[PATCH RFC 4/5] backfill: add --dry-run option

[Pablo Sabater <pabloosabaterr@gmail.com>]

Users have no way to know how many blobs git backfill is going to
download before running it.

Add a new --dry-run option to the backfill command. The objects are
walked as usual, but instead of fetching each batch of missing blobs
they are only counted, and the total is printed at the end.

A subsequent commit will also print their size when the server supports
the object-info capability.

Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
---
 Documentation/git-backfill.adoc |  6 +++++-
 builtin/backfill.c              | 37 +++++++++++++++++++++++++++++++++----
 t/t5620-backfill.sh             | 26 ++++++++++++++++++++++++++
 3 files changed, 64 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-backfill.adoc b/Documentation/git-backfill.adoc
index 82d6a1969d..08f19fea17 100644
--- a/Documentation/git-backfill.adoc
+++ b/Documentation/git-backfill.adoc
@@ -9,7 +9,7 @@ git-backfill - Download missing objects in a partial clone
 SYNOPSIS
 --------
 [synopsis]
-git backfill [--min-batch-size=<n>] [--[no-]sparse] [--[no-]include-edges] [<revision-range>]
+git backfill [--min-batch-size=<n>] [--[no-]sparse] [--[no-]include-edges] [--dry-run] [<revision-range>]
 
 DESCRIPTION
 -----------
@@ -70,6 +70,10 @@ OPTIONS
 	--onto TARGET A..B`, where A..B normally excludes A but you need
 	the blobs from A as well.  `--include-edges` is the default.
 
+`--dry-run`::
+	Do not download any objects. Instead, print the number of
+	missing blobs that would be downloaded.
+
 `<revision-range>`::
 	Backfill only blobs reachable from commits in the specified
 	revision range.  When no _<revision-range>_ is specified, it
diff --git a/builtin/backfill.c b/builtin/backfill.c
index e71e0f4742..6019112966 100644
--- a/builtin/backfill.c
+++ b/builtin/backfill.c
@@ -26,7 +26,7 @@
 #include "path-walk.h"
 
 static const char * const builtin_backfill_usage[] = {
-	N_("git backfill [--min-batch-size=<n>] [--[no-]sparse] [--[no-]include-edges] [<revision-range>]"),
+	N_("git backfill [--min-batch-size=<n>] [--[no-]sparse] [--[no-]include-edges] [--dry-run] [<revision-range>]"),
 	NULL
 };
 
@@ -36,6 +36,8 @@ struct backfill_context {
 	size_t min_batch_size;
 	int sparse;
 	int include_edges;
+	int dry_run;
+	size_t total_batch_nr;
 	struct rev_info revs;
 };
 
@@ -58,6 +60,15 @@ static void download_batch(struct backfill_context *ctx)
 	odb_reprepare(ctx->repo->objects);
 }
 
+static void dry_run_batch(struct backfill_context *ctx)
+{
+	if (!ctx->current_batch.nr)
+		return;
+
+	ctx->total_batch_nr += ctx->current_batch.nr;
+	oid_array_clear(&ctx->current_batch);
+}
+
 static int fill_missing_blobs(const char *path UNUSED,
 			      struct oid_array *list,
 			      enum object_type type,
@@ -73,8 +84,12 @@ static int fill_missing_blobs(const char *path UNUSED,
 			oid_array_append(&ctx->current_batch, &list->oid[i]);
 	}
 
-	if (ctx->current_batch.nr >= ctx->min_batch_size)
-		download_batch(ctx);
+	if (ctx->current_batch.nr >= ctx->min_batch_size) {
+		if (ctx->dry_run)
+			dry_run_batch(ctx);
+		else
+			download_batch(ctx);
+	}
 
 	return 0;
 }
@@ -131,10 +146,23 @@ static int do_backfill(struct backfill_context *ctx)
 
 	ret = walk_objects_by_path(&info);
 
+	if (ret)
+		goto end;
+
 	/* Download the objects that did not fill a batch. */
-	if (!ret)
+	if (!ctx->dry_run) {
 		download_batch(ctx);
+		goto end;
+	}
+
+	dry_run_batch(ctx);
+
+	printf(Q_("After backfill, %" PRIuMAX " blob would be fetched.\n",
+		  "After backfill, %" PRIuMAX " blobs would be fetched.\n",
+		  (unsigned long)ctx->total_batch_nr),
+	       (uintmax_t)ctx->total_batch_nr);
 
+end:
 	path_walk_info_clear(&info);
 	return ret;
 }
@@ -157,6 +185,7 @@ int cmd_backfill(int argc, const char **argv, const char *prefix, struct reposit
 			 N_("Restrict the missing objects to the current sparse-checkout")),
 		OPT_BOOL(0, "include-edges", &ctx.include_edges,
 			 N_("Include blobs from boundary commits in the backfill")),
+		OPT__DRY_RUN(&ctx.dry_run, N_("Preview the number of blobs to be fetched")),
 		OPT_END(),
 	};
 	struct repo_config_values *cfg = repo_config_values(the_repository);
diff --git a/t/t5620-backfill.sh b/t/t5620-backfill.sh
index 7462280470..e76fa6081b 100755
--- a/t/t5620-backfill.sh
+++ b/t/t5620-backfill.sh
@@ -141,6 +141,32 @@ test_expect_success 'do partial clone 2, backfill min batch size' '
 	test_line_count = 0 revs2
 '
 
+test_expect_success '--dry-run reports missing blobs without fetching them' '
+	test_when_finished "rm -rf backfill-dry-run dry-trace" &&
+	git clone --no-checkout --filter=blob:none \
+		--single-branch --branch=main \
+		"file://$(pwd)/srv.bare" backfill-dry-run &&
+
+	GIT_TRACE2_EVENT="$(pwd)/dry-trace" git \
+		-C backfill-dry-run backfill --dry-run >out &&
+
+	test_grep "48 blobs would be fetched" out &&
+	test_grep ! fetch_count dry-trace &&
+	git -C backfill-dry-run rev-list --quiet --objects --missing=print HEAD >missing &&
+	test_line_count = 48 missing
+'
+
+test_expect_success '--dry-run with no missing blobs' '
+	test_when_finished rm -rf backfill-dry-run &&
+	git clone --no-checkout --filter=blob:none \
+		--single-branch --branch=main \
+		"file://$(pwd)/srv.bare" backfill-dry-run &&
+	git -C backfill-dry-run backfill &&
+
+	git -C backfill-dry-run backfill --dry-run >out &&
+	test_grep "0 blobs would be fetched" out
+'
+
 test_expect_success 'backfill --sparse without sparse-checkout fails' '
 	git init not-sparse &&
 	test_must_fail git -C not-sparse backfill --sparse 2>err &&

-- 
2.54.0