[RFC PATCH] upload_pack.c: make deepen-not more tree-ish

[Andrew Wansink <andy@halogix.com>]

This unlocks `git clone --shallow-exclude=<commit-sha1>`

git-clone only accepts --shallow-excude arguments where
the argument is a branch or tag because upload_pack only
searches deepen-not arguments for branches and tags.

Make process_deepen_not search for commit objects if no
branch or tag is found then add them to the deepen_not
list.

Signed-off-by: Andrew Wansink <wansink@uber.com>
---

At Uber we have a lot of patches in CI simultaneously,
the CI jobs will frequently clone the monorepo multiple
times for each patch.  They do this to calculate diffs
between a patch and its parent commit.

One optimisation in this flow is to clone only to a specific
depth, this may or may not work, depending on how old the 
patch is.  In this case we have to --unshallow or discard
the shallow clone and fully clone the repo.

This patch would allow us to clone to exactly the depth we
need to find a patch's parent commit.

 t/t5500-fetch-pack.sh | 30 ++++++++++++++++++++++++++++++
 upload-pack.c         | 35 +++++++++++++++++++++++++++++++----
 2 files changed, 61 insertions(+), 4 deletions(-)

diff --git a/t/t5500-fetch-pack.sh b/t/t5500-fetch-pack.sh
index d18f2823d86..8d5045cc1b9 100755
--- a/t/t5500-fetch-pack.sh
+++ b/t/t5500-fetch-pack.sh
@@ -899,6 +899,36 @@ test_expect_success 'shallow clone exclude tag two' '
 	)
 '
 
+test_expect_success 'shallow clone exclude commit' '
+	test_create_repo shallow-exclude-commit &&
+	(
+	cd shallow-exclude-commit &&
+	test_commit one &&
+	test_commit two &&
+	test_commit three &&
+	commit_two_sha1=$(git log -n 1 --pretty=tformat:%h HEAD^) &&
+	git clone --shallow-exclude=${commit_two_sha1} "file://$(pwd)/." ../shallow3-by-commit &&
+	git -C ../shallow3-by-commit log --pretty=tformat:%s HEAD >actual &&
+	git log -n 1 --pretty=tformat:%s HEAD >expected &&
+	test_cmp expected actual
+	)
+'
+
+test_expect_success 'shallow clone exclude commit^' '
+	test_create_repo shallow-exclude-commit-carat &&
+	(
+	cd shallow-exclude-commit-carat &&
+	test_commit one &&
+	test_commit two &&
+	test_commit three &&
+	commit_two_sha1=$(git log -n 1 --pretty=tformat:%h HEAD^) &&
+	git clone --shallow-exclude=${commit_two_sha1}^ "file://$(pwd)/." ../shallow23-by-commit &&
+	git -C ../shallow23-by-commit log --pretty=tformat:%s HEAD >actual &&
+	git log -n 2 --pretty=tformat:%s HEAD >expected &&
+	test_cmp expected actual
+	)
+'
+
 test_expect_success 'fetch exclude tag one' '
 	git -C shallow12 fetch --shallow-exclude one origin &&
 	git -C shallow12 log --pretty=tformat:%s origin/main >actual &&
diff --git a/upload-pack.c b/upload-pack.c
index 551f22ffa5d..0c8594f4744 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -985,10 +985,37 @@ static int process_deepen_not(const char *line, struct string_list *deepen_not,
 	if (skip_prefix(line, "deepen-not ", &arg)) {
 		char *ref = NULL;
 		struct object_id oid;
-		if (expand_ref(the_repository, arg, strlen(arg), &oid, &ref) != 1)
-			die("git upload-pack: ambiguous deepen-not: %s", line);
-		string_list_append(deepen_not, ref);
-		free(ref);
+
+		switch (expand_ref(the_repository, arg, strlen(arg), &oid, &ref)) {
+		case 1:
+			// tag or branch matching arg found
+			string_list_append(deepen_not, ref);
+			free(ref);
+			break;
+		case 0: {
+			// no tags or branches matching arg
+			struct object *obj = NULL;
+			struct commit *commit = NULL;
+
+			if (get_oid(arg, &oid))
+				die("git upload-pack: deepen-not: no ref or object %s", arg);
+
+			obj = parse_object(the_repository, &oid);
+			if (!obj)
+				die("git upload-pack: deepen-not: object could not be parsed: %s", arg);
+
+			commit = (struct commit *)peel_to_type(arg, 0, obj, OBJ_COMMIT);
+			if (!commit)
+				die("git upload-pack: deepen-not: object not a commit: %s", arg);
+
+			string_list_append(deepen_not, oid_to_hex(&commit->object.oid));
+			break;
+		}
+		default:
+			// more than 1 tag or branch matches arg
+			die("git upload-pack: ambiguous deepen-not: %s", arg);
+		}
+
 		*deepen_rev_list = 1;
 		return 1;
 	}
-- 
2.39.1