Merge pull request #11330 from NixOS/packfile

libgit2, GitRepo: Write (thin) packfiles
2024-09-18 10:30:23 -04:00 · 2024-09-16 14:42:59 +02:00 · 2024-09-16 14:42:59 +02:00 · 799abea0c4
parent 176334d8be 459d02672c
commit 799abea0c4
9 changed files with 1379 additions and 5 deletions
--- a/packaging/dependencies.nix
+++ b/packaging/dependencies.nix
@ -115,6 +115,29 @@ scope: {
    version = inputs.libgit2.lastModifiedDate;
    cmakeFlags = attrs.cmakeFlags or []
      ++ [ "-DUSE_SSH=exec" ];
+    nativeBuildInputs = attrs.nativeBuildInputs or []
+      # gitMinimal does not build on Windows. See packbuilder patch.
+      ++ lib.optionals (!stdenv.hostPlatform.isWindows) [
+        # Needed for `git apply`; see `prePatch`
+        pkgs.buildPackages.gitMinimal
+      ];
+    # Only `git apply` can handle git binary patches
+    prePatch = attrs.prePatch or ""
+      + lib.optionalString (!stdenv.hostPlatform.isWindows) ''
+        patch() {
+          git apply
+        }
+      '';
+    patches = attrs.patches or []
+      ++ [
+        ./patches/libgit2-mempack-thin-packfile.patch
+      ]
+      # gitMinimal does not build on Windows, but fortunately this patch only
+      # impacts interruptibility
+      ++ lib.optionals (!stdenv.hostPlatform.isWindows) [
+        # binary patch; see `prePatch`
+        ./patches/libgit2-packbuilder-callback-interruptible.patch
+      ];
  });

