Share packs across repository instances

Opening the same repository multiple times will currently open the same
file multiple times, as well as map the same region of the file multiple
times. This is not necessary, as the packfile data is immutable.

Instead of opening and closing packfiles directly, introduce an
indirection and allocate packfiles globally. This does mean locking on
each packfile open, but we already use this lock for the global mwindow
list so it doesn't introduce a new contention point.

src/indexer.c

@@ -18,6 +18,8 @@
 #include "oidmap.h"
 #include "zstream.h"
 
+extern git_mutex git__mwindow_mutex;
+
 #define UINT31_MAX (0x7FFFFFFF)
 
 struct entry {
@@ -1044,6 +1046,11 @@ void git_indexer_free(git_indexer *idx)
 	}
 
 	git_vector_free_deep(&idx->deltas);
-	git_packfile_free(idx->pack);
+
+	if (!git_mutex_lock(&git__mwindow_mutex)) {
+		git_packfile_free(idx->pack);
+		git_mutex_unlock(&git__mwindow_mutex);
+	}
+
 	git__free(idx);
 }

src/mwindow.c

@@ -11,6 +11,10 @@
 #include "fileops.h"
 #include "map.h"
 #include "global.h"
+#include "strmap.h"
+#include "pack.h"
+
+GIT__USE_STRMAP;
 
 #define DEFAULT_WINDOW_SIZE \
 	(sizeof(void*) >= 8 \
@@ -26,20 +30,126 @@ size_t git_mwindow__mapped_limit = DEFAULT_MAPPED_LIMIT;
 /* Whenever you want to read or modify this, grab git__mwindow_mutex */
 static git_mwindow_ctl mem_ctl;
 
-/*
- * Free all the windows in a sequence, typically because we're done
- * with the file
+/* Global list of mwindow files, to open packs once across repos */
+git_strmap *git__pack_cache = NULL;
+
+/**
+ * Run under mwindow lock
  */
-void git_mwindow_free_all(git_mwindow_file *mwf)
+int git_mwindow_files_init(void)
 {
-	git_mwindow_ctl *ctl = &mem_ctl;
-	size_t i;
+	if (git__pack_cache)
+		return 0;
+
+	return git_strmap_alloc(&git__pack_cache);
+}
+
+void git_mwindow_files_free(void)
+{
+	git_strmap *tmp = git__pack_cache;
+
+	git__pack_cache = NULL;
+	git_strmap_free(tmp);
+}
+
+int git_mwindow_get_pack(struct git_pack_file **out, const char *path)
+{
+	int error;
+	char *packname;
+	git_strmap_iter pos;
+	struct git_pack_file *pack;
+
+	if ((error = git_packfile__name(&packname, path)) < 0)
+		return error;
+
+	if (git_mutex_lock(&git__mwindow_mutex) < 0)
+		return -1;
+
+	if (git_mwindow_files_init() < 0) {
+		git_mutex_unlock(&git__mwindow_mutex);
+		return -1;
+	}
+
+	pos = git_strmap_lookup_index(git__pack_cache, packname);
+	git__free(packname);
+
+	if (git_strmap_valid_index(git__pack_cache, pos)) {
+		pack = git_strmap_value_at(git__pack_cache, pos);
+		git_atomic_inc(&pack->refcount);
+
+		git_mutex_unlock(&git__mwindow_mutex);
+		*out = pack;
+		return 0;
+	}
+
+	/* If we didn't find it, we need to create it */
+	if ((error = git_packfile_alloc(&pack, path)) < 0) {
+		git_mutex_unlock(&git__mwindow_mutex);
+		return error;
+	}
+
+	git_atomic_inc(&pack->refcount);
+
+	git_strmap_insert(git__pack_cache, pack->pack_name, pack, error);
+	git_mutex_unlock(&git__mwindow_mutex);
+
+	if (error < 0)
+		return -1;
 
