[PATCH 5/5] backfill: assume --sparse when sparse-checkout is enabled

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <derrickstolee@github.com>

The previous change introduced the '--[no-]sparse' option for the 'git
backfill' command, but did not assume it as enabled by default. However,
this is likely the behavior that users will most often want to happen.
Without this default, users with a small sparse-checkout may be confused
when 'git backfill' downloads every version of every object in the full
history.

However, this is left as a separate change so this decision can be reviewed
independently of the value of the '--[no-]sparse' option.

Add a test of adding the '--sparse' option to a repo without sparse-checkout
to make it clear that supplying it without a sparse-checkout is an error.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
 Documentation/git-backfill.txt |  3 ++-
 builtin/backfill.c             |  7 +++++++
 t/t5620-backfill.sh            | 13 ++++++++++++-
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-backfill.txt b/Documentation/git-backfill.txt
index ecf2ac428ce..066ec6b161a 100644
--- a/Documentation/git-backfill.txt
+++ b/Documentation/git-backfill.txt
@@ -48,7 +48,8 @@ OPTIONS
 
 --[no-]sparse::
 	Only download objects if they appear at a path that matches the
-	current sparse-checkout.
+	current sparse-checkout. If the sparse-checkout feature is enabled,
+	then `--sparse` is assumed and can be disabled with `--no-sparse`.
 
 SEE ALSO
 --------
diff --git a/builtin/backfill.c b/builtin/backfill.c
index cdee87e38af..225764f17e8 100644
--- a/builtin/backfill.c
+++ b/builtin/backfill.c
@@ -1,3 +1,6 @@
+/* We need this macro to access core_apply_sparse_checkout */
+#define USE_THE_REPOSITORY_VARIABLE
+
 #include "builtin.h"
 #include "git-compat-util.h"
 #include "config.h"
@@ -5,6 +8,7 @@
 #include "repository.h"
 #include "commit.h"
 #include "dir.h"
+#include "environment.h"
 #include "hex.h"
 #include "tree.h"
 #include "tree-walk.h"
@@ -136,5 +140,8 @@ int cmd_backfill(int argc, const char **argv, const char *prefix, struct reposit
 
 	repo_config(repo, git_default_config, NULL);
 
+	if (ctx.sparse < 0)
+		ctx.sparse = core_apply_sparse_checkout;
+
 	return do_backfill(&ctx);
 }
diff --git a/t/t5620-backfill.sh b/t/t5620-backfill.sh
index c2acd1339bd..eecf03d5199 100755
--- a/t/t5620-backfill.sh
+++ b/t/t5620-backfill.sh
@@ -77,6 +77,12 @@ test_expect_success 'do partial clone 2, backfill batch size' '
 	test_line_count = 0 revs2
 '
 
+test_expect_success 'backfill --sparse without sparse-checkout fails' '
+	git init not-sparse &&
+	test_must_fail git -C not-sparse backfill --sparse 2>err &&
+	grep "problem loading sparse-checkout" err
+'
+
 test_expect_success 'backfill --sparse' '
 	git clone --sparse --filter=blob:none		\
 		--single-branch --branch=main 		\
@@ -105,7 +111,12 @@ test_expect_success 'backfill --sparse' '
 	test_trace2_data promisor fetch_count 8 <sparse-trace2 &&
 	test_trace2_data path-walk paths 15 <sparse-trace2 &&
 	git -C backfill3 rev-list --quiet --objects --missing=print HEAD >missing &&
-	test_line_count = 24 missing
+	test_line_count = 24 missing &&
+
+	# Disabling the --sparse option (on by default) will download everything
+	git -C backfill3 backfill --no-sparse &&
+	git -C backfill3 rev-list --quiet --objects --missing=print HEAD >missing &&
+	test_line_count = 0 missing
 '
 
 test_expect_success 'backfill --sparse without cone mode' '
-- 
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