  busybox-sandbox-shell = pkgs.busybox-sandbox-shell or (pkgs.busybox.override {
--- a/packaging/patches/libgit2-mempack-thin-packfile.patch
+++ b/packaging/patches/libgit2-mempack-thin-packfile.patch
@ -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);
+
+}
--- a/packaging/patches/libgit2-packbuilder-callback-interruptible.patch
+++ b/packaging/patches/libgit2-packbuilder-callback-interruptible.patch
@ -0,0 +1,930 @@
+commit e9823c5da4fa977c46bcb97167fbdd0d70adb5ff
+Author: Robert Hensing <robert@roberthensing.nl>
+Date:   Mon Aug 26 20:07:04 2024 +0200
+
+    Make packbuilder interruptible using progress callback
+    
+    Forward errors from packbuilder->progress_cb
+    
+    This allows the callback to terminate long-running operations when
+    the application is interrupted.
+
+diff --git a/include/git2/pack.h b/include/git2/pack.h
+index 0f6bd2ab9..bee72a6c0 100644
+--- a/include/git2/pack.h
+++ b/include/git2/pack.h
+@@ -247,6 +247,9 @@ typedef int GIT_CALLBACK(git_packbuilder_progress)(
+  * @param progress_cb Function to call with progress information during
+  * pack building. Be aware that this is called inline with pack building
+  * operations, so performance may be affected.
+ * When progress_cb returns an error, the pack building process will be
+ * aborted and the error will be returned from the invoked function.
+ * `pb` must then be freed.
+  * @param progress_cb_payload Payload for progress callback.
+  * @return 0 or an error code
+  */
+diff --git a/src/libgit2/pack-objects.c b/src/libgit2/pack-objects.c
+index b2d80cba9..7c331c2d5 100644
+--- a/src/libgit2/pack-objects.c
+++ b/src/libgit2/pack-objects.c
+@@ -932,6 +932,9 @@ static int report_delta_progress(
+ {
+ 	int ret;
+ 
+	if (pb->failure)
+		return pb->failure;
+
+ 	if (pb->progress_cb) {
+ 		uint64_t current_time = git_time_monotonic();
+ 		uint64_t elapsed = current_time - pb->last_progress_report_time;
+@@ -943,8 +946,10 @@ static int report_delta_progress(
+ 				GIT_PACKBUILDER_DELTAFICATION,
+ 				count, pb->nr_objects, pb->progress_cb_payload);
+ 
+-			if (ret)
+			if (ret) {
+				pb->failure = ret;
+ 				return git_error_set_after_callback(ret);
+			}
+ 		}
+ 	}
+ 
+@@ -976,7 +981,10 @@ static int find_deltas(git_packbuilder *pb, git_pobject **list,
+ 		}
+ 
+ 		pb->nr_deltified += 1;
+-		report_delta_progress(pb, pb->nr_deltified, false);
+		if ((error = report_delta_progress(pb, pb->nr_deltified, false)) < 0) {
+				GIT_ASSERT(git_packbuilder__progress_unlock(pb) == 0);
+				goto on_error;
+		}
+ 
+ 		po = *list++;
+ 		(*list_size)--;
+@@ -1124,6 +1132,10 @@ struct thread_params {
+ 	size_t depth;
+ 	size_t working;
+ 	size_t data_ready;
+
+	/* A pb->progress_cb can stop the packing process by returning an error.
+	   When that happens, all threads observe the error and stop voluntarily. */
+	bool stopped;
+ };
+ 
+ static void *threaded_find_deltas(void *arg)
+@@ -1133,7 +1145,12 @@ static void *threaded_find_deltas(void *arg)
+ 	while (me->remaining) {
+ 		if (find_deltas(me->pb, me->list, &me->remaining,
+ 				me->window, me->depth) < 0) {
+-			; /* TODO */
+			me->stopped = true;
+			GIT_ASSERT_WITH_RETVAL(git_packbuilder__progress_lock(me->pb) == 0, NULL);
+			me->working = false;
+			git_cond_signal(&me->pb->progress_cond);
+			GIT_ASSERT_WITH_RETVAL(git_packbuilder__progress_unlock(me->pb) == 0, NULL);
+			return NULL;
+ 		}
+ 
+ 		GIT_ASSERT_WITH_RETVAL(git_packbuilder__progress_lock(me->pb) == 0, NULL);
+@@ -1175,8 +1192,7 @@ static int ll_find_deltas(git_packbuilder *pb, git_pobject **list,
+ 		pb->nr_threads = git__online_cpus();
+ 
+ 	if (pb->nr_threads <= 1) {
+-		find_deltas(pb, list, &list_size, window, depth);
+-		return 0;
+		return find_deltas(pb, list, &list_size, window, depth);
+ 	}
+ 
+ 	p = git__mallocarray(pb->nr_threads, sizeof(*p));
+@@ -1195,6 +1211,7 @@ static int ll_find_deltas(git_packbuilder *pb, git_pobject **list,
+ 		p[i].depth = depth;
+ 		p[i].working = 1;
+ 		p[i].data_ready = 0;
+		p[i].stopped = 0;
+ 
+ 		/* try to split chunks on "path" boundaries */
+ 		while (sub_size && sub_size < list_size &&
+@@ -1262,7 +1279,7 @@ static int ll_find_deltas(git_packbuilder *pb, git_pobject **list,
+ 			    (!victim || victim->remaining < p[i].remaining))
+ 				victim = &p[i];
+ 
+-		if (victim) {
+		if (victim && !target->stopped) {
+ 			sub_size = victim->remaining / 2;
+ 			list = victim->list + victim->list_size - sub_size;
+ 			while (sub_size && list[0]->hash &&
+@@ -1286,7 +1303,7 @@ static int ll_find_deltas(git_packbuilder *pb, git_pobject **list,
+ 		}
+ 		target->list_size = sub_size;
+ 		target->remaining = sub_size;
+-		target->working = 1;
+		target->working = 1; /* even when target->stopped, so that we don't process this thread again */
+ 		GIT_ASSERT(git_packbuilder__progress_unlock(pb) == 0);
+ 
+ 		if (git_mutex_lock(&target->mutex)) {
+@@ -1299,7 +1316,7 @@ static int ll_find_deltas(git_packbuilder *pb, git_pobject **list,
+ 		git_cond_signal(&target->cond);
+ 		git_mutex_unlock(&target->mutex);
+ 
+-		if (!sub_size) {
+		if (target->stopped || !sub_size) {
+ 			git_thread_join(&target->thread, NULL);
+ 			git_cond_free(&target->cond);
+ 			git_mutex_free(&target->mutex);
+@@ -1308,7 +1325,7 @@ static int ll_find_deltas(git_packbuilder *pb, git_pobject **list,
+ 	}
+ 
+ 	git__free(p);
+-	return 0;
+	return pb->failure;
+ }
+ 
+ #else
+@@ -1319,6 +1336,7 @@ int git_packbuilder__prepare(git_packbuilder *pb)
+ {
+ 	git_pobject **delta_list;
+ 	size_t i, n = 0;
+	int error;
+ 
+ 	if (pb->nr_objects == 0 || pb->done)
