Start converting `struct odb_source_loose` into a proper pluggable
`struct odb_source` by embedding the base struct and assigning it the
new `ODB_SOURCE_LOOSE` type. Furthermore, wire up lifecycle management
of this source by implementing the `free` callback and taking ownership
of the chdir notifications.

Note that the loose source is not yet functional as a standalone `struct
odb_source`, as it's missing all of the callback implementations. These
will be wired up in subsequent commits.

Signed-off-by: Patrick Steinhardt <redacted>
---
 object-file.c      | 17 -----------------
 object-file.h      |  2 --
 odb/source-files.c |  2 +-
 odb/source-loose.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 odb/source-loose.h | 14 ++++++++++++++
 odb/source.h       |  3 +++
 6 files changed, 63 insertions(+), 20 deletions(-)

diff --git a/object-file.c b/object-file.c
index 7a1908bfc0..977d959d33 100644
--- a/object-file.c
+++ b/object-file.c

@@ -2041,14 +2041,6 @@ static struct oidtree *odb_source_loose_cache(struct odb_source *source,
 	return files->loose->cache;
 }
 
-static void odb_source_loose_clear_cache(struct odb_source_loose *loose)
-{
-	oidtree_clear(loose->cache);
-	FREE_AND_NULL(loose->cache);
-	memset(&loose->subdir_seen, 0,
-	       sizeof(loose->subdir_seen));
-}
-
 void odb_source_loose_reprepare(struct odb_source *source)
 {
 	struct odb_source_files *files = odb_source_files_downcast(source);

@@ -2205,15 +2197,6 @@ struct odb_transaction *odb_transaction_files_begin(struct odb_source *source)
 	return &transaction->base;
 }
 
-void odb_source_loose_free(struct odb_source_loose *loose)
-{
-	if (!loose)
-		return;
-	odb_source_loose_clear_cache(loose);
-	loose_object_map_clear(&loose->map);
-	free(loose);
-}
-
 struct odb_loose_read_stream {
 	struct odb_read_stream base;
 	git_zstream z;

diff --git a/object-file.h b/object-file.h
index 1d8312cf7f..02c9680980 100644
--- a/object-file.h
+++ b/object-file.h

@@ -21,8 +21,6 @@ struct object_info;
 struct odb_read_stream;
 struct odb_source;
 
-void odb_source_loose_free(struct odb_source_loose *loose);
-
 /* Reprepare the loose source by emptying the loose object cache. */
 void odb_source_loose_reprepare(struct odb_source *source);
 

diff --git a/odb/source-files.c b/odb/source-files.c
index 185cc6903e..ccc637311b 100644
--- a/odb/source-files.c
+++ b/odb/source-files.c

@@ -27,7 +27,7 @@ static void odb_source_files_free(struct odb_source *source)
 {
 	struct odb_source_files *files = odb_source_files_downcast(source);
 	chdir_notify_unregister(NULL, odb_source_files_reparent, files);
-	odb_source_loose_free(files->loose);
+	odb_source_free(&files->loose->base);
 	packfile_store_free(files->packed);
 	odb_source_release(&files->base);
 	free(files);

diff --git a/odb/source-loose.c b/odb/source-loose.c
index c9e7414814..92e18f5adb 100644
--- a/odb/source-loose.c
+++ b/odb/source-loose.c

@@ -1,10 +1,55 @@
 #include "git-compat-util.h"
+#include "abspath.h"
+#include "chdir-notify.h"
+#include "loose.h"
+#include "odb.h"
+#include "odb/source-files.h"
 #include "odb/source-loose.h"
+#include "oidtree.h"
+
+void odb_source_loose_clear_cache(struct odb_source_loose *loose)
+{
+	oidtree_clear(loose->cache);
+	FREE_AND_NULL(loose->cache);
+	memset(&loose->subdir_seen, 0,
+	       sizeof(loose->subdir_seen));
+}
+
+static void odb_source_loose_reparent(const char *name UNUSED,
+				      const char *old_cwd,
+				      const char *new_cwd,
+				      void *cb_data)
+{
+	struct odb_source_loose *loose = cb_data;
+	char *path = reparent_relative_path(old_cwd, new_cwd,
+					    loose->base.path);
+	free(loose->base.path);
+	loose->base.path = path;
+}
+
+static void odb_source_loose_free(struct odb_source *source)
+{
+	struct odb_source_loose *loose = odb_source_loose_downcast(source);
+	odb_source_loose_clear_cache(loose);
+	loose_object_map_clear(&loose->map);
+	chdir_notify_unregister(NULL, odb_source_loose_reparent, loose);
+	odb_source_release(&loose->base);
+	free(loose);
+}
 
 struct odb_source_loose *odb_source_loose_new(struct odb_source_files *files)
 {
 	struct odb_source_loose *loose;
+
 	CALLOC_ARRAY(loose, 1);
+	odb_source_init(&loose->base, files->base.odb, ODB_SOURCE_LOOSE,
+			files->base.path, files->base.local);
 	loose->files = files;
+
+	loose->base.free = odb_source_loose_free;
+
+	if (!is_absolute_path(loose->base.path))
+		chdir_notify_register(NULL, odb_source_loose_reparent, loose);
+
 	return loose;
 }

diff --git a/odb/source-loose.h b/odb/source-loose.h
index bf61e767c8..441da9e418 100644
--- a/odb/source-loose.h
+++ b/odb/source-loose.h

@@ -12,6 +12,7 @@ struct oidtree;
  * file per object. This source is part of the files source.
  */
 struct odb_source_loose {
+	struct odb_source base;
 	struct odb_source_files *files;
 
 	/*

@@ -32,4 +33,17 @@ struct odb_source_loose {
 
 struct odb_source_loose *odb_source_loose_new(struct odb_source_files *files);
 
+/*
+ * Cast the given object database source to the loose backend. This will cause
+ * a BUG in case the source uses doesn't use this backend.

+ */
+static inline struct odb_source_loose *odb_source_loose_downcast(struct odb_source *source)
+{
+	if (source->type != ODB_SOURCE_LOOSE)
+		BUG("trying to downcast source of type '%d' to loose", source->type);
+	return container_of(source, struct odb_source_loose, base);
+}
+
+void odb_source_loose_clear_cache(struct odb_source_loose *loose);
+
 #endif

diff --git a/odb/source.h b/odb/source.h
index 0a440884e4..8bcb67787e 100644
--- a/odb/source.h
+++ b/odb/source.h

@@ -14,6 +14,9 @@ enum odb_source_type {
 	/* The "files" backend that uses loose objects and packfiles. */
 	ODB_SOURCE_FILES,
 
+	/* The "loose" backend that uses loose objects, only. */
+	ODB_SOURCE_LOOSE,
+
 	/* The "in-memory" backend that stores objects in memory. */
 	ODB_SOURCE_INMEMORY,
 };