config: learn `git_protected_config()`

`uploadpack.packObjectsHook` is the only 'protected configuration only'
variable today, but we've noted that `safe.directory` and the upcoming
`safe.bareRepository` should also be 'protected configuration only'. So,
for consistency, we'd like to have a single implementation for protected
configuration.

The primary constraints are:

1. Reading from protected configuration should be fast. Nearly all "git"
   commands inside a bare repository will read both `safe.directory` and
   `safe.bareRepository`, so we cannot afford to be slow.

2. Protected configuration must be readable when the gitdir is not
   known. `safe.directory` and `safe.bareRepository` both affect
   repository discovery and the gitdir is not known at that point [1].

The chosen implementation in this commit is to read protected
configuration and cache the values in a global configset. This is
similar to the caching behavior we get with the_repository->config.

Introduce git_protected_config(), which reads protected configuration
and caches them in the global configset protected_config. Then, refactor
`uploadpack.packObjectsHook` to use git_protected_config().

The protected configuration functions are named similarly to their
non-protected counterparts, e.g. git_protected_config_check_init() vs
git_config_check_init().

In light of constraint 1, this implementation can still be improved.
git_protected_config() iterates through every variable in
protected_config, which is wasteful, but it makes the conversion simple
because it matches existing patterns. We will likely implement constant
time lookup functions for protected configuration in a future series
(such functions already exist for non-protected configuration, i.e.
repo_config_get_*()).

An alternative that avoids introducing another configset is to continue
to read all config using git_config(), but only accept values that have
the correct config scope [2]. This technically fulfills constraint 2,
because git_config() simply ignores the local and worktree config when
the gitdir is not known. However, this would read incomplete config into
the_repository->config, which would need to be reset when the gitdir is
known and git_config() needs to read the local and worktree config.
Resetting the_repository->config might be reasonable while we only have
these 'protected configuration only' variables, but it's not clear
whether this extends well to future variables.

[1] In this case, we do have a candidate gitdir though, so with a little
refactoring, it might be possible to provide a gitdir.
[2] This is how `uploadpack.packObjectsHook` was implemented prior to
this commit.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -81,6 +81,17 @@ static enum config_scope current_parsing_scope;
 static int pack_compression_seen;
 static int zlib_compression_seen;
 
+/*
+ * Config that comes from trusted scopes, namely:
+ * - CONFIG_SCOPE_SYSTEM (e.g. /etc/gitconfig)
+ * - CONFIG_SCOPE_GLOBAL (e.g. $HOME/.gitconfig, $XDG_CONFIG_HOME/git)
+ * - CONFIG_SCOPE_COMMAND (e.g. "-c" option, environment variables)
+ *
+ * This is declared here for code cleanliness, but unlike the other
+ * static variables, this does not hold config parser state.
+ */
+static struct config_set protected_config;
+
 static int config_file_fgetc(struct config_source *conf)
 {
 	return getc_unlocked(conf->u.file);
@@ -2378,6 +2389,11 @@ int git_configset_add_file(struct config_set *cs, const char *filename)
 	return git_config_from_file(config_set_callback, filename, cs);
 }
 
+int git_configset_add_parameters(struct config_set *cs)
+{
+	return git_config_from_parameters(config_set_callback, cs);
+}
+
 int git_configset_get_value(struct config_set *cs, const char *key, const char **value)
 {
 	const struct string_list *values = NULL;
@@ -2619,6 +2635,33 @@ int repo_config_get_pathname(struct repository *repo,
 	return ret;
 }
 
+/* Read values into protected_config. */
+static void read_protected_config(void)
+{
+	char *xdg_config = NULL, *user_config = NULL, *system_config = NULL;
+
+	git_configset_init(&protected_config);
+
+	system_config = git_system_config();
+	git_global_config(&user_config, &xdg_config);
+
+	git_configset_add_file(&protected_config, system_config);
+	git_configset_add_file(&protected_config, xdg_config);
+	git_configset_add_file(&protected_config, user_config);
+	git_configset_add_parameters(&protected_config);
+
+	free(system_config);
+	free(xdg_config);
+	free(user_config);
+}
+
+void git_protected_config(config_fn_t fn, void *data)
+{
+	if (!protected_config.hash_initialized)
+		read_protected_config();
+	configset_iter(&protected_config, fn, data);
+}
+
 /* Functions used historically to read configuration from 'the_repository' */
 void git_config(config_fn_t fn, void *data)
 {

config.h

@@ -446,6 +446,15 @@ void git_configset_init(struct config_set *cs);
  */
 int git_configset_add_file(struct config_set *cs, const char *filename);
 