+ 		return 0; /* nothing to do */
+@@ -1327,8 +1345,10 @@ int git_packbuilder__prepare(git_packbuilder *pb)
+ 	 * Although we do not report progress during deltafication, we
+ 	 * at least report that we are in the deltafication stage
+ 	 */
+-	if (pb->progress_cb)
+-			pb->progress_cb(GIT_PACKBUILDER_DELTAFICATION, 0, pb->nr_objects, pb->progress_cb_payload);
+	if (pb->progress_cb) {
+		if ((error = pb->progress_cb(GIT_PACKBUILDER_DELTAFICATION, 0, pb->nr_objects, pb->progress_cb_payload)) < 0)
+			return git_error_set_after_callback(error);
+	}
+ 
+ 	delta_list = git__mallocarray(pb->nr_objects, sizeof(*delta_list));
+ 	GIT_ERROR_CHECK_ALLOC(delta_list);
+@@ -1345,31 +1365,33 @@ int git_packbuilder__prepare(git_packbuilder *pb)
+ 
+ 	if (n > 1) {
+ 		git__tsort((void **)delta_list, n, type_size_sort);
+-		if (ll_find_deltas(pb, delta_list, n,
+		if ((error = ll_find_deltas(pb, delta_list, n,
+ 				   GIT_PACK_WINDOW + 1,
+-				   GIT_PACK_DEPTH) < 0) {
+				   GIT_PACK_DEPTH)) < 0) {
+ 			git__free(delta_list);
+-			return -1;
+			return error;
+ 		}
+ 	}
+ 
+-	report_delta_progress(pb, pb->nr_objects, true);
+	error = report_delta_progress(pb, pb->nr_objects, true);
+ 
+ 	pb->done = true;
+ 	git__free(delta_list);
+-	return 0;
+	return error;
+ }
+ 
+-#define PREPARE_PACK if (git_packbuilder__prepare(pb) < 0) { return -1; }
+#define PREPARE_PACK error = git_packbuilder__prepare(pb); if (error < 0) { return error; }
+ 
+ int git_packbuilder_foreach(git_packbuilder *pb, int (*cb)(void *buf, size_t size, void *payload), void *payload)
+ {
+	int error;
+ 	PREPARE_PACK;
+ 	return write_pack(pb, cb, payload);
+ }
+ 
+ int git_packbuilder__write_buf(git_str *buf, git_packbuilder *pb)
+ {
+	int error;
+ 	PREPARE_PACK;
+ 
+ 	return write_pack(pb, &write_pack_buf, buf);
+diff --git a/src/libgit2/pack-objects.h b/src/libgit2/pack-objects.h
+index bbc8b9430..380a28ebe 100644
+--- a/src/libgit2/pack-objects.h
+++ b/src/libgit2/pack-objects.h
+@@ -100,6 +100,10 @@ struct git_packbuilder {
+ 	uint64_t last_progress_report_time;
+ 
+ 	bool done;
+
+	/* A non-zero error code in failure causes all threads to shut themselves
+	   down. Some functions will return this error code.  */
+	volatile int failure;
+ };
+ 
+ int git_packbuilder__write_buf(git_str *buf, git_packbuilder *pb);
+diff --git a/tests/libgit2/pack/cancel.c b/tests/libgit2/pack/cancel.c
+new file mode 100644
+index 000000000..a0aa9716a
+--- /dev/null
+++ b/tests/libgit2/pack/cancel.c
+@@ -0,0 +1,240 @@
+#include "clar_libgit2.h"
+#include "futils.h"
+#include "pack.h"
+#include "hash.h"
+#include "iterator.h"
+#include "vector.h"
+#include "posix.h"
+#include "hash.h"
+#include "pack-objects.h"
+
+static git_repository *_repo;
+static git_revwalk *_revwalker;
+static git_packbuilder *_packbuilder;
+static git_indexer *_indexer;
+static git_vector _commits;
+static int _commits_is_initialized;
+static git_indexer_progress _stats;
+
+extern bool git_disable_pack_keep_file_checks;
+
+static void pack_packbuilder_init(const char *sandbox) {
+	_repo = cl_git_sandbox_init(sandbox);
+	/* cl_git_pass(p_chdir(sandbox)); */
+	cl_git_pass(git_revwalk_new(&_revwalker, _repo));
+	cl_git_pass(git_packbuilder_new(&_packbuilder, _repo));
+	cl_git_pass(git_vector_init(&_commits, 0, NULL));
+	_commits_is_initialized = 1;
+	memset(&_stats, 0, sizeof(_stats));
+	p_fsync__cnt = 0;
+}
+
+void test_pack_cancel__initialize(void)
+{
+	pack_packbuilder_init("small.git");
+}
+
+void test_pack_cancel__cleanup(void)
+{
+	git_oid *o;
+	unsigned int i;
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_FSYNC_GITDIR, 0));
+	cl_git_pass(git_libgit2_opts(GIT_OPT_DISABLE_PACK_KEEP_FILE_CHECKS, false));
+
+	if (_commits_is_initialized) {
+		_commits_is_initialized = 0;
+		git_vector_foreach(&_commits, i, o) {
+			git__free(o);
+		}
+		git_vector_free(&_commits);
+	}
+
+	git_packbuilder_free(_packbuilder);
+	_packbuilder = NULL;
+
+	git_revwalk_free(_revwalker);
+	_revwalker = NULL;
+
+	git_indexer_free(_indexer);
+	_indexer = NULL;
+
+	/* cl_git_pass(p_chdir("..")); */
+	cl_git_sandbox_cleanup();
+	_repo = NULL;
+}
+
+static int seed_packbuilder(void)
+{
+	int error;
+	git_oid oid, *o;
+	unsigned int i;
+
+	git_revwalk_sorting(_revwalker, GIT_SORT_TIME);
+	cl_git_pass(git_revwalk_push_ref(_revwalker, "HEAD"));
+
+	while (git_revwalk_next(&oid, _revwalker) == 0) {
+		o = git__malloc(sizeof(git_oid));
+		cl_assert(o != NULL);
+		git_oid_cpy(o, &oid);
+		cl_git_pass(git_vector_insert(&_commits, o));
+	}
+
+	git_vector_foreach(&_commits, i, o) {
+		if((error = git_packbuilder_insert(_packbuilder, o, NULL)) < 0)
+			return error;
+	}
+
+	git_vector_foreach(&_commits, i, o) {
+		git_object *obj;
+		cl_git_pass(git_object_lookup(&obj, _repo, o, GIT_OBJECT_COMMIT));
+		error = git_packbuilder_insert_tree(_packbuilder,
+					git_commit_tree_id((git_commit *)obj));
+		git_object_free(obj);
+		if (error < 0)
+			return error;
+	}
+
+	return 0;
+}
+
+static int fail_stage;
+
+static int packbuilder_cancel_after_n_calls_cb(int stage, uint32_t current, uint32_t total, void *payload)
+{
+
+	/* Force the callback to run again on the next opportunity regardless
+	   of how fast we're running. */
+	_packbuilder->last_progress_report_time = 0;
+
+	if (stage == fail_stage) {
+		int *calls = (int *)payload;
+		int n = *calls;
+		/* Always decrement, including past zero. This way the error is only
+		   triggered once, making sure it is picked up immediately. */
+		--*calls;
+		if (n == 0)
+			return GIT_EUSER;
+	}
+
+	return 0;
