odb: introduce "in-memory" source

Next to our typical object database sources, each object database also
has an implicit source of "cached" objects. These cached objects only
exist in memory and some use cases:

  - They contain evergreen objects that we expect to always exist, like
    for example the empty tree.

  - They can be used to store temporary objects that we don't want to
    persist to disk, which is used by git-blame(1) to create a fake
    worktree commit.

Overall, their use is somewhat restricted though. For example, we don't
provide the ability to use it as a temporary object database source that
allows the user to write objects, but discard them after Git exists. So
while these cached objects behave almost like a source, they aren't used
as one.

This is about to change over the following commits, where we will turn
cached objects into a new "in-memory" source. This will allow us to use
it exactly the same as any other source by providing the same common
interface as the "files" source.

For now, the in-memory source only hosts the cached objects and doesn't
provide any logic yet. This will change with subsequent commits, where
we move respective functionality into the source.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1218,6 +1218,7 @@ LIB_OBJS += object.o
 LIB_OBJS += odb.o
 LIB_OBJS += odb/source.o
 LIB_OBJS += odb/source-files.o
+LIB_OBJS += odb/source-inmemory.o
 LIB_OBJS += odb/streaming.o
 LIB_OBJS += odb/transaction.o
 LIB_OBJS += oid-array.o

meson.build

@@ -404,6 +404,7 @@ libgit_sources = [
   'odb.c',
   'odb/source.c',
   'odb/source-files.c',
+  'odb/source-inmemory.c',
   'odb/streaming.c',
   'odb/transaction.c',
   'oid-array.c',

odb.c

@@ -14,6 +14,7 @@
 #include "object-file.h"
 #include "object-name.h"
 #include "odb.h"
+#include "odb/source-inmemory.h"
 #include "packfile.h"
 #include "path.h"
 #include "promisor-remote.h"
@@ -53,9 +54,9 @@ static const struct cached_object *find_cached_object(struct object_database *ob
 		.type = OBJ_TREE,
 		.buf = "",
 	};
-	const struct cached_object_entry *co = object_store->cached_objects;
+	const struct cached_object_entry *co = object_store->inmemory_objects->objects;
 
-	for (size_t i = 0; i < object_store->cached_object_nr; i++, co++)
+	for (size_t i = 0; i < object_store->inmemory_objects->objects_nr; i++, co++)
 		if (oideq(&co->oid, oid))
 			return &co->value;
 
@@ -792,9 +793,10 @@ int odb_pretend_object(struct object_database *odb,
 	    find_cached_object(odb, oid))
 		return 0;
 
-	ALLOC_GROW(odb->cached_objects,
-		   odb->cached_object_nr + 1, odb->cached_object_alloc);
-	co = &odb->cached_objects[odb->cached_object_nr++];
+	ALLOC_GROW(odb->inmemory_objects->objects,
+		   odb->inmemory_objects->objects_nr + 1,
+		   odb->inmemory_objects->objects_alloc);
+	co = &odb->inmemory_objects->objects[odb->inmemory_objects->objects_nr++];
 	co->value.size = len;
 	co->value.type = type;
 	co_buf = xmalloc(len);
@@ -1083,6 +1085,7 @@ struct object_database *odb_new(struct repository *repo,
 	o->sources = odb_source_new(o, primary_source, true);
 	o->sources_tail = &o->sources->next;
 	o->alternate_db = xstrdup_or_null(secondary_sources);
+	o->inmemory_objects = odb_source_inmemory_new(o);
 
 	free(to_free);
 
@@ -1123,9 +1126,11 @@ void odb_free(struct object_database *o)
 	odb_close(o);
 	odb_free_sources(o);
 
-	for (size_t i = 0; i < o->cached_object_nr; i++)
-		free((char *) o->cached_objects[i].value.buf);
-	free(o->cached_objects);
+	for (size_t i = 0; i < o->inmemory_objects->objects_nr; i++)
+		free((char *) o->inmemory_objects->objects[i].value.buf);
+	free(o->inmemory_objects->objects);
+	free(o->inmemory_objects->base.path);
+	free(o->inmemory_objects);
 
 	string_list_clear(&o->submodule_source_paths, 0);
 

odb.h

@@ -8,6 +8,7 @@
 #include "thread-utils.h"
 
 struct cached_object_entry;
+struct odb_source_inmemory;
 struct packed_git;
 struct repository;
 struct strbuf;
@@ -80,8 +81,7 @@ struct object_database {
 	 * to write them into the object store (e.g. a browse-only
 	 * application).
 	 */
-	struct cached_object_entry *cached_objects;
-	size_t cached_object_nr, cached_object_alloc;
+	struct odb_source_inmemory *inmemory_objects;
 
 	/*
 	 * A fast, rough count of the number of objects in the repository.

odb/source-inmemory.c

@@ -0,0 +1,12 @@
+#include "git-compat-util.h"
+#include "odb/source-inmemory.h"
+
+struct odb_source_inmemory *odb_source_inmemory_new(struct object_database *odb)
+{
+	struct odb_source_inmemory *source;
+
+	CALLOC_ARRAY(source, 1);
+	odb_source_init(&source->base, odb, ODB_SOURCE_INMEMORY, "source", false);
+
+	return source;
+}

odb/source-inmemory.h

@@ -0,0 +1,35 @@
+#ifndef ODB_SOURCE_INMEMORY_H
+#define ODB_SOURCE_INMEMORY_H
+
+#include "odb/source.h"
+
+struct cached_object_entry;
+
+/*
+ * An inmemory source that you can write objects to that shall be made
+ * available for reading, but that shouldn't ever be persisted to disk. Note
+ * that any objects written to this source will be stored in memory, so the
+ * number of objects you can store is limited by available system memory.
+ */
+struct odb_source_inmemory {
+	struct odb_source base;
+
+	struct cached_object_entry *objects;
+	size_t objects_nr, objects_alloc;
+};
+
+/* Create a new in-memory object database source. */
+struct odb_source_inmemory *odb_source_inmemory_new(struct object_database *odb);
+
+/*
+ * Cast the given object database source to the inmemory backend. This will
+ * cause a BUG in case the source doesn't use this backend.
+ */
+static inline struct odb_source_inmemory *odb_source_inmemory_downcast(struct odb_source *source)
+{
+	if (source->type != ODB_SOURCE_INMEMORY)
+		BUG("trying to downcast source of type '%d' to inmemory", source->type);
+	return container_of(source, struct odb_source_inmemory, base);
+}
+
+#endif

odb/source.h

@@ -13,6 +13,9 @@ enum odb_source_type {
 
 	/* The "files" backend that uses loose objects and packfiles. */
 	ODB_SOURCE_FILES,
+
+	/* The "inmemory" backend that stores objects in memory. */
+	ODB_SOURCE_INMEMORY,
 };
 
 struct object_id;