[RFC PATCHv2 07/17] update submodules: introduce submodule_is_interesting

[Stefan Beller <sbeller@google.com>]

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.

I am not particular happy with the name `submodule_is_interesting`,
in internal iterations I had `submodule_requires_check_for_update`
and `submodule_needs_update`, but I was even less happy with those
names. Maybe `submodule_interesting_for_update`?

Generally this is to answer "Am I allowed to touch the submodule
at all?" or: "Does the user expect me to touch it?"
which includes all of creation/deletion/update.

This patch is based off a prior attempt by Jens Lehmann to add
submodules to checkout.

Signed-off-by: Stefan Beller <sbeller@google.com>
---
 submodule.c | 23 +++++++++++++++++++++++
 submodule.h |  9 +++++++++
 2 files changed, 32 insertions(+)

diff --git a/submodule.c b/submodule.c
index 1ba398ba3b..62e9ef3872 100644
--- a/submodule.c
+++ b/submodule.c
@@ -516,6 +516,29 @@ 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 submodule_is_interesting(const char *path)
+{
+	const struct submodule *sub;
+
+	if (!submodules_interesting_for_update())
+		return 0;
+
+	sub = submodule_from_path(null_sha1, path);
+	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 21236b095c..7d890e0464 100644
--- a/submodule.h
+++ b/submodule.h
@@ -54,6 +54,15 @@ 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);
+
+/**
+ * When updating the working tree, we need to check if the submodule needs
+ * updating. We do not require a check if we are already sure that the
+ * submodule doesn't need updating, e.g. when we are not interested in submodules
+ * or the submodule is marked uninteresting by being not initialized.
+ */
+extern int submodule_is_interesting(const char *path);
+extern int submodules_interesting_for_update(void);
 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,
-- 
2.11.0.rc2.28.g2673dad