+}
+
+static void test_cancel(int n)
+{
+
+	int calls_remaining = n;
+	int err;
+	git_buf buf = GIT_BUF_INIT;
+
+	/* Switch to a small repository, so that `packbuilder_cancel_after_n_calls_cb`
+	   can hack the time to call the callback on every opportunity. */
+
+	cl_git_pass(git_packbuilder_set_callbacks(_packbuilder, &packbuilder_cancel_after_n_calls_cb, &calls_remaining));
+	err = seed_packbuilder();
+	if (!err)
+		err = git_packbuilder_write_buf(&buf, _packbuilder);
+
+	cl_assert_equal_i(GIT_EUSER, err);
+}
+void test_pack_cancel__cancel_after_add_0(void)
+{
+	fail_stage = GIT_PACKBUILDER_ADDING_OBJECTS;
+	test_cancel(0);
+}
+
+void test_pack_cancel__cancel_after_add_1(void)
+{
+	cl_skip();
+	fail_stage = GIT_PACKBUILDER_ADDING_OBJECTS;
+	test_cancel(1);
+}
+
+void test_pack_cancel__cancel_after_delta_0(void)
+{
+	fail_stage = GIT_PACKBUILDER_DELTAFICATION;
+	test_cancel(0);
+}
+
+void test_pack_cancel__cancel_after_delta_1(void)
+{
+	fail_stage = GIT_PACKBUILDER_DELTAFICATION;
+	test_cancel(1);
+}
+
+void test_pack_cancel__cancel_after_delta_0_threaded(void)
+{
+#ifdef GIT_THREADS
+	git_packbuilder_set_threads(_packbuilder, 8);
+	fail_stage = GIT_PACKBUILDER_DELTAFICATION;
+	test_cancel(0);
+#else
+	cl_skip();
+#endif
+}
+
+void test_pack_cancel__cancel_after_delta_1_threaded(void)
+{
+#ifdef GIT_THREADS
+	git_packbuilder_set_threads(_packbuilder, 8);
+	fail_stage = GIT_PACKBUILDER_DELTAFICATION;
+	test_cancel(1);
+#else
+	cl_skip();
+#endif
+}
+
+static int foreach_cb(void *buf, size_t len, void *payload)
+{
+	git_indexer *idx = (git_indexer *) payload;
+	cl_git_pass(git_indexer_append(idx, buf, len, &_stats));
+	return 0;
+}
+
+void test_pack_cancel__foreach(void)
+{
+	git_indexer *idx;
+
+	seed_packbuilder();
+
+#ifdef GIT_EXPERIMENTAL_SHA256
+	cl_git_pass(git_indexer_new(&idx, ".", GIT_OID_SHA1, NULL));
+#else
+	cl_git_pass(git_indexer_new(&idx, ".", 0, NULL, NULL));
+#endif
+
+	cl_git_pass(git_packbuilder_foreach(_packbuilder, foreach_cb, idx));
+	cl_git_pass(git_indexer_commit(idx, &_stats));
+	git_indexer_free(idx);
+}
+
+static int foreach_cancel_cb(void *buf, size_t len, void *payload)
+{
+	git_indexer *idx = (git_indexer *)payload;
+	cl_git_pass(git_indexer_append(idx, buf, len, &_stats));
+	return (_stats.total_objects > 2) ? -1111 : 0;
+}
+
+void test_pack_cancel__foreach_with_cancel(void)
+{
+	git_indexer *idx;
+
+	seed_packbuilder();
+
+#ifdef GIT_EXPERIMENTAL_SHA256
+	cl_git_pass(git_indexer_new(&idx, ".", GIT_OID_SHA1, NULL));
+#else
+	cl_git_pass(git_indexer_new(&idx, ".", 0, NULL, NULL));
+#endif
+
+	cl_git_fail_with(
+		git_packbuilder_foreach(_packbuilder, foreach_cancel_cb, idx), -1111);
+	git_indexer_free(idx);
+}
+
+void test_pack_cancel__keep_file_check(void)
+{
+	assert(!git_disable_pack_keep_file_checks);
+	cl_git_pass(git_libgit2_opts(GIT_OPT_DISABLE_PACK_KEEP_FILE_CHECKS, true));
+	assert(git_disable_pack_keep_file_checks);
+}
+diff --git a/tests/resources/small.git/HEAD b/tests/resources/small.git/HEAD
+new file mode 100644
+index 0000000000000000000000000000000000000000..cb089cd89a7d7686d284d8761201649346b5aa1c
+GIT binary patch
+literal 23
+ecmXR)O|w!cN=+-)&qz&7Db~+TEG|hc;sO9;xClW2
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/config b/tests/resources/small.git/config
+new file mode 100644
+index 0000000000000000000000000000000000000000..07d359d07cf1ed0c0074fdad71ffff5942f0adfa
+GIT binary patch
+literal 66
+zcmaz}&M!)h<>D+#Eyyp<EXgmbOv^9IO)M!(Eh^5;&r`5fFyP`$%gjm5%}+@M@=A(I
+MQ@J>k5{uv*03B5png9R*
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/description b/tests/resources/small.git/description
+new file mode 100644
+index 0000000000000000000000000000000000000000..498b267a8c7812490d6479839c5577eaaec79d62
+GIT binary patch
+literal 73
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+Q$yWgB0LrH#Y0~2Y0PnOK(EtDd
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/hooks/applypatch-msg.sample b/tests/resources/small.git/hooks/applypatch-msg.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..dcbf8167fa503f96ff6a39c68409007eadc9b1f3
+GIT binary patch
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+
+literal 0
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+
+diff --git a/tests/resources/small.git/hooks/commit-msg.sample b/tests/resources/small.git/hooks/commit-msg.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..f3780f92349638ebe32f6baf24c7c3027675d7c9
+GIT binary patch
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+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/hooks/fsmonitor-watchman.sample b/tests/resources/small.git/hooks/fsmonitor-watchman.sample
+new file mode 100755
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+GIT binary patch
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+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/hooks/post-update.sample b/tests/resources/small.git/hooks/post-update.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..9f95e41a39cf974958277c8881ac6cce405ebb20
+GIT binary patch
+literal 246
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+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/hooks/pre-applypatch.sample b/tests/resources/small.git/hooks/pre-applypatch.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..625837e25f91421b8809a097f4a3103dd387ef31
+GIT binary patch
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+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/hooks/pre-commit.sample b/tests/resources/small.git/hooks/pre-commit.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..10b39b2e26981b8f87ea424e735ef87359066dbb
