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libgit2, GitRepo: Write thin packfiles

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libgit2 didn't write thin ones, hence the patch.

This should improve performance on systems with weak I/O in ~/.cache,
especially in terms of operations per second, or where system calls
are slower. (macOS, VMs?)
This commit is contained in:
Robert Hensing 2024-08-19 13:21:44 +02:00
parent cb4b9be458
commit 5dd6c4f062
4 changed files with 343 additions and 0 deletions
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2
packaging/dependencies.nix
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@ -115,6 +115,8 @@ scope: {
version = inputs.libgit2.lastModifiedDate;
cmakeFlags = attrs.cmakeFlags or []
++ [ "-DUSE_SSH=exec" ];
patches = attrs.patches or []
++ [ ./patches/libgit2-mempack-thin-packfile.patch ];
});
busybox-sandbox-shell = pkgs.busybox-sandbox-shell or (pkgs.busybox.override {

282
packaging/patches/libgit2-mempack-thin-packfile.patch Normal file
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@ -0,0 +1,282 @@
commit 9bacade4a3ef4b6b26e2c02f549eef0e9eb9eaa2
Author: Robert Hensing <robert@roberthensing.nl>
Date: Sun Aug 18 20:20:36 2024 +0200
Add unoptimized git_mempack_write_thin_pack
diff --git a/include/git2/sys/mempack.h b/include/git2/sys/mempack.h
index 17da590a3..3688bdd50 100644
--- a/include/git2/sys/mempack.h
+++ b/include/git2/sys/mempack.h
@@ -44,6 +44,29 @@ GIT_BEGIN_DECL
*/
GIT_EXTERN(int) git_mempack_new(git_odb_backend **out);
+/**
+ * Write a thin packfile with the objects in the memory store.
+ *
+ * A thin packfile is a packfile that does not contain its transitive closure of
+ * references. This is useful for efficiently distributing additions to a
+ * repository over the network, but also finds use in the efficient bulk
+ * addition of objects to a repository, locally.
+ *
+ * This operation performs the (shallow) insert operations into the
+ * `git_packbuilder`, but does not write the packfile to disk;
+ * see `git_packbuilder_write_buf`.
+ *
+ * It also does not reset the memory store; see `git_mempack_reset`.
+ *
+ * @note This function may or may not write trees and blobs that are not
+ * referenced by commits. Currently everything is written, but this
+ * behavior may change in the future as the packer is optimized.
+ *
+ * @param backend The mempack backend
+ * @param pb The packbuilder to use to write the packfile
+ */
+GIT_EXTERN(int) git_mempack_write_thin_pack(git_odb_backend *backend, git_packbuilder *pb);
+
/**
* Dump all the queued in-memory writes to a packfile.
*
diff --git a/src/libgit2/odb_mempack.c b/src/libgit2/odb_mempack.c
index 6f27f45f8..0b61e2b66 100644
--- a/src/libgit2/odb_mempack.c
+++ b/src/libgit2/odb_mempack.c
@@ -132,6 +132,35 @@ cleanup:
return err;
}
+int git_mempack_write_thin_pack(git_odb_backend *backend, git_packbuilder *pb)
+{
+ struct memory_packer_db *db = (struct memory_packer_db *)backend;
+ const git_oid *oid;
+ size_t iter = 0;
+ int err = -1;
+
+ /* TODO: Implement the recency heuristics.
+ For this it probably makes sense to only write what's referenced
+ through commits, an option I've carved out for you in the docs.
+ wrt heuristics: ask your favorite LLM to translate https://git-scm.com/docs/pack-heuristics/en
+ to actual normal reference documentation. */
+ while (true) {
+ err = git_oidmap_iterate(NULL, db->objects, &iter, &oid);
+ if (err == GIT_ITEROVER) {
+ err = 0;
+ break;
+ }
+ if (err != 0)
+ return err;
+
+ err = git_packbuilder_insert(pb, oid, NULL);
+ if (err != 0)
+ return err;
+ }
+
+ return 0;
+}
+
int git_mempack_dump(
git_buf *pack,
git_repository *repo,
diff --git a/tests/libgit2/mempack/thinpack.c b/tests/libgit2/mempack/thinpack.c
new file mode 100644
index 000000000..604a4dda2
--- /dev/null
+++ b/tests/libgit2/mempack/thinpack.c
@@ -0,0 +1,196 @@
+#include "clar_libgit2.h"
+#include "git2/indexer.h"
+#include "git2/odb_backend.h"
+#include "git2/tree.h"
+#include "git2/types.h"
+#include "git2/sys/mempack.h"
+#include "git2/sys/odb_backend.h"
+#include "util.h"
+
+static git_repository *_repo;
+static git_odb_backend * _mempack_backend;
+
+void test_mempack_thinpack__initialize(void)
+{
+ git_odb *odb;
+
+ _repo = cl_git_sandbox_init_new("mempack_thinpack_repo");
+
+ cl_git_pass(git_mempack_new(&_mempack_backend));
+ cl_git_pass(git_repository_odb(&odb, _repo));
+ cl_git_pass(git_odb_add_backend(odb, _mempack_backend, 999));
+ git_odb_free(odb);
+}
+
+void _mempack_thinpack__cleanup(void)
+{
+ cl_git_sandbox_cleanup();
+}
+
+/*
+ Generating a packfile for an unchanged repo works and produces an empty packfile.
