[PATCH] Mark `cat-file` sparse-index compatible

["Kevin Lyles via GitGitGadget" <gitgitgadget@gmail.com>]

From: Kevin Lyles <klyles+github@epic.com>

`cat-file` will expand a sparse index to a full index when needed, but
is currently marked as needing a full index (or rather, not marked as
not needing a full index). This results in much slower `cat-file`
operations when working within the sparse index, since we expand the
index whether it's needed or not.

Mark `cat-file` as not needing a full index, so that you only pay the
cost of expanding the sparse index to a full index when you request a
file outside of the sparse index.

Add tests to ensure both that:
- `cat-file` returns the correct file contents whether or not the file
  is in the sparse index
- `cat-file` warns about expanding to the full index any time you
  request something outside of the sparse index

Signed-off-by: Kevin Lyles <klyles+github@epic.com>
---
    Mark cat-file sparse-index compatible
    
    Please note that this is my first contribution to git. I've tried to
    follow the instructions about how to correctly submit a patch (I'm using
    GitGitGadget as getting Outlook to do plain text e-mail correctly seems
    impossible), but please let me know if I've missed something.
    
    I've worded the commit message itself as though I'm definitely correct
    about how cat-file behaves, since I assume we want the final commit
    message to be definite. However, this change felt a little too easy and
    I can't help but feel that I might have missed something. So, even
    though this is just one commit, I'm also including this cover letter
    going into more detail about the parts that don't need to be part of the
    commit history.
    
    My motivation for making this change is purely performance. We have a
    large repository that we enable the sparse index for, and I am
    developing a pre-commit hook that (among other things) uses git cat-file
    to get the staged contents of certain files. Without this change,
    getting the contents of a single small file from the index can take
    upwards of 10 seconds due to the index expansion. After this change, it
    only takes ~0.3 seconds unless the file is outside of the sparse index.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1770%2Fklylesatepic%2Fkl%2Fmark-cat-file-sparse-index-compatible-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1770/klylesatepic/kl/mark-cat-file-sparse-index-compatible-v1
Pull-Request: https://github.com/git/git/pull/1770

 builtin/cat-file.c                       |  3 +
 t/t1092-sparse-checkout-compatibility.sh | 71 ++++++++++++++++++++++--
 2 files changed, 69 insertions(+), 5 deletions(-)

diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index 18fe58d6b8b..1afdfb5cbae 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -1047,6 +1047,9 @@ int cmd_cat_file(int argc, const char **argv, const char *prefix)
 	if (batch.buffer_output < 0)
 		batch.buffer_output = batch.all_objects;
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	/* Return early if we're in batch mode? */
 	if (batch.enabled) {
 		if (opt_cw)
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index a2c0e1b4dcc..0f36246ae84 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -179,22 +179,26 @@ init_repos_as_submodules () {
 }
 
 run_on_sparse () {
+	tee run_on_sparse-checkout run_on_sparse-index &&
+
 	(
 		cd sparse-checkout &&
 		GIT_PROGRESS_DELAY=100000 "$@" >../sparse-checkout-out 2>../sparse-checkout-err
-	) &&
+	) <run_on_sparse-checkout &&
 	(
 		cd sparse-index &&
 		GIT_PROGRESS_DELAY=100000 "$@" >../sparse-index-out 2>../sparse-index-err
-	)
+	) <run_on_sparse-index
 }
 
 run_on_all () {
+	tee run_on_all-full run_on_all-sparse &&
+
 	(
 		cd full-checkout &&
 		GIT_PROGRESS_DELAY=100000 "$@" >../full-checkout-out 2>../full-checkout-err
-	) &&
-	run_on_sparse "$@"
+	) <run_on_all-full &&
+	run_on_sparse "$@" <run_on_all-sparse
 }
 
 test_all_match () {
@@ -221,7 +225,7 @@ test_sparse_unstaged () {
 	done
 }
 
-# Usage: test_sprase_checkout_set "<c1> ... <cN>" "<s1> ... <sM>"
+# Usage: test_sparse_checkout_set "<c1> ... <cN>" "<s1> ... <sM>"
 # Verifies that "git sparse-checkout set <c1> ... <cN>" succeeds and
 # leaves the sparse index in a state where <s1> ... <sM> are sparse
 # directories (and <c1> ... <cN> are not).
@@ -2345,4 +2349,61 @@ test_expect_success 'advice.sparseIndexExpanded' '
 	grep "The sparse index is expanding to a full index" err
 '
 
+test_expect_success 'cat-file -p' '
+	init_repos &&
+	echo "new content" >>full-checkout/deep/a &&
+	echo "new content" >>sparse-checkout/deep/a &&
+	echo "new content" >>sparse-index/deep/a &&
+	run_on_all git add deep/a &&
+
+	test_all_match git cat-file -p HEAD:deep/a &&
+	ensure_not_expanded cat-file -p HEAD:deep/a &&
+	test_all_match git cat-file -p HEAD:folder1/a &&
+	ensure_not_expanded cat-file -p HEAD:folder1/a &&
+
+	test_all_match git cat-file -p :deep/a &&
+	ensure_not_expanded cat-file -p :deep/a &&
+	run_on_all git cat-file -p :folder1/a &&
+	test_cmp full-checkout-out sparse-checkout-out &&
+	test_cmp full-checkout-out sparse-index-out &&
+	test_cmp full-checkout-err sparse-checkout-err &&
+	ensure_expanded cat-file -p :folder1/a'
+
+test_expect_success 'cat-file --batch' '
+	init_repos &&
+	echo "new content" >>full-checkout/deep/a &&
+	echo "new content" >>sparse-checkout/deep/a &&
+	echo "new content" >>sparse-index/deep/a &&
+	run_on_all git add deep/a &&
+
+	cat <<-\EOF | test_all_match git cat-file --batch &&
+	HEAD:deep/a
+	:deep/a
+	EOF
+	cat <<-\EOF | ensure_not_expanded cat-file --batch &&
+	HEAD:deep/a
+	:deep/a
+	EOF
+
+	echo "HEAD:folder1/a" | test_all_match git cat-file --batch &&
+	echo "HEAD:folder1/a" | ensure_not_expanded cat-file --batch &&
+
+	echo ":folder1/a" | run_on_all git cat-file --batch &&
+	test_cmp full-checkout-out sparse-checkout-out &&
+	test_cmp full-checkout-out sparse-index-out &&
+	test_cmp full-checkout-err sparse-checkout-err &&
+	echo ":folder1/a" | ensure_expanded cat-file --batch &&
+
+	cat <<-\EOF | run_on_all git cat-file --batch &&
+	:deep/a
+	:folder1/a
+	EOF
+	test_cmp full-checkout-out sparse-checkout-out &&
+	test_cmp full-checkout-out sparse-index-out &&
+	test_cmp full-checkout-err sparse-checkout-err &&
+	cat <<-\EOF | ensure_expanded cat-file --batch
+	:deep/a
+	:folder1/a
+	EOF'
+
 test_done

base-commit: 80ccd8a2602820fdf896a8e8894305225f86f61d
-- 
gitgitgadget