[PATCH 06/30] 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             |  4 ++++
 t/t5620-backfill.sh            | 13 ++++++++++++-
 3 files changed, 18 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 de75471cf44..82a18e58a41 100644
--- a/builtin/backfill.c
+++ b/builtin/backfill.c
@@ -5,6 +5,7 @@
 #include "repository.h"
 #include "commit.h"
 #include "dir.h"
+#include "environment.h"
 #include "hex.h"
 #include "tree.h"
 #include "tree-walk.h"
@@ -133,5 +134,8 @@ int cmd_backfill(int argc, const char **argv, const char *prefix)
 
 	git_config(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 c7bb27b72c1..1fa2e90f8cf 100755
--- a/t/t5620-backfill.sh
+++ b/t/t5620-backfill.sh
@@ -80,6 +80,12 @@ 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 		\
@@ -108,7 +114,12 @@ 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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