+/**
+ * Parses command line options and environment variables, and adds the
+ * variable-value pairs to the `config_set`. Returns 0 on success, or -1
+ * if there is an error in parsing. The caller decides whether to free
+ * the incomplete configset or continue using it when the function
+ * returns -1.
+ */
+int git_configset_add_parameters(struct config_set *cs);
+
 /**
  * Finds and returns the value list, sorted in order of increasing priority
  * for the configuration variable `key` and config set `cs`. When the
@@ -505,6 +514,13 @@ int repo_config_get_maybe_bool(struct repository *repo,
 int repo_config_get_pathname(struct repository *repo,
 			     const char *key, const char **dest);
 
+/*
+ * Functions for reading protected config. By definition, protected
+ * config ignores repository config, so these do not take a `struct
+ * repository` parameter.
+ */
+void git_protected_config(config_fn_t fn, void *data);
+
 /**
  * Querying For Specific Variables
  * -------------------------------

t/t5544-pack-objects-hook.sh

@@ -56,7 +56,12 @@ test_expect_success 'hook does not run from repo config' '
 	! grep "hook running" stderr &&
 	test_path_is_missing .git/hook.args &&
 	test_path_is_missing .git/hook.stdin &&
-	test_path_is_missing .git/hook.stdout
+	test_path_is_missing .git/hook.stdout &&
+
+	# check that global config is used instead
+	test_config_global uploadpack.packObjectsHook ./hook &&
+	git clone --no-local . dst2.git 2>stderr &&
+	grep "hook running" stderr
 '
 
 test_expect_success 'hook works with partial clone' '

upload-pack.c

@@ -1321,18 +1321,27 @@ static int upload_pack_config(const char *var, const char *value, void *cb_data)
 		data->advertise_sid = git_config_bool(var, value);
 	}
 
-	if (current_config_scope() != CONFIG_SCOPE_LOCAL &&
-	    current_config_scope() != CONFIG_SCOPE_WORKTREE) {
-		if (!strcmp("uploadpack.packobjectshook", var))
-			return git_config_string(&data->pack_objects_hook, var, value);
-	}
-
 	if (parse_object_filter_config(var, value, data) < 0)
 		return -1;
 
 	return parse_hide_refs_config(var, value, "uploadpack");
 }
 
+static int upload_pack_protected_config(const char *var, const char *value, void *cb_data)
+{
+	struct upload_pack_data *data = cb_data;
+
+	if (!strcmp("uploadpack.packobjectshook", var))
+		return git_config_string(&data->pack_objects_hook, var, value);
+	return 0;
+}
+
+static void get_upload_pack_config(struct upload_pack_data *data)
+{
+	git_config(upload_pack_config, data);
+	git_protected_config(upload_pack_protected_config, data);
+}
+
 void upload_pack(const int advertise_refs, const int stateless_rpc,
 		 const int timeout)
 {
@@ -1340,8 +1349,7 @@ void upload_pack(const int advertise_refs, const int stateless_rpc,
 	struct upload_pack_data data;
 
 	upload_pack_data_init(&data);
-
-	git_config(upload_pack_config, &data);
+	get_upload_pack_config(&data);
 
 	data.stateless_rpc = stateless_rpc;
 	data.timeout = timeout;
@@ -1695,8 +1703,7 @@ int upload_pack_v2(struct repository *r, struct packet_reader *request)
 
 	upload_pack_data_init(&data);
 	data.use_sideband = LARGE_PACKET_MAX;
-
-	git_config(upload_pack_config, &data);
+	get_upload_pack_config(&data);
 
 	while (state != FETCH_DONE) {
 		switch (state) {