+GIT binary patch
+literal 1706
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+diff --git a/tests/resources/small.git/hooks/pre-merge-commit.sample b/tests/resources/small.git/hooks/pre-merge-commit.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..d8e8ad4bcf8337f23c78a98537ecab4831301202
+GIT binary patch
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+literal 0
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+diff --git a/tests/resources/small.git/hooks/pre-push.sample b/tests/resources/small.git/hooks/pre-push.sample
+new file mode 100755
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+diff --git a/tests/resources/small.git/hooks/pre-rebase.sample b/tests/resources/small.git/hooks/pre-rebase.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..193ee3f127387f469894675f0d621b5bb9c275da
+GIT binary patch
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+literal 0
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+diff --git a/tests/resources/small.git/hooks/pre-receive.sample b/tests/resources/small.git/hooks/pre-receive.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..32b597f8c32399017c3d7d51134d1f6490ad9575
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+diff --git a/tests/resources/small.git/hooks/prepare-commit-msg.sample b/tests/resources/small.git/hooks/prepare-commit-msg.sample
+new file mode 100755
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+diff --git a/tests/resources/small.git/hooks/push-to-checkout.sample b/tests/resources/small.git/hooks/push-to-checkout.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..a80611e18896f212c390d845e49a3f6d5693b41d
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+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/hooks/sendemail-validate.sample b/tests/resources/small.git/hooks/sendemail-validate.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..effa6f54f2a5c1f0cebe258bf4805904be4b4852
+GIT binary patch
+literal 2365
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+zk+$48h#x9Ika!=nv$M0y{clD}-j1x(hDWbnzP?l2k8j?TH{brS+Nf21yPm)Nczma>
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+fi$|vMHn?H<rred|1&u#kOoJ`%vx)-*g-ZSfgGvdP
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/hooks/update.sample b/tests/resources/small.git/hooks/update.sample
+new file mode 100755
+index 0000000000000000000000000000000000000000..c3430f6fe27e572a1cebdb2f2bec70add0642a60
+GIT binary patch
+literal 3707
+zcmb_fU2oeq6n(aS#jRq*br9K3y0z;^QgkUc^kKlTrB6js&@yE)mPu8l<0$L?`wmIT
+zrsXba))@gJi@e|G+<SfSXe`CeSQ}OG@sr8ZTUlQ{dzM}Q@O-hBi}GeUom`#X%FiYH
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+z`9sa`0lR_azMX|=t_(FvG@%w2);9P}SG0u&K1(*oX3};~=<<!N*F$UTSxD{V&6mgL
+ztw9Ntc2eOF@c!OJytNC4T^#)Ien7-`dI{6s#co~0f^E3J<0Ty)l3xq2u@Y8DXTK>v
+zozoWSr{{+!h}zGpumG3H`=AvWpm^9kW;J$Jp^Xl*?8ckr`fqN%c|Z;VC0|cM4vSrk
+zH_O8Yvh85nvJp^;``wo8=z0f`FWg?`>gO#y1hjX1{}rTlg9rwIKia8eyGexA3GnuR
+z`Rg~XZoW;0pA)vI8=p5!+6sIn#C^FCvR>ffv39h6SCNi9v);%WD;WZ`of_MgwyRWy
+z-yY%n*Y>X8<Sf+RyB|Sr2YG@1w~%U$o`(5EDkJ|3$u=eMZA&_wxEsy<Xxh2k^tP?4
+RGx&ZHQs^8zuIpu!=pQhfp8Eg*
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/info/exclude b/tests/resources/small.git/info/exclude
+new file mode 100644
+index 0000000000000000000000000000000000000000..a5196d1be8fb59edf8062bef36d3a602e0812139
+GIT binary patch
+literal 240
+zcmXYqK?=e!6h!x)VxWtv*mf_t5?px$aS_{}Hj?C*<cHXeXE&AZhT*-L3ZoI&*l1%Z
+zqG?zr3TvQGZ__}H4(u*%p*rI=cU!%ya5ugfGATh66$IJHgu1Gs0-<N;$V+SsdE)?u
+zIq;i$f#WE4f$_MWicZjMEob9LWKMR#iwZq54~QgST^6=i%u0lUkJu-_J**QBMq}ZG
+Wth_)NDbl|`oQr&HAFQ5h`0jqAIZ*BZ
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/objects/88/f9a6eaa2cf008b9bc92847178621f21fa99f3e b/tests/resources/small.git/objects/88/f9a6eaa2cf008b9bc92847178621f21fa99f3e
+new file mode 100644
+index 0000000000000000000000000000000000000000..7d80b8d78e0dc55669d831a6638f48ec9fed0982
+GIT binary patch
+literal 50
+zcmV-20L}k+0V^p=O;s>9W-v4`Ff%bx$Vkn}$!Ay}rnY6F$m-Kg*KD_+;Lx#g4|^&N
+I02NaX#p`nv=Kufz
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/objects/af/5626b4a114abcb82d63db7c8082c3c4756e51b b/tests/resources/small.git/objects/af/5626b4a114abcb82d63db7c8082c3c4756e51b
+new file mode 100644
+index 0000000000000000000000000000000000000000..822bc151862ec3763cf2d3fa2372b93bbd3a4b65
+GIT binary patch
+literal 30
+mcmb<m^geacKghr&@q@?NlP9kSYMj?U<r(;diNWtH+YSKNt_|)0
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/objects/c6/97d4f7a6eac8d7b131673c340bd3cc5bac14d4 b/tests/resources/small.git/objects/c6/97d4f7a6eac8d7b131673c340bd3cc5bac14d4
+new file mode 100644
+index 0000000000000000000000000000000000000000..5adfa88cc6b00bb19f2031b8ab61f6d07f9ccdb8
+GIT binary patch
+literal 130
+zcmV-|0Db>>0i}&W3IZ_@1U=^!a~EV1casc=c+{&un1qQN*i9hD|0|m(2n|iwp*q%W
+z%N;b$hu%cM`$TMo*~EnC1BFP&Pfj~;jZVKXQ96s_PhV<-XAROi+@-v8dBLUa`!;GB
+k^i<X>XlEv8$>R)1G>9th&t3j;s7J{?^9n<zzF|~BaA?ar-~a#s
+
+literal 0
+HcmV?d00001
+
+diff --git a/tests/resources/small.git/refs/heads/master b/tests/resources/small.git/refs/heads/master
+new file mode 100644
+index 0000000000000000000000000000000000000000..4eb36d22298f060fd324155ab854d9d6486fc498
+GIT binary patch
+literal 41
+ucmV~$!4Uu;2m`Rc)5uXl$AMP&AHjriQg~T$i(A>|7U^`%mXoWC24Q^m!3%@{
+
+literal 0
+HcmV?d00001
+
--- a/src/libfetchers/git-utils.cc
+++ b/src/libfetchers/git-utils.cc
@ -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>
@ -159,11 +166,60 @@ static Object peelToTreeOrBlob(git_object * obj)
        return peelObject<Object>(obj, GIT_OBJECT_TREE);
 }

