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

> 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;

>  	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