[WORK-IN-PROGRESS] Introduce the path walk API into Git for Windows

.gitignore

@@ -19,6 +19,7 @@
 /git-apply
 /git-archimport
 /git-archive
+/git-backfill
 /git-bisect
 /git-blame
 /git-branch

Documentation/git-backfill.txt

@@ -0,0 +1,60 @@
+git-backfill(1)
+===============
+
+NAME
+----
+git-backfill - Download missing objects in a partial clone
+
+
+SYNOPSIS
+--------
+[verse]
+'git backfill' [--batch-size=<n>] [--[no-]sparse]
+
+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.
+
+OPTIONS
+-------
+
+--batch-size=<n>::
+	Specify a minimum size for a batch of missing objects to request
+	from the server. This size may be exceeded by the last set of
+	blobs seen at a given path. Default batch size is 16,000.
+
+--[no-]sparse::
+	Only download objects if they appear at a path that matches the
+	current sparse-checkout. If the sparse-checkout feature is enabled,
+	then `--sparse` is assumed and can be disabled with `--no-sparse`.
+
+SEE ALSO
+--------
+linkgit:git-clone[1].
+
+GIT
+---
+Part of the linkgit:git[1] suite

Makefile

@@ -1097,6 +1097,7 @@ LIB_OBJS += parse-options.o
 LIB_OBJS += patch-delta.o
 LIB_OBJS += patch-ids.o
 LIB_OBJS += path.o
+LIB_OBJS += path-walk.o
 LIB_OBJS += pathspec.o
 LIB_OBJS += pkt-line.o
 LIB_OBJS += preload-index.o
@@ -1204,6 +1205,7 @@ BUILTIN_OBJS += builtin/am.o
 BUILTIN_OBJS += builtin/annotate.o
 BUILTIN_OBJS += builtin/apply.o
 BUILTIN_OBJS += builtin/archive.o
+BUILTIN_OBJS += builtin/backfill.o
 BUILTIN_OBJS += builtin/bisect.o
 BUILTIN_OBJS += builtin/blame.o
 BUILTIN_OBJS += builtin/branch.o

builtin.h

@@ -127,6 +127,7 @@ int cmd_am(int argc, const char **argv, const char *prefix);
 int cmd_annotate(int argc, const char **argv, const char *prefix);
 int cmd_apply(int argc, const char **argv, const char *prefix);
 int cmd_archive(int argc, const char **argv, const char *prefix);
+int cmd_backfill(int argc, const char **argv, const char *prefix);
 int cmd_bisect(int argc, const char **argv, const char *prefix);
 int cmd_blame(int argc, const char **argv, const char *prefix);
 int cmd_branch(int argc, const char **argv, const char *prefix);

builtin/backfill.c

@@ -0,0 +1,141 @@
+#include "builtin.h"
+#include "git-compat-util.h"
+#include "config.h"
+#include "parse-options.h"
+#include "repository.h"
+#include "commit.h"
+#include "dir.h"
+#include "environment.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 [--batch-size=<n>] [--[no-]sparse]"),
+	NULL
+};
+
+struct backfill_context {
+	struct repository *repo;
+	struct oid_array current_batch;
+	size_t batch_size;
+	int sparse;
+};
+
+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,
+			      struct oid_array *list,
+			      enum object_type type,
+			      void *data)
+{
+	struct backfill_context *ctx = data;
+
+	if (type != OBJ_BLOB)
+		BUG("fill_missing_blobs only takes blob objects");
+
+	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(the_repository,
+					     &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;
+
+	if (ctx->sparse) {
+		CALLOC_ARRAY(info.pl, 1);
+		if (get_sparse_checkout_patterns(info.pl))
+			return error(_("problem loading sparse-checkout"));
+	}
+
+	repo_init_revisions(ctx->repo, &revs, "");
+	handle_revision_arg("HEAD", &revs, 0, 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 backfill_context ctx = {
+		.repo = the_repository,
+		.current_batch = OID_ARRAY_INIT,
+		.batch_size = 16000,
+		.sparse = 0,
+	};
+	struct option options[] = {
+		OPT_INTEGER(0, "batch-size", &ctx.batch_size,
+			    N_("Minimun number of objects to request at a time")),
+		OPT_BOOL(0, "sparse", &ctx.sparse,
+			 N_("Restrict the missing objects to the current sparse-checkout")),
+		OPT_END(),
+	};
+
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(builtin_backfill_usage, options);
+
+	argc = parse_options(argc, argv, prefix, options, builtin_backfill_usage,
+			     0);
+
+	git_config(git_default_config, NULL);
+
+	if (ctx.sparse < 0)
+		ctx.sparse = core_apply_sparse_checkout;
+
+	return do_backfill(&ctx);
+}

command-list.txt

@@ -60,6 +60,7 @@ git-annotate                            ancillaryinterrogators
 git-apply                               plumbingmanipulators            complete
 git-archimport                          foreignscminterface
 git-archive                             mainporcelain
+git-backfill                            mainporcelain           history
 git-bisect                              mainporcelain           info
 git-blame                               ancillaryinterrogators          complete
 git-branch                              mainporcelain           history

git.c

@@ -502,6 +502,7 @@ static struct cmd_struct commands[] = {
 	{ "annotate", cmd_annotate, RUN_SETUP },
 	{ "apply", cmd_apply, RUN_SETUP_GENTLY },
 	{ "archive", cmd_archive, RUN_SETUP_GENTLY },
+	{ "backfill", cmd_backfill, RUN_SETUP },
 	{ "bisect", cmd_bisect, RUN_SETUP },
 	{ "blame", cmd_blame, RUN_SETUP },
 	{ "branch", cmd_branch, RUN_SETUP | DELAY_PAGER_CONFIG },