		return NULL;
 
 	key = xstrfmt("submodule.%s.branch", sub->name);
-	if (repo_config_get_string_const(the_repository, key, &branch))
+	if (repo_config_get_string_tmp(the_repository, key, &branch))
 		branch = sub->branch;
 	free(key);
 

config.c

@@ -2020,6 +2020,20 @@ int git_configset_get_string(struct config_set *cs, const char *key, char **dest
 	return git_configset_get_string_const(cs, key, (const char **)dest);
 }
 
+int git_configset_get_string_tmp(struct config_set *cs, const char *key,
+				 const char **dest)
+{
+	const char *value;
+	if (!git_configset_get_value(cs, key, &value)) {
+		if (!value)
+			return config_error_nonbool(key);
+		*dest = value;
+		return 0;
+	} else {
+		return 1;
+	}
+}
+
 int git_configset_get_int(struct config_set *cs, const char *key, int *dest)
 {
 	const char *value;
@@ -2165,6 +2179,17 @@ int repo_config_get_string(struct repository *repo,
 	return repo_config_get_string_const(repo, key, (const char **)dest);
 }
 
+int repo_config_get_string_tmp(struct repository *repo,
+			       const char *key, const char **dest)
+{
+	int ret;
+	git_config_check_init(repo);
+	ret = git_configset_get_string_tmp(repo->config, key, dest);
+	if (ret < 0)
+		git_die_config(key, NULL);
+	return ret;
+}
+
 int repo_config_get_int(struct repository *repo,
 			const char *key, int *dest)
 {
@@ -2242,6 +2267,11 @@ int git_config_get_string(const char *key, char **dest)
 	return repo_config_get_string(the_repository, key, dest);
 }
 
+int git_config_get_string_tmp(const char *key, const char **dest)
+{
+	return repo_config_get_string_tmp(the_repository, key, dest);
+}
+
 int git_config_get_int(const char *key, int *dest)
 {
 	return repo_config_get_int(the_repository, key, dest);

config.h

@@ -460,6 +460,7 @@ int git_configset_get_value(struct config_set *cs, const char *key, const char *
 
 int git_configset_get_string_const(struct config_set *cs, const char *key, const char **dest);
 int git_configset_get_string(struct config_set *cs, const char *key, char **dest);
+int git_configset_get_string_tmp(struct config_set *cs, const char *key, const char **dest);
 int git_configset_get_int(struct config_set *cs, const char *key, int *dest);
 int git_configset_get_ulong(struct config_set *cs, const char *key, unsigned long *dest);
 int git_configset_get_bool(struct config_set *cs, const char *key, int *dest);
@@ -478,6 +479,8 @@ int repo_config_get_string_const(struct repository *repo,
 				 const char *key, const char **dest);
 int repo_config_get_string(struct repository *repo,
 			   const char *key, char **dest);
+int repo_config_get_string_tmp(struct repository *repo,
+			       const char *key, const char **dest);
 int repo_config_get_int(struct repository *repo,
 			const char *key, int *dest);
 int repo_config_get_ulong(struct repository *repo,
@@ -537,6 +540,13 @@ int git_config_get_string_const(const char *key, const char **dest);
  */
 int git_config_get_string(const char *key, char **dest);
 
+/**
+ * Similar to `git_config_get_string_const`, but does not allocate any new
+ * memory; on success `dest` will point to memory owned by the config
+ * machinery, which could be invalidated if it is discarded and reloaded.
+ */
+int git_config_get_string_tmp(const char *key, const char **dest);
+
 /**
  * Finds and parses the value to an integer for the configuration variable
  * `key`. Dies on error; otherwise, stores the value of the parsed integer in

connect.c

@@ -1052,7 +1052,7 @@ static const char *get_ssh_command(void)
 	if ((ssh = getenv("GIT_SSH_COMMAND")))
 		return ssh;
 
-	if (!git_config_get_string_const("core.sshcommand", &ssh))
+	if (!git_config_get_string_tmp("core.sshcommand", &ssh))
 		return ssh;
 
 	return NULL;
@@ -1071,7 +1071,7 @@ static void override_ssh_variant(enum ssh_variant *ssh_variant)
 {
 	const char *variant = getenv("GIT_SSH_VARIANT");
 
-	if (!variant && git_config_get_string_const("ssh.variant", &variant))
+	if (!variant && git_config_get_string_tmp("ssh.variant", &variant))
 		return;
 
 	if (!strcmp(variant, "auto"))

editor.c

@@ -40,7 +40,7 @@ const char *git_sequence_editor(void)
 	const char *editor = getenv("GIT_SEQUENCE_EDITOR");
 
 	if (!editor)
-		git_config_get_string_const("sequence.editor", &editor);
+		git_config_get_string_tmp("sequence.editor", &editor);
 	if (!editor)
 		editor = git_editor();
 

help.c

@@ -375,7 +375,7 @@ void list_cmds_by_config(struct string_list *list)
 {
 	const char *cmd_list;
 
-	if (git_config_get_string_const("completion.commands", &cmd_list))
+	if (git_config_get_string_tmp("completion.commands", &cmd_list))
 		return;
 
 	string_list_sort(list);

protocol.c

@@ -21,7 +21,7 @@ enum protocol_version get_protocol_version_config(void)
 	const char *git_test_k = "GIT_TEST_PROTOCOL_VERSION";
 	const char *git_test_v;
 
-	if (!git_config_get_string_const("protocol.version", &value)) {
+	if (!git_config_get_string_tmp("protocol.version", &value)) {
 		enum protocol_version version = parse_protocol_version(value);
 
 		if (version == protocol_unknown_version)

submodule.c

@@ -194,7 +194,7 @@ void set_diffopt_flags_from_submodule_config(struct diff_options *diffopt,
 		char *key;
 
 		key = xstrfmt("submodule.%s.ignore", submodule->name);
-		if (repo_config_get_string_const(the_repository, key, &ignore))
+		if (repo_config_get_string_tmp(the_repository, key, &ignore))
 			ignore = submodule->ignore;
 		free(key);
 
@@ -1299,7 +1299,7 @@ static int get_fetch_recurse_config(const struct submodule *submodule,
 
 		int fetch_recurse = submodule->fetch_recurse;
 		key = xstrfmt("submodule.%s.fetchRecurseSubmodules", submodule->name);
-		if (!repo_config_get_string_const(spf->r, key, &value)) {
+		if (!repo_config_get_string_tmp(spf->r, key, &value)) {
 			fetch_recurse = parse_fetch_recurse_submodules_arg(key, value);
 		}
 		free(key);

t/helper/test-config.c

@@ -126,7 +126,7 @@ int cmd__config(int argc, const char **argv)
 			goto exit1;
 		}
 	} else if (argc == 3 && !strcmp(argv[1], "get_string")) {
-		if (!git_config_get_string_const(argv[2], &v)) {
+		if (!git_config_get_string_tmp(argv[2], &v)) {
 			printf("%s\n", v);
 			goto exit0;
 		} else {