+struct PackBuilderContext {
+    std::exception_ptr exception;
+
+    void handleException(const char * activity, int errCode)
+    {
+        switch (errCode) {
+            case GIT_OK:
+                break;
+            case GIT_EUSER:
+                if (!exception)
+                    panic("PackBuilderContext::handleException: user error, but exception was not set");
+
+                std::rethrow_exception(exception);
+            default:
+                throw Error("%s: %i, %s", Uncolored(activity), errCode, git_error_last()->message);
+        }
+    }
+};
+
+extern "C" {
+
+/**
+ * A `git_packbuilder_progress` implementation that aborts the pack building if needed.
+ */
+static int packBuilderProgressCheckInterrupt(int stage, uint32_t current, uint32_t total, void *payload)
+{
+    PackBuilderContext & args = * (PackBuilderContext *) payload;
+    try {
+        checkInterrupt();
+        return GIT_OK;
+    } catch (const std::exception & e) {
+        args.exception = std::current_exception();
+        return GIT_EUSER;
+    }
+};
+static git_packbuilder_progress PACKBUILDER_PROGRESS_CHECK_INTERRUPT = &packBuilderProgressCheckInterrupt;
+
+} // extern "C"
+
 struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 {
    /** Location of the repository on disk. */
    std::filesystem::path path;
+    /**
+     * libgit2 repository. Note that new objects are not written to disk,
+     * because we are using a mempack backend. For writing to disk, see
+     * `flush()`, which is also called by `GitFileSystemObjectSink::sync()`.
+     */
    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 +233,17 @@ 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);
        }
+
+        ObjectDb odb;
+        if (git_repository_odb(Setter(odb), repo.get()))
+            throw Error("getting Git object database: %s", git_error_last()->message);
+
+        // mempack_backend will be owned by the repository, so we are not expected to free it ourselves.
+        if (git_mempack_new(&mempack_backend))
+            throw Error("creating mempack backend: %s", git_error_last()->message);
+
+        if (git_odb_add_backend(odb.get(), mempack_backend, 999))
+            throw Error("adding mempack backend to Git object database: %s", git_error_last()->message);
    }