+	*out = pack;
+	return 0;
+}
+
+int git_mwindow_put_pack(struct git_pack_file *pack)
+{
+	int count;
+	git_strmap_iter pos;
+
+	if (git_mutex_lock(&git__mwindow_mutex) < 0)
+		return -1;
+
+	if (git_mwindow_files_init() < 0) {
+		git_mutex_unlock(&git__mwindow_mutex);
+		return -1;
+	}
+
+	pos = git_strmap_lookup_index(git__pack_cache, pack->pack_name);
+	if (!git_strmap_valid_index(git__pack_cache, pos)) {
+		git_mutex_unlock(&git__mwindow_mutex);
+		return GIT_ENOTFOUND;
+	}
+
+	count = git_atomic_dec(&pack->refcount);
+	if (count == 0) {
+		git_strmap_delete_at(git__pack_cache, pos);
+		git_packfile_free(pack);
+	}
+
+	git_mutex_unlock(&git__mwindow_mutex);
+	return 0;
+}
+
+void git_mwindow_free_all(git_mwindow_file *mwf)
+{
 	if (git_mutex_lock(&git__mwindow_mutex)) {
 		giterr_set(GITERR_THREAD, "unable to lock mwindow mutex");
 		return;
 	}
 
+	git_mwindow_free_all_locked(mwf);
+
+	git_mutex_unlock(&git__mwindow_mutex);
+}
+
+/*
+ * Free all the windows in a sequence, typically because we're done
+ * with the file
+ */
+void git_mwindow_free_all_locked(git_mwindow_file *mwf)
+{
+	git_mwindow_ctl *ctl = &mem_ctl;
+	size_t i;
+
 	/*
 	 * Remove these windows from the global list
 	 */
@@ -67,8 +177,6 @@ void git_mwindow_free_all(git_mwindow_file *mwf)
 		mwf->windows = w->next;
 		git__free(w);
 	}
-
-	git_mutex_unlock(&git__mwindow_mutex);
 }
 
 /*

src/mwindow.h

@@ -36,10 +36,18 @@ typedef struct git_mwindow_ctl {
 } git_mwindow_ctl;
 
 int git_mwindow_contains(git_mwindow *win, git_off_t offset);
-void git_mwindow_free_all(git_mwindow_file *mwf);
+void git_mwindow_free_all(git_mwindow_file *mwf); /* locks */
+void git_mwindow_free_all_locked(git_mwindow_file *mwf); /* run under lock */
 unsigned char *git_mwindow_open(git_mwindow_file *mwf, git_mwindow **cursor, git_off_t offset, size_t extra, unsigned int *left);
 int git_mwindow_file_register(git_mwindow_file *mwf);
 void git_mwindow_file_deregister(git_mwindow_file *mwf);
 void git_mwindow_close(git_mwindow **w_cursor);
 
+int git_mwindow_files_init(void);
+void git_mwindow_files_free(void);
+
+struct git_pack_file; /* just declaration to avoid cyclical includes */
+int git_mwindow_get_pack(struct git_pack_file **out, const char *path);
+int git_mwindow_put_pack(struct git_pack_file *pack);
+
 #endif

src/odb_pack.c

@@ -210,7 +210,7 @@ static int packfile_load__cb(void *data, git_buf *path)
 			return 0;
 	}
 
-	error = git_packfile_alloc(&pack, path->ptr);
+	error = git_mwindow_get_pack(&pack, path->ptr);
 
 	/* ignore missing .pack file as git does */
 	if (error == GIT_ENOTFOUND) {
@@ -605,7 +605,7 @@ static void pack_backend__free(git_odb_backend *_backend)
 
 	for (i = 0; i < backend->packs.length; ++i) {
 		struct git_pack_file *p = git_vector_get(&backend->packs, i);
-		git_packfile_free(p);
+		git_mwindow_put_pack(p);
 	}
 
 	git_vector_free(&backend->packs);
@@ -647,7 +647,7 @@ int git_odb_backend_one_pack(git_odb_backend **backend_out, const char *idx)
 	if (pack_backend__alloc(&backend, 1) < 0)
 		return -1;
 
