attr: attr.allowInvalidSource config to allow invalid revision

44451a2 (attr: teach "--attr-source=<tree>" global option to "git",
2023-05-06) provided the ability to pass in a treeish as the attr
source. When a revision does not resolve to a valid tree is passed, Git
will die. GitLab keeps bare repositories and always reads attributes
from the default branch, so we pass in HEAD to --attr-source.

With empty repositories however, HEAD does not point to a valid treeish,
causing Git to die. This means we would need to check for a valid
treeish each time. To avoid this, let's add a configuration that allows
Git to simply ignore --attr-source if it does not resolve to a valid
tree.

Documentation/config.txt

@@ -371,6 +371,8 @@ other popular tools, and describe them in your documentation.
 
 include::config/advice.txt[]
 
+include::config/attr.txt[]
+
 include::config/core.txt[]
 
 include::config/add.txt[]

Documentation/config/attr.txt

@@ -0,0 +1,6 @@
+attr.allowInvalidSource::
+	If `--attr-source` cannot resolve to a valid tree object, ignore
+	`--attr-source` instead of erroring out, and fall back to looking for
+	attributes in the default locations. Useful when passing `HEAD` into
+	`attr-source` since it allows `HEAD` to point to an unborn branch in
+	cases like an empty repository.

attr.c

@@ -1208,8 +1208,15 @@ static void compute_default_attr_source(struct object_id *attr_source)
 	if (!default_attr_source_tree_object_name || !is_null_oid(attr_source))
 		return;
 
-	if (repo_get_oid_treeish(the_repository, default_attr_source_tree_object_name, attr_source))
-		die(_("bad --attr-source or GIT_ATTR_SOURCE"));
+
+	if (repo_get_oid_treeish(the_repository, default_attr_source_tree_object_name, attr_source)) {
+		int allow_invalid_attr_source = 0;
+
+		git_config_get_bool("attr.allowinvalidsource", &allow_invalid_attr_source);
+
+		if (!allow_invalid_attr_source)
+			die(_("bad --attr-source or GIT_ATTR_SOURCE"));
+	}
 }
 
 static struct object_id *default_attr_source(void)

t/t0003-attributes.sh

@@ -342,6 +342,42 @@ test_expect_success 'bare repository: check that .gitattribute is ignored' '
 	)
 '
 
+bad_attr_source_err="fatal: bad --attr-source or GIT_ATTR_SOURCE"
+
+test_expect_success 'attr.allowInvalidSource when HEAD is unborn' '
+	test_when_finished rm -rf empty &&
+	echo $bad_attr_source_err >expect_err &&
+	echo "f/path: test: unspecified" >expect &&
+	git init empty &&
+	test_must_fail git -C empty --attr-source=HEAD check-attr test -- f/path 2>err &&
+	test_cmp expect_err err &&
+	git -C empty -c attr.allowInvalidSource=true --attr-source=HEAD check-attr test -- f/path >actual 2>err &&
+	test_must_be_empty err &&
+	test_cmp expect actual
+'
+
+test_expect_success 'attr.allowInvalidSource when --attr-source points to non-existing ref' '
+	test_when_finished rm -rf empty &&
+	echo $bad_attr_source_err >expect_err &&
+	echo "f/path: test: unspecified" >expect &&
+	git init empty &&
+	test_must_fail git -C empty --attr-source=refs/does/not/exist check-attr test -- f/path 2>err &&
+	test_cmp expect_err err &&
+	git -C empty -c attr.allowInvalidSource=true --attr-source=refs/does/not/exist check-attr test -- f/path >actual 2>err &&
+	test_must_be_empty err &&
+	test_cmp expect actual
+'
+
+test_expect_success 'bad attr source defaults to reading .gitattributes file' '
+	test_when_finished rm -rf empty &&
+	git init empty &&
+	echo "f/path test=val" >empty/.gitattributes &&
+	echo "f/path: test: val" >expect &&
+	git -C empty -c attr.allowInvalidSource=true --attr-source=HEAD check-attr test -- f/path >actual 2>err &&
+	test_must_be_empty err &&
+	test_cmp expect actual
+'
+
 test_expect_success 'bare repository: with --source' '
 	(
 		cd bare.git &&