backfill: basic functionality and tests

The default behavior of 'git backfill' is to fetch all missing blobs that
are reachable from HEAD. Document and test this behavior.

The implementation is a very simple use of the path-walk API, initializing
the revision walk at HEAD to start the path-walk from all commits reachable
from HEAD. Ignore the object arrays that correspond to tree entries,
assuming that they are all present already.

The path-walk API provides lists of objects in batches according to a
common path, but that list could be very small. We want to balance the
number of requests to the server with the ability to have the process
interrupted with minimal repeated work to catch up in the next run.
Based on some experiments (detailed in the next change) a minimum batch
size of 50,000 is selected for the default.

This batch size is a _minimum_. As the path-walk API emits lists of blob
IDs, they are collected into a list of objects for a request to the
server. When that list is at least the minimum batch size, then the
request is sent to the server for the new objects. However, the list of
blob IDs from the path-walk API could be much longer than the batch
size. At this moment, it is unclear if there is a benefit to split the
list when there are too many objects at the same path.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Documentation/git-backfill.txt

@@ -14,6 +14,30 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
+Blobless partial clones are created using `git clone --filter=blob:none`
+and then configure the local repository such that the Git client avoids
+downloading blob objects unless they are required for a local operation.
+This initially means that the clone and later fetches download reachable
+commits and trees but no blobs. Later operations that change the `HEAD`
+pointer, such as `git checkout` or `git merge`, may need to download
+missing blobs in order to complete their operation.
+
+In the worst cases, commands that compute blob diffs, such as `git blame`,
+become very slow as they download the missing blobs in single-blob
+requests to satisfy the missing object as the Git command needs it. This
+leads to multiple download requests and no ability for the Git server to
+provide delta compression across those objects.
+
+The `git backfill` command provides a way for the user to request that
+Git downloads the missing blobs (with optional filters) such that the
+missing blobs representing historical versions of files can be downloaded
+in batches. The `backfill` command attempts to optimize the request by
+grouping blobs that appear at the same path, hopefully leading to good
+delta compression in the packfile sent by the server.
+
+By default, `git backfill` downloads all blobs reachable from the `HEAD`
+commit. This set can be restricted or expanded using various options.
+
 SEE ALSO
 --------
 linkgit:git-clone[1].

Documentation/technical/api-path-walk.txt

@@ -60,4 +60,5 @@ Examples
 --------
 
 See example usages in:
-	`t/helper/test-path-walk.c`
+	`t/helper/test-path-walk.c`,
+	`builtin/backfill.c`

builtin/backfill.c

@@ -1,16 +1,116 @@
 #include "builtin.h"
+#include "git-compat-util.h"
 #include "config.h"
 #include "parse-options.h"
 #include "repository.h"
+#include "commit.h"
+#include "hex.h"
+#include "tree.h"
+#include "tree-walk.h"
 #include "object.h"
+#include "object-store-ll.h"
+#include "oid-array.h"
+#include "oidset.h"
+#include "promisor-remote.h"
+#include "strmap.h"
+#include "string-list.h"
+#include "revision.h"
+#include "trace2.h"
+#include "progress.h"
+#include "packfile.h"
+#include "path-walk.h"
 
 static const char * const builtin_backfill_usage[] = {
 	N_("git backfill [<options>]"),
 	NULL
 };
 
+struct backfill_context {
+	struct repository *repo;
+	struct oid_array current_batch;
+	size_t batch_size;
+};
+
+static void clear_backfill_context(struct backfill_context *ctx)
+{
+	oid_array_clear(&ctx->current_batch);
+}
+
+static void download_batch(struct backfill_context *ctx)
+{
+	promisor_remote_get_direct(ctx->repo,
+				   ctx->current_batch.oid,
+				   ctx->current_batch.nr);
+	oid_array_clear(&ctx->current_batch);
+
+	/*
+	 * We likely have a new packfile. Add it to the packed list to
+	 * avoid possible duplicate downloads of the same objects.
+	 */
+	reprepare_packed_git(ctx->repo);
+}
+
+static int fill_missing_blobs(const char *path UNUSED,
+			      struct oid_array *list,
+			      enum object_type type,
+			      void *data)
+{
+	struct backfill_context *ctx = data;
+
+	if (type != OBJ_BLOB)
+		return 0;
+
+	for (size_t i = 0; i < list->nr; i++) {
+		off_t size = 0;
+		struct object_info info = OBJECT_INFO_INIT;
+		info.disk_sizep = &size;
+		if (oid_object_info_extended(ctx->repo,
+					     &list->oid[i],
+					     &info,
+					     OBJECT_INFO_FOR_PREFETCH) ||
+		    !size)
+			oid_array_append(&ctx->current_batch, &list->oid[i]);
+	}
+
+	if (ctx->current_batch.nr >= ctx->batch_size)
+		download_batch(ctx);
+
+	return 0;
+}
+
+static int do_backfill(struct backfill_context *ctx)
+{
+	struct rev_info revs;
+	struct path_walk_info info = PATH_WALK_INFO_INIT;
+	int ret;
+
+	repo_init_revisions(ctx->repo, &revs, "");
+	handle_revision_arg("HEAD", &revs, 0, 0);
+
+	info.blobs = 1;
+	info.tags = info.commits = info.trees = 0;
+
+	info.revs = &revs;
+	info.path_fn = fill_missing_blobs;
+	info.path_fn_data = ctx;
+
+	ret = walk_objects_by_path(&info);
+
+	/* Download the objects that did not fill a batch. */
+	if (!ret)
+		download_batch(ctx);
+
+	clear_backfill_context(ctx);
+	return ret;
+}
+
 int cmd_backfill(int argc, const char **argv, const char *prefix, struct repository *repo)
 {
+	struct backfill_context ctx = {
+		.repo = repo,
+		.current_batch = OID_ARRAY_INIT,
+		.batch_size = 50000,
+	};
 	struct option options[] = {
 		OPT_END(),
 	};
@@ -23,7 +123,5 @@ int cmd_backfill(int argc, const char **argv, const char *prefix, struct reposit
 
