When adding an alternative object database source we not only have to
consider the added source itself, but we also have to add _its_ sources
to our database. We implement this via mutual recursion:

  1. We first call `link_alt_odb_entries()`.

  2. `link_alt_odb_entries()` calls `parse_alternates()`.

  3. We then add each parsed alternate via `odb_add_source()`.

  4. `odb_add_source()` calls `link_alt_odb_entries()` again.

This flow is somewhat hard to follow, but more importantly it means that
parsing of alternates is somewhat tied to the recursive behaviour.

Refactor the function to remove the mutual recursion between adding
sources and parsing alternates. The parsing step thus becomes completely
oblivious to the fact that there is recursive behaviour going on at all.
Instead, the recursion is handled exclusively by `odb_add_source()`,
which now recurses with itself.

This refactoring allows us to move parsing of alternates into object
database sources in a subsequent step.

Signed-off-by: Patrick Steinhardt <redacted>
---
 odb.c | 60 +++++++++++++++++++++++++++---------------------------------
 1 file changed, 27 insertions(+), 33 deletions(-)

diff --git a/odb.c b/odb.c
index 94cff19221..27f3c8e263 100644
--- a/odb.c
+++ b/odb.c

@@ -147,9 +147,8 @@ static bool odb_is_source_usable(struct object_database *o, const char *path)
  * of the object ID, an extra slash for the first level indirection, and
  * the terminating NUL.
  */
-static void read_info_alternates(struct object_database *odb,
-				 const char *relative_base,
-				 int depth);
+static void read_info_alternates(const char *relative_base,
+				 struct strvec *out);
 
 static struct odb_source *odb_source_new(struct object_database *odb,
 					 const char *path,

@@ -171,6 +170,7 @@ static struct odb_source *odb_add_source(struct object_database *odb,
 					 int depth)
 {
 	struct odb_source *alternate = NULL;
+	struct strvec sources = STRVEC_INIT;
 	khiter_t pos;
 	int ret;
 

@@ -189,9 +189,17 @@ static struct odb_source *odb_add_source(struct object_database *odb,
 	kh_value(odb->source_by_path, pos) = alternate;
 
 	/* recursively add alternates */
-	read_info_alternates(odb, alternate->path, depth + 1);
+	read_info_alternates(alternate->path, &sources);
+	if (sources.nr && depth + 1 > 5) {
+		error(_("%s: ignoring alternate object stores, nesting too deep"),
+		      source);
+	} else {
+		for (size_t i = 0; i < sources.nr; i++)
+			odb_add_source(odb, sources.v[i], depth + 1);
+	}

@@ -622,13 +610,19 @@ int odb_for_each_alternate(struct object_database *odb,
 
 void odb_prepare_alternates(struct object_database *odb)
 {
+	struct strvec sources = STRVEC_INIT;
+
 	if (odb->loaded_alternates)
 		return;
 
-	link_alt_odb_entries(odb, odb->alternate_db, PATH_SEP, NULL, 0);
+	parse_alternates(odb->alternate_db, PATH_SEP, NULL, &sources);
+	read_info_alternates(odb->sources->path, &sources);
+	for (size_t i = 0; i < sources.nr; i++)
+		odb_add_source(odb, sources.v[i], 0);