attr: read attributes from HEAD when bare repo

The motivation for 44451a2 (attr: teach "--attr-source=<tree>" global
option to "git" , 2023-05-06), was to make it possible to use
gitattributes with bare repositories.

To make it easier to read gitattributes in bare repositories however,
let's just make HEAD:.gitattributes the default. This is in line with
how mailmap works, 8c473ce (mailmap: default mailmap.blob in bare
repositories, 2012-12-13).

Signed-off-by: John Cai <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: john-cai

attr.c

@@ -1173,6 +1173,7 @@ static void collect_some_attrs(struct index_state *istate,
 }
 
 static const char *default_attr_source_tree_object_name;
+static int ignore_bad_attr_tree;
 
 void set_git_attr_source(const char *tree_object_name)
 {
@@ -1184,10 +1185,19 @@ static void compute_default_attr_source(struct object_id *attr_source)
 	if (!default_attr_source_tree_object_name)
 		default_attr_source_tree_object_name = getenv(GIT_ATTR_SOURCE_ENVIRONMENT);
 
+	if (!default_attr_source_tree_object_name &&
+	    startup_info->have_repository &&
+	    is_bare_repository()) {
+		default_attr_source_tree_object_name = "HEAD";
+		ignore_bad_attr_tree = 1;
+	}
+
 	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))
+	if (repo_get_oid_treeish(the_repository,
+				 default_attr_source_tree_object_name,
+				 attr_source) && !ignore_bad_attr_tree)
 		die(_("bad --attr-source or GIT_ATTR_SOURCE"));
 }
 

t/t0003-attributes.sh

@@ -342,6 +342,17 @@ test_expect_success 'bare repository: check that .gitattribute is ignored' '
 	)
 '
 
+
+test_expect_success 'bare repo defaults to reading .gitattributes from HEAD' '
+	test_when_finished rm -rf test bare_with_gitattribute &&
+	git init test &&
+	test_commit -C test gitattributes .gitattributes "f/path test=val" &&
+	git clone --bare test bare_with_gitattribute &&
+	echo "f/path: test: val" >expect &&
+	git -C bare_with_gitattribute check-attr test -- f/path >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'bare repository: with --source' '
 	(
 		cd bare.git &&

t/t5001-archive-attr.sh

@@ -138,7 +138,7 @@ test_expect_success 'git archive with worktree attributes, bare' '
 '
 
 test_expect_missing	bare-worktree/ignored
-test_expect_exists	bare-worktree/ignored-by-tree
+test_expect_missing	bare-worktree/ignored-by-tree
 test_expect_exists	bare-worktree/ignored-by-worktree
 
 test_expect_success 'export-subst' '