 	repo_config(repo, git_default_config, NULL);
 
-	die(_("not implemented"));
-
-	return 0;
+	return do_backfill(&ctx);
 }

t/t5620-backfill.sh

@@ -0,0 +1,94 @@
+#!/bin/sh
+
+test_description='git backfill on partial clones'
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+. ./test-lib.sh
+
+# We create objects in the 'src' repo.
+test_expect_success 'setup repo for object creation' '
+	echo "{print \$1}" >print_1.awk &&
+	echo "{print \$2}" >print_2.awk &&
+
+	git init src &&
+
+	mkdir -p src/a/b/c &&
+	mkdir -p src/d/e &&
+
+	for i in 1 2
+	do
+		for n in 1 2 3 4
+		do
+			echo "Version $i of file $n" > src/file.$n.txt &&
+			echo "Version $i of file a/$n" > src/a/file.$n.txt &&
+			echo "Version $i of file a/b/$n" > src/a/b/file.$n.txt &&
+			echo "Version $i of file a/b/c/$n" > src/a/b/c/file.$n.txt &&
+			echo "Version $i of file d/$n" > src/d/file.$n.txt &&
+			echo "Version $i of file d/e/$n" > src/d/e/file.$n.txt &&
+			git -C src add . &&
+			git -C src commit -m "Iteration $n" || return 1
+		done
+	done
+'
+
+# Clone 'src' into 'srv.bare' so we have a bare repo to be our origin
+# server for the partial clone.
+test_expect_success 'setup bare clone for server' '
+	git clone --bare "file://$(pwd)/src" srv.bare &&
+	git -C srv.bare config --local uploadpack.allowfilter 1 &&
+	git -C srv.bare config --local uploadpack.allowanysha1inwant 1
+'
+
+# do basic partial clone from "srv.bare"
+test_expect_success 'do partial clone 1, backfill gets all objects' '
+	git clone --no-checkout --filter=blob:none	\
+		--single-branch --branch=main 		\
+		"file://$(pwd)/srv.bare" backfill1 &&
+
+	# Backfill with no options gets everything reachable from HEAD.
+	GIT_TRACE2_EVENT="$(pwd)/backfill-file-trace" git \
+		-C backfill1 backfill &&
+
+	# We should have engaged the partial clone machinery
+	test_trace2_data promisor fetch_count 48 <backfill-file-trace &&
+
+	# No more missing objects!
+	git -C backfill1 rev-list --quiet --objects --missing=print HEAD >revs2 &&
+	test_line_count = 0 revs2
+'
+
+. "$TEST_DIRECTORY"/lib-httpd.sh
+start_httpd
+
+test_expect_success 'create a partial clone over HTTP' '
+	SERVER="$HTTPD_DOCUMENT_ROOT_PATH/server" &&
+	rm -rf "$SERVER" repo &&
+	git clone --bare "file://$(pwd)/src" "$SERVER" &&
+	test_config -C "$SERVER" uploadpack.allowfilter 1 &&
+	test_config -C "$SERVER" uploadpack.allowanysha1inwant 1 &&
+
+	git clone --no-checkout --filter=blob:none \
+		"$HTTPD_URL/smart/server" backfill-http
+'
+
+test_expect_success 'backfilling over HTTP succeeds' '
+	GIT_TRACE2_EVENT="$(pwd)/backfill-http-trace" git \
+		-C backfill-http backfill &&
+
+	# We should have engaged the partial clone machinery
+	test_trace2_data promisor fetch_count 48 <backfill-http-trace &&
+
+	# Confirm all objects are present, none missing.
+	git -C backfill-http rev-list --objects --all >rev-list-out &&
+	awk "{print \$1;}" <rev-list-out >oids &&
+	GIT_TRACE2_EVENT="$(pwd)/walk-trace" git -C backfill-http \
+		cat-file --batch-check <oids >batch-out &&
+	! grep missing batch-out
+'
+
+# DO NOT add non-httpd-specific tests here, because the last part of this
+# test script is only executed when httpd is available and enabled.
+
+test_done