+ Even if we allow this scenario to be detected, it shouldn't misbehave if the
+ application is unaware of it.
+*/
+void test_mempack_thinpack__empty(void)
+{
+ git_packbuilder *pb;
+ int version;
+ int n;
+ git_buf buf = GIT_BUF_INIT;
+
+ git_packbuilder_new(&pb, _repo);
+
+ cl_git_pass(git_mempack_write_thin_pack(_mempack_backend, pb));
+ cl_git_pass(git_packbuilder_write_buf(&buf, pb));
+ cl_assert_in_range(12, buf.size, 1024 /* empty packfile is >0 bytes, but certainly not that big */);
+ cl_assert(buf.ptr[0] == 'P');
+ cl_assert(buf.ptr[1] == 'A');
+ cl_assert(buf.ptr[2] == 'C');
+ cl_assert(buf.ptr[3] == 'K');
+ version = (buf.ptr[4] << 24) | (buf.ptr[5] << 16) | (buf.ptr[6] << 8) | buf.ptr[7];
+ /* Subject to change. https://git-scm.com/docs/pack-format: Git currently accepts version number 2 or 3 but generates version 2 only.*/
+ cl_assert_equal_i(2, version);
+ n = (buf.ptr[8] << 24) | (buf.ptr[9] << 16) | (buf.ptr[10] << 8) | buf.ptr[11];
+ cl_assert_equal_i(0, n);
+ git_buf_dispose(&buf);
+
+ git_packbuilder_free(pb);
+}
+
+#define LIT_LEN(x) x, sizeof(x) - 1
+
+/*
+ Check that git_mempack_write_thin_pack produces a thin packfile.
+*/
+void test_mempack_thinpack__thin(void)
+{
+ /* Outline:
+ - Create tree 1
+ - Flush to packfile A
+ - Create tree 2
+ - Flush to packfile B
+
+ Tree 2 has a new blob and a reference to a blob from tree 1.
+
+ Expectation:
+ - Packfile B is thin and does not contain the objects from packfile A
+ */
+
+
+ git_oid oid_blob_1;
+ git_oid oid_blob_2;
+ git_oid oid_blob_3;
+ git_oid oid_tree_1;
+ git_oid oid_tree_2;
+ git_treebuilder *tb;
+
+ git_packbuilder *pb;
+ git_buf buf = GIT_BUF_INIT;
+ git_indexer *indexer;
+ git_indexer_progress stats;
+ char pack_dir_path[1024];
+
+ char sbuf[1024];
+ const char * repo_path;
+ const char * pack_name_1;
+ const char * pack_name_2;
+ git_str pack_path_1 = GIT_STR_INIT;
+ git_str pack_path_2 = GIT_STR_INIT;
+ git_odb_backend * pack_odb_backend_1;
+ git_odb_backend * pack_odb_backend_2;
+
+
+ cl_assert_in_range(0, snprintf(pack_dir_path, sizeof(pack_dir_path), "%s/objects/pack", git_repository_path(_repo)), sizeof(pack_dir_path));
+
+ /* Create tree 1 */
+
+ cl_git_pass(git_blob_create_from_buffer(&oid_blob_1, _repo, LIT_LEN("thinpack blob 1")));
+ cl_git_pass(git_blob_create_from_buffer(&oid_blob_2, _repo, LIT_LEN("thinpack blob 2")));
+
+
+ cl_git_pass(git_treebuilder_new(&tb, _repo, NULL));
+ cl_git_pass(git_treebuilder_insert(NULL, tb, "blob1", &oid_blob_1, GIT_FILEMODE_BLOB));
+ cl_git_pass(git_treebuilder_insert(NULL, tb, "blob2", &oid_blob_2, GIT_FILEMODE_BLOB));
+ cl_git_pass(git_treebuilder_write(&oid_tree_1, tb));
+
+ /* Flush */
+
+ cl_git_pass(git_packbuilder_new(&pb, _repo));
+ cl_git_pass(git_mempack_write_thin_pack(_mempack_backend, pb));
+ cl_git_pass(git_packbuilder_write_buf(&buf, pb));
+ cl_git_pass(git_indexer_new(&indexer, pack_dir_path, 0, NULL, NULL));
+ cl_git_pass(git_indexer_append(indexer, buf.ptr, buf.size, &stats));
+ cl_git_pass(git_indexer_commit(indexer, &stats));
+ pack_name_1 = strdup(git_indexer_name(indexer));
+ cl_assert(pack_name_1);
+ git_buf_dispose(&buf);
+ git_mempack_reset(_mempack_backend);
+ git_indexer_free(indexer);
+ git_packbuilder_free(pb);
+
+ /* Create tree 2 */
+
+ cl_git_pass(git_treebuilder_clear(tb));
+ /* blob 1 won't be used, but we add it anyway to test that just "declaring" an object doesn't