    operator git_repository * ()
@ -184,6 +251,62 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
        return repo.get();
    }

+    void flush() override {
+        checkInterrupt();
+
+        git_buf buf = GIT_BUF_INIT;
+        Finally _disposeBuf { [&] { git_buf_dispose(&buf); } };
+        PackBuilder packBuilder;
+        PackBuilderContext packBuilderContext;
+        git_packbuilder_new(Setter(packBuilder), *this);
+        git_packbuilder_set_callbacks(packBuilder.get(), PACKBUILDER_PROGRESS_CHECK_INTERRUPT, &packBuilderContext);
+        git_packbuilder_set_threads(packBuilder.get(), 0 /* autodetect */);
+
+        packBuilderContext.handleException(
+            "preparing packfile",
+            git_mempack_write_thin_pack(mempack_backend, packBuilder.get())
+        );
+        checkInterrupt();
+        packBuilderContext.handleException(
+            "writing packfile",
+            git_packbuilder_write_buf(&buf, packBuilder.get())
+        );
+        checkInterrupt();
+
+        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 (performance): could the indexing be done in a separate thread?
+        //                     we'd need a more streaming variation of
+        //                     git_packbuilder_write_buf, or incur the cost of
+        //                     copying parts of the buffer to a separate thread.
+        //                     (synchronously on the git_packbuilder_write_buf 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);
+
+        // TODO: provide index callback for checkInterrupt() termination
+        //       though this is about an order of magnitude faster than the packbuilder
+        //       expect up to 1 sec latency due to uninterruptible git_indexer_append.
+        constexpr size_t chunkSize = 128 * 1024;
+        for (size_t offset = 0; offset < buf.size; offset += chunkSize) {
+            if (git_indexer_append(indexer.get(), buf.ptr + offset, std::min(chunkSize, buf.size - offset), &stats))
+                throw Error("appending to git packfile index: %s", git_error_last()->message);
+            checkInterrupt();
+        }
+
+        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);
+
+        checkInterrupt();
+    }
+
    uint64_t getRevCount(const Hash & rev) override
    {
        std::unordered_set<git_oid> done;
@ -1006,12 +1129,14 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
            git_tree_entry_filemode(entry));
    }

