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

From

Pablo Sabater <pabloosabaterr@gmail.com>

Date

Sep 30, 2026, 00:21 UTC

Message-ID

<20260930-backfill-dryrun-v1-4-1128f247ee01@gmail.com>

In-Reply-To

<20260930-backfill-dryrun-v1-0-1128f247ee01@gmail.com>

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