+ necessarily cause its inclusion in the next thin packfile. It must only be included if new. */
+ cl_git_pass(git_blob_create_from_buffer(&oid_blob_1, _repo, LIT_LEN("thinpack blob 1")));
+ cl_git_pass(git_blob_create_from_buffer(&oid_blob_3, _repo, LIT_LEN("thinpack blob 3")));
+ cl_git_pass(git_treebuilder_insert(NULL, tb, "blob1", &oid_blob_1, GIT_FILEMODE_BLOB));
+ cl_git_pass(git_treebuilder_insert(NULL, tb, "blob3", &oid_blob_3, GIT_FILEMODE_BLOB));
+ cl_git_pass(git_treebuilder_write(&oid_tree_2, tb));
+
+ /* Flush */
+
+ cl_git_pass(git_packbuilder_new(&pb, _repo));
+ cl_git_pass(git_mempack_write_thin_pack(_mempack_backend, pb));
+ cl_git_pass(git_packbuilder_write_buf(&buf, pb));
+ cl_git_pass(git_indexer_new(&indexer, pack_dir_path, 0, NULL, NULL));
+ cl_git_pass(git_indexer_append(indexer, buf.ptr, buf.size, &stats));
+ cl_git_pass(git_indexer_commit(indexer, &stats));
+ pack_name_2 = strdup(git_indexer_name(indexer));
+ cl_assert(pack_name_2);
+ git_buf_dispose(&buf);
+ git_mempack_reset(_mempack_backend);
+ git_indexer_free(indexer);
+ git_packbuilder_free(pb);
+ git_treebuilder_free(tb);
+
+ /* Assertions */
+
+ assert(pack_name_1);
+ assert(pack_name_2);
+
+ repo_path = git_repository_path(_repo);
+
+ snprintf(sbuf, sizeof(sbuf), "objects/pack/pack-%s.pack", pack_name_1);
+ git_str_joinpath(&pack_path_1, repo_path, sbuf);
+ snprintf(sbuf, sizeof(sbuf), "objects/pack/pack-%s.pack", pack_name_2);
+ git_str_joinpath(&pack_path_2, repo_path, sbuf);
+
+ /* If they're the same, something definitely went wrong. */
+ cl_assert(strcmp(pack_name_1, pack_name_2) != 0);
+
+ cl_git_pass(git_odb_backend_one_pack(&pack_odb_backend_1, pack_path_1.ptr));
+ cl_assert(pack_odb_backend_1->exists(pack_odb_backend_1, &oid_blob_1));
+ cl_assert(pack_odb_backend_1->exists(pack_odb_backend_1, &oid_blob_2));
+ cl_assert(!pack_odb_backend_1->exists(pack_odb_backend_1, &oid_blob_3));
+ cl_assert(pack_odb_backend_1->exists(pack_odb_backend_1, &oid_tree_1));
+ cl_assert(!pack_odb_backend_1->exists(pack_odb_backend_1, &oid_tree_2));
+
+ cl_git_pass(git_odb_backend_one_pack(&pack_odb_backend_2, pack_path_2.ptr));
+ /* blob 1 is already in the packfile 1, so packfile 2 must not include it, in order to be _thin_. */
+ cl_assert(!pack_odb_backend_2->exists(pack_odb_backend_2, &oid_blob_1));
+ cl_assert(!pack_odb_backend_2->exists(pack_odb_backend_2, &oid_blob_2));
+ cl_assert(pack_odb_backend_2->exists(pack_odb_backend_2, &oid_blob_3));
+ cl_assert(!pack_odb_backend_2->exists(pack_odb_backend_2, &oid_tree_1));
+ cl_assert(pack_odb_backend_2->exists(pack_odb_backend_2, &oid_tree_2));
+
+ pack_odb_backend_1->free(pack_odb_backend_1);
+ pack_odb_backend_2->free(pack_odb_backend_2);
+ free((void *)pack_name_1);
+ free((void *)pack_name_2);
+ git_str_dispose(&pack_path_1);
+ git_str_dispose(&pack_path_2);
+
+}

57
src/libfetchers/git-utils.cc
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@ -13,13 +13,17 @@
#include <git2/describe.h>
#include <git2/errors.h>
#include <git2/global.h>
#include <git2/indexer.h>
#include <git2/object.h>
#include <git2/odb.h>
#include <git2/refs.h>
#include <git2/remote.h>
#include <git2/repository.h>
#include <git2/revparse.h>
#include <git2/status.h>
#include <git2/submodule.h>
#include <git2/sys/odb_backend.h>
#include <git2/sys/mempack.h>
#include <git2/tree.h>
#include <iostream>
@ -76,6 +80,9 @@ typedef std::unique_ptr<git_status_list, Deleter<git_status_list_free>> StatusLi