-    Hash sync() override
+    Hash flush() override
    {
        updateBuilders({});

        auto [oid, _name] = popBuilder();

+        repo->flush();
+
        return toHash(oid);
    }
 };
--- a/src/libfetchers/git-utils.hh
+++ b/src/libfetchers/git-utils.hh
@ -7,12 +7,16 @@ namespace nix {

 namespace fetchers { struct PublicKey; }

+/**
+ * A sink that writes into a Git repository. Note that nothing may be written
+ * until `flush()` is called.
+ */
 struct GitFileSystemObjectSink : ExtendedFileSystemObjectSink
 {
    /**
     * Flush builder and return a final Git hash.
     */
-    virtual Hash sync() = 0;
+    virtual Hash flush() = 0;
 };

 struct GitRepo
@ -80,6 +84,8 @@ struct GitRepo

    virtual ref<GitFileSystemObjectSink> getFileSystemObjectSink() = 0;

+    virtual void flush() = 0;
+
    virtual void fetch(
        const std::string & url,
        const std::string & refspec,
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@ -261,11 +261,12 @@ struct GitArchiveInputScheme : InputScheme
        auto tarballCache = getTarballCache();
        auto parseSink = tarballCache->getFileSystemObjectSink();
        auto lastModified = unpackTarfileToSink(archive, *parseSink);
+        auto tree = parseSink->flush();

        act.reset();

        TarballInfo tarballInfo {
-            .treeHash = tarballCache->dereferenceSingletonDirectory(parseSink->sync()),
+            .treeHash = tarballCache->dereferenceSingletonDirectory(tree),
            .lastModified = lastModified
        };

--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@ -170,6 +170,7 @@ static DownloadTarballResult downloadTarball_(
    auto tarballCache = getTarballCache();
    auto parseSink = tarballCache->getFileSystemObjectSink();
    auto lastModified = unpackTarfileToSink(archive, *parseSink);
+    auto tree = parseSink->flush();

    act.reset();

@ -184,7 +185,7 @@ static DownloadTarballResult downloadTarball_(
    } else {
        infoAttrs.insert_or_assign("etag", res->etag);
        infoAttrs.insert_or_assign("treeHash",
-            tarballCache->dereferenceSingletonDirectory(parseSink->sync()).gitRev());
+            tarballCache->dereferenceSingletonDirectory(tree).gitRev());
        infoAttrs.insert_or_assign("lastModified", uint64_t(lastModified));
        if (res->immutableUrl)
            infoAttrs.insert_or_assign("immutableUrl", *res->immutableUrl);
--- a/src/libutil/unix/signals-impl.hh
+++ b/src/libutil/unix/signals-impl.hh
@ -84,6 +84,12 @@ static inline bool getInterrupted()
    return unix::_isInterrupted;
 }

+/**
+ * Throw `Interrupted` exception if the process has been interrupted.
+ *
+ * Call this in long-running loops and between slow operations to terminate
+ * them as needed.
+ */
 void inline checkInterrupt()
 {
    using namespace unix;
--- a/tests/unit/libfetchers/git-utils.cc
+++ b/tests/unit/libfetchers/git-utils.cc
@ -77,7 +77,7 @@ TEST_F(GitUtilsTest, sink_basic)

    // sink->createHardlink("foo-1.1/links/foo-2", CanonPath("foo-1.1/hello"));

-    auto result = repo->dereferenceSingletonDirectory(sink->sync());
+    auto result = repo->dereferenceSingletonDirectory(sink->flush());
    auto accessor = repo->getAccessor(result, false);
    auto entries = accessor->readDirectory(CanonPath::root);
    ASSERT_EQ(entries.size(), 5);