attr: add attr.tree for setting the treeish to read attributes from

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. In the context of serving Git repositories as bare repos like we
do at GitLab however, it would be easier to point --attr-source to HEAD
for all commands by setting it once.

Add a new config attr.tree that allows this.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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 +1,7 @@
+attr.tree::
+	A reference to a tree in the repository from which to read attributes,
+	instead of the `.gitattributes` file in the working tree. In a bare
+	repository, this defaults to `HEAD:.gitattributes`. If the value does
+	not resolve to a valid tree object, an empty tree is used instead.
+	When the `GIT_ATTR_SOURCE` environment variable or `--attr-source`
+	command line option are used, this configuration variable has no effect.

attr.c

@@ -24,6 +24,8 @@
 #include "tree-walk.h"
 #include "object-name.h"
 
+const char *git_attr_tree;
+
 const char git_attr__true[] = "(builtin)true";
 const char git_attr__false[] = "\0(builtin)false";
 static const char git_attr__unknown[] = "(builtin)unknown";
@@ -1185,6 +1187,11 @@ 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 && git_attr_tree) {
+		default_attr_source_tree_object_name = git_attr_tree;
+		ignore_bad_attr_tree = 1;
+	}
+
 	if (!default_attr_source_tree_object_name &&
 	    startup_info->have_repository &&
 	    is_bare_repository()) {

attr.h

@@ -236,4 +236,6 @@ const char *git_attr_global_file(void);
 /* Return whether the system gitattributes file is enabled and should be used. */
 int git_attr_system_is_enabled(void);
 
+extern const char *git_attr_tree;
+
 #endif /* ATTR_H */

config.c

@@ -18,6 +18,7 @@
 #include "repository.h"
 #include "lockfile.h"
 #include "mailmap.h"
+#include "attr.h"
 #include "exec-cmd.h"
 #include "strbuf.h"
 #include "quote.h"
@@ -1904,6 +1905,18 @@ static int git_default_mailmap_config(const char *var, const char *value)
 	return 0;
 }
 
+static int git_default_attr_config(const char *var, const char *value)
+{
+	if (!strcmp(var, "attr.tree"))
+		return git_config_string(&git_attr_tree, var, value);
+
+	/*
+	 * Add other attribute related config variables here and to
+	 * Documentation/config/attr.txt.
+	 */
+	return 0;
+}
+
 int git_default_config(const char *var, const char *value,
 		       const struct config_context *ctx, void *cb)
 {
@@ -1927,6 +1940,9 @@ int git_default_config(const char *var, const char *value,
 	if (starts_with(var, "mailmap."))
 		return git_default_mailmap_config(var, value);
 
+	if (starts_with(var, "attr."))
+		return git_default_attr_config(var, value);
+
 	if (starts_with(var, "advice.") || starts_with(var, "color.advice"))
 		return git_default_advice_config(var, value);
 

t/t0003-attributes.sh

@@ -40,6 +40,10 @@ attr_check_source () {
 	test_cmp expect actual &&
 	test_must_be_empty err
 
+	git $git_opts -c "attr.tree=$source" check-attr test -- "$path" >actual 2>err &&
+	test_cmp expect actual &&
+	test_must_be_empty err
+
 	GIT_ATTR_SOURCE="$source" git $git_opts check-attr test -- "$path" >actual 2>err &&
 	test_cmp expect actual &&
 	test_must_be_empty err
@@ -342,6 +346,43 @@ 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-source is bad' '
+	test_when_finished rm -rf empty &&
+	git init empty &&
+	(
+		cd empty &&
+		echo "$bad_attr_source_err" >expect_err &&
+		test_must_fail git --attr-source=HEAD check-attr test -- f/path 2>err &&
+		test_cmp expect_err err
+	)
+'
+
+test_expect_success 'attr.tree when HEAD is unborn' '
+	test_when_finished rm -rf empty &&
+	git init empty &&
+	(
+		cd empty &&
+		echo "f/path: test: unspecified" >expect &&
+		git -c attr.tree=HEAD 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 &&
+	(
+		cd empty &&
+		echo "f/path test=val" >.gitattributes &&
+		echo "f/path: test: val" >expect &&
+		git -c attr.tree=HEAD check-attr test -- f/path >actual 2>err &&
+		test_must_be_empty err &&
+		test_cmp expect actual
+	)
+'
 
 test_expect_success 'bare repo defaults to reading .gitattributes from HEAD' '
 	test_when_finished rm -rf test bare_with_gitattribute &&
@@ -353,6 +394,26 @@ test_expect_success 'bare repo defaults to reading .gitattributes from HEAD' '
 	test_cmp expect actual
 '
 
+test_expect_success 'precedence of --attr-source, GIT_ATTR_SOURCE, then attr.tree' '
+	test_when_finished rm -rf empty &&
+	git init empty &&
+	(
+		cd empty &&
+		git checkout -b attr-source &&
+		test_commit "val1" .gitattributes "f/path test=val1" &&
+		git checkout -b attr-tree &&
+		test_commit "val2" .gitattributes "f/path test=val2" &&
+		git checkout attr-source &&
+		echo "f/path: test: val1" >expect &&
+		GIT_ATTR_SOURCE=attr-source git -c attr.tree=attr-tree --attr-source=attr-source \
+		check-attr test -- f/path >actual &&
+		test_cmp expect actual &&
+		GIT_ATTR_SOURCE=attr-source git -c attr.tree=attr-tree \
+		check-attr test -- f/path >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'bare repository: with --source' '
 	(
 		cd bare.git &&