When the .gitmodules file is not available in the working tree, try
using HEAD:.gitmodules 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.

Signed-off-by: Antonio Ospite <a...@ao2.it>
---
 builtin/submodule--helper.c | 17 ++++++++++++++++-
 cache.h                     |  1 +
 submodule-config.c          | 16 ++++++++++++++--
 3 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index c388f4ee6f..616d5de0a9 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -2089,8 +2089,23 @@ static int module_config(int argc, const char **argv, 
const char *prefix)
                return print_config_from_gitmodules(argv[1]);
 
        /* Equivalent to ACTION_SET in builtin/config.c */
-       if (argc == 3)
+       if (argc == 3) {
+               struct object_id oid;
+
+               /*
+                * If the .gitmodules file is not in the working tree but it
+                * is in the current branch, stop, as writing new values and
+                * staging them would blindly overwrite ALL the old content.
+                *
+                * This still makes it possible to create a brand new
+                * .gitmodules when neither GITMODULES_FILE nor
+                * GITMODULES_HEAD exist.
+                */
+               if (!file_exists(GITMODULES_FILE) && get_oid(GITMODULES_HEAD, 
&oid) >= 0)
+                       die(_("please make sure that the .gitmodules file in 
the current branch is checked out"));
+
                return config_set_in_gitmodules_file_gently(argv[1], argv[2]);
+       }
 
        return 0;
 }
diff --git a/cache.h b/cache.h
index 8b447652a7..8f75cafbb6 100644
--- a/cache.h
+++ b/cache.h
@@ -424,6 +424,7 @@ static inline enum object_type object_type(unsigned int 
mode)
 #define INFOATTRIBUTES_FILE "info/attributes"
 #define ATTRIBUTE_MACRO_PREFIX "[attr]"
 #define GITMODULES_FILE ".gitmodules"
+#define GITMODULES_HEAD "HEAD:.gitmodules"
 #define GIT_NOTES_REF_ENVIRONMENT "GIT_NOTES_REF"
 #define GIT_NOTES_DEFAULT_REF "refs/notes/commits"
 #define GIT_NOTES_DISPLAY_REF_ENVIRONMENT "GIT_NOTES_DISPLAY_REF"
diff --git a/submodule-config.c b/submodule-config.c
index 702d40dd6b..cf08264220 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 +604,19 @@ 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 (get_oid(GITMODULES_HEAD, &oid) >= 0)
+                       config_source.blob = GITMODULES_HEAD;
+
+               config_with_options(fn, data, &config_source, &opts);
+
                free(file);
        }
 }
-- 
2.18.0

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