typedef std::unique_ptr<git_remote, Deleter<git_remote_free>> Remote;
typedef std::unique_ptr<git_config, Deleter<git_config_free>> GitConfig;
typedef std::unique_ptr<git_config_iterator, Deleter<git_config_iterator_free>> ConfigIterator;
typedef std::unique_ptr<git_odb, Deleter<git_odb_free>> ObjectDb;
typedef std::unique_ptr<git_packbuilder, Deleter<git_packbuilder_free>> PackBuilder;
typedef std::unique_ptr<git_indexer, Deleter<git_indexer_free>> Indexer;
// A helper to ensure that we don't leak objects returned by libgit2.
template<typename T>
@ -164,6 +171,11 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
/** Location of the repository on disk. */
std::filesystem::path path;
Repository repo;
/**
* In-memory object store for efficient batched writing to packfiles.
* Owned by `repo`.
*/
git_odb_backend * mempack_backend;
GitRepoImpl(std::filesystem::path _path, bool create, bool bare)
: path(std::move(_path))
@ -177,6 +189,16 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
if (git_repository_init(Setter(repo), path.string().c_str(), bare))
throw Error("creating Git repository '%s': %s", path, git_error_last()->message);
}
git_odb * odb;
if (git_repository_odb(&odb, repo.get()))
throw Error("getting Git object database: %s", git_error_last()->message);
// TODO: release mempack_backend?
if (git_mempack_new(&mempack_backend))
throw Error("creating mempack backend: %s", git_error_last()->message);
if (git_odb_add_backend(odb, mempack_backend, 999))
throw Error("adding mempack backend to Git object database: %s", git_error_last()->message);
}
operator git_repository * ()
@ -184,6 +206,39 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
return repo.get();
}
void flush() override {
git_buf buf = GIT_BUF_INIT;
try {
PackBuilder packBuilder;
git_packbuilder_new(Setter(packBuilder), *this);
git_mempack_write_thin_pack(mempack_backend, packBuilder.get());
git_packbuilder_write_buf(&buf, packBuilder.get());
std::string repo_path = std::string(git_repository_path(repo.get()));
while (!repo_path.empty() && repo_path.back() == '/')
repo_path.pop_back();
std::string pack_dir_path = repo_path + "/objects/pack";
// TODO: could the indexing be done in a separate thread?
Indexer indexer;
git_indexer_progress stats;
if (git_indexer_new(Setter(indexer), pack_dir_path.c_str(), 0, nullptr, nullptr))
throw Error("creating git packfile indexer: %s", git_error_last()->message);
if (git_indexer_append(indexer.get(), buf.ptr, buf.size, &stats))
throw Error("appending to git packfile index: %s", git_error_last()->message);
if (git_indexer_commit(indexer.get(), &stats))
throw Error("committing git packfile index: %s", git_error_last()->message);
if (git_mempack_reset(mempack_backend))
throw Error("resetting git mempack backend: %s", git_error_last()->message);
git_buf_dispose(&buf);
} catch (...) {
git_buf_dispose(&buf);
throw;
}
}
uint64_t getRevCount(const Hash & rev) override
{
std::unordered_set<git_oid> done;
@ -1008,6 +1063,8 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
auto [oid, _name] = popBuilder();
repo->flush();
return toHash(oid);
}
};

2
src/libfetchers/git-utils.hh
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@ -80,6 +80,8 @@ struct GitRepo
virtual ref<GitFileSystemObjectSink> getFileSystemObjectSink() = 0;
virtual void flush() = 0;
virtual void fetch(
const std::string & url,
const std::string & refspec,