submodule: support reading .gitmodules when it's not in the working tree

When the .gitmodules file is not available in the working tree, try
using the content from the index and from the current branch. This
covers the case when the file is part of the repository but for some
reason it is not checked out, for example because of a sparse checkout.

This makes it possible to use at least the 'git submodule' commands
which *read* the gitmodules configuration file without fully populating
the working tree.

Writing to .gitmodules will still require that the file is checked out,
so check for that before calling config_set_in_gitmodules_file_gently.

Add a similar check also in git-submodule.sh::cmd_add() to anticipate
the eventual failure of the "git submodule add" command when .gitmodules
is not safely writeable; this prevents the command from leaving the
repository in a spurious state (e.g. the submodule repository was cloned
but .gitmodules was not updated because
config_set_in_gitmodules_file_gently failed).

Moreover, since config_from_gitmodules() now accesses the global object
store, it is necessary to protect all code paths which call the function
against concurrent access to the global object store. Currently this
only happens in builtin/grep.c::grep_submodules(), so call
grep_read_lock() before invoking code involving
config_from_gitmodules().

Finally, add t7418-submodule-sparse-gitmodules.sh to verify that reading
from .gitmodules succeeds and that writing to it fails when the file is
not checked out.

NOTE: there is one rare case where this new feature does not work
properly yet: nested submodules without .gitmodules in their working
tree.  This has been documented with a warning and a test_expect_failure
item in t7814, and in this case the current behavior is not altered: no
config is read.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 29 insertions, 2 deletions

diff --git a/submodule-config.c b/submodule-config.c
index 9faf141e4..6869394c3 100644
--- a/submodule-config.c
+++ b/submodule-config.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "dir.h"
 #include "repository.h"
 #include "config.h"
 #include "submodule-config.h"
@@ -603,8 +604,34 @@ static void submodule_cache_check_init(struct repository *repo)
 static void config_from_gitmodules(config_fn_t fn, struct repository *repo, void *data)
 {
 	if (repo->worktree) {
-		char *file = repo_worktree_path(repo, GITMODULES_FILE);
-		git_config_from_file(fn, file, data);
+		struct git_config_source config_source = { 0 };
+		const struct config_options opts = { 0 };
+		struct object_id oid;
+		char *file;
+
+		file = repo_worktree_path(repo, GITMODULES_FILE);
+		if (file_exists(file)) {
+			config_source.file = file;
+		} else if (repo->submodule_prefix) {
+			/*
+			 * When get_oid and config_with_options, used below,
+			 * become able to work on a specific repository, this
+			 * warning branch can be removed.
+			 */
+			warning("nested submodules without %s in the working tree are not supported yet",
+				GITMODULES_FILE);
+			goto out;
+		} else if (get_oid(GITMODULES_INDEX, &oid) >= 0) {
+			config_source.blob = GITMODULES_INDEX;
+		} else if (get_oid(GITMODULES_HEAD, &oid) >= 0) {
+			config_source.blob = GITMODULES_HEAD;
+		} else {
+			goto out;
+		}
+
+		config_with_options(fn, data, &config_source, &opts);
+
+out:
 		free(file);
 	}
 }