In later patches we introduce the --recurse-submodule flag for commands
that modify the working directory, e.g. git-checkout.

It is potentially expensive to check if a submodule needs an update,
because a common theme to interact with submodules is to spawn a child
process for each interaction.

So let's introduce a function that pre checks if a submodule needs
to be checked for an update.

Signed-off-by: Stefan Beller <redacted>
---
 submodule.c | 26 ++++++++++++++++++++++++++
 submodule.h |  8 ++++++++
 2 files changed, 34 insertions(+)

diff --git a/submodule.c b/submodule.c
index c0060c29f2..4c33374ae8 100644
--- a/submodule.c
+++ b/submodule.c

@@ -551,6 +551,32 @@ void set_config_update_recurse_submodules(int value)
 	config_update_recurse_submodules = value;
 }
 
+int submodules_interesting_for_update(void)
+{
+	/*
+	 * Update can't be "none", "merge" or "rebase",
+	 * treat any value as OFF, except an explicit ON.
+	 */
+	return config_update_recurse_submodules == RECURSE_SUBMODULES_ON;
+}
+
+int is_interesting_submodule(const struct cache_entry *ce)

+{
+	const struct submodule *sub;
+
+	if (!S_ISGITLINK(ce->ce_mode))
+		return 0;
+
+	if (!submodules_interesting_for_update())
+		return 0;
+
+	sub = submodule_from_path(null_sha1, ce->name);
+	if (!sub)
+		return 0;
+
+	return sub->update_strategy.type != SM_UPDATE_NONE;
+}
+
 static int has_remote(const char *refname, const struct object_id *oid,
 		      int flags, void *cb_data)
 {

diff --git a/submodule.h b/submodule.h
index c4e1ac828e..84b67a7c4a 100644
--- a/submodule.h
+++ b/submodule.h

@@ -59,6 +59,14 @@ extern void show_submodule_inline_diff(FILE *f, const char *path,
 		const struct diff_options *opt);
 extern void set_config_fetch_recurse_submodules(int value);
 extern void set_config_update_recurse_submodules(int value);
+
+/*
+ * Traditionally Git ignored changes made for submodules.
+ * This function checks if we are interested in the given submodule
+ * for any kind of operation.
+ */
+extern int submodules_interesting_for_update(void);
+extern int is_interesting_submodule(const struct cache_entry *ce);
 extern void check_for_new_submodule_commits(unsigned char new_sha1[20]);
 extern int fetch_populated_submodules(const struct argv_array *options,
 			       const char *prefix, int command_line_option,

-- 
2.12.0.rc0.16.gd1691994b4.dirty