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>

1 files changed, 17 insertions, 10 deletions

diff --git a/upload-pack.c b/upload-pack.c
index 3a851b3606..09f48317b0 100644
--- a/upload-pack.c
+++ b/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) {