-	if (git_packfile_alloc(&packfile, idx) < 0 ||
+	if (git_mwindow_get_pack(&packfile, idx) < 0 ||
 		git_vector_insert(&backend->packs, packfile) < 0)
 	{
 		pack_backend__free((git_odb_backend *)backend);
@@ -664,6 +664,9 @@ int git_odb_backend_pack(git_odb_backend **backend_out, const char *objects_dir)
 	struct pack_backend *backend = NULL;
 	git_buf path = GIT_BUF_INIT;
 
+	if (git_mwindow_files_init() < 0)
+		return -1;
+
 	if (pack_backend__alloc(&backend, 8) < 0)
 		return -1;
 

src/pack.c

@@ -968,7 +968,7 @@ void git_packfile_free(struct git_pack_file *p)
 
 	cache_free(&p->bases);
 
-	git_mwindow_free_all(&p->mwf);
+	git_mwindow_free_all_locked(&p->mwf);
 
 	if (p->mwf.fd >= 0)
 		p_close(p->mwf.fd);
@@ -1063,6 +1063,23 @@ cleanup:
 	return -1;
 }
 
+int git_packfile__name(char **out, const char *path)
+{
+	size_t path_len;
+	git_buf buf = GIT_BUF_INIT;
+
+	path_len = strlen(path);
+
+	if (path_len < strlen(".idx"))
+		return git_odb__error_notfound("invalid packfile path", NULL);
+
+	if (git_buf_printf(&buf, "%.*s.pack", (int)(path_len - strlen(".idx")), path) < 0)
+		return -1;
+
+	*out = git_buf_detach(&buf);
+	return 0;
+}
+
 int git_packfile_alloc(struct git_pack_file **pack_out, const char *path)
 {
 	struct stat st;

src/pack.h

@@ -90,6 +90,7 @@ struct git_pack_file {
 	git_mwindow_file mwf;
 	git_map index_map;
 	git_mutex lock; /* protect updates to mwf and index_map */
+	git_atomic refcount;
 
 	uint32_t num_objects;
 	uint32_t num_bad_objects;
@@ -123,6 +124,8 @@ typedef struct git_packfile_stream {
 
 size_t git_packfile__object_header(unsigned char *hdr, size_t size, git_otype type);
 
+int git_packfile__name(char **out, const char *path);
+
 int git_packfile_unpack_header(
 		size_t *size_p,
 		git_otype *type_p,

tests/pack/sharing.c

+#include "clar_libgit2.h"
+#include <git2.h>
+#include "strmap.h"
+#include "mwindow.h"
+#include "pack.h"
+
+extern git_strmap *git__pack_cache;
+
+void test_pack_sharing__open_two_repos(void)
+{
+	git_repository *repo1, *repo2;
+	git_object *obj1, *obj2;
+	git_oid id;
+	git_strmap_iter pos;
+	void *data;
+	int error;
+
+	cl_git_pass(git_repository_open(&repo1, cl_fixture("testrepo.git")));
+	cl_git_pass(git_repository_open(&repo2, cl_fixture("testrepo.git")));
+
+	git_oid_fromstr(&id, "a65fedf39aefe402d3bb6e24df4d4f5fe4547750");
+
+	cl_git_pass(git_object_lookup(&obj1, repo1, &id, GIT_OBJ_ANY));
+	cl_git_pass(git_object_lookup(&obj2, repo2, &id, GIT_OBJ_ANY));
+
+	pos = 0;
+	while ((error = git_strmap_next(&data, &pos, git__pack_cache)) == 0) {
+		struct git_pack_file *pack = (struct git_pack_file *) data;
+
+		cl_assert_equal_i(2, pack->refcount.val);
+	}
+
+	cl_assert_equal_i(3, git_strmap_num_entries(git__pack_cache));
+
+	git_object_free(obj1);
+	git_object_free(obj2);
+	git_repository_free(repo1);
+	git_repository_free(repo2);
+
+	/* we don't want to keep the packs open after the repos go away */
+	cl_assert_equal_i(0, git_strmap_num_entries(git__pack_cache));
+}