[PATCH 1/2] diff: use HEAD for attributes when using bare repository

["John Cai via GitGitGadget" <gitgitgadget@gmail.com>]

From: John Cai <johncai86@gmail.com>

(a4cf900e diff: teach diff to read algorithm from diff driver,
2022-02-20) does not support bare repositories. Since running diff
on bare repositories is often done on Git servers, it would be useful to
allow bare repositories to also take advantage of setting the algorithm
via the diff driver.

Teach diff to check against the attributes from HEAD if a bare
repository is being operated on. This change also allows custom diff
drivers to work with bare repositories.

Signed-off-by: John Cai <johncai86@gmail.com>
---
 Documentation/gitattributes.txt |  4 ++++
 diff.c                          | 33 +++++++++++++++++++++++++++++----
 t/lib-diff-alternative.sh       | 10 ++++++++++
 t/t4018-diff-funcname.sh        | 11 +++++++++++
 userdiff.c                      |  9 ++++++++-
 userdiff.h                      |  4 ++++
 6 files changed, 66 insertions(+), 5 deletions(-)

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 39bfbca1ffe..15488bd92b2 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -758,6 +758,8 @@ with the above configuration, i.e. `j-c-diff`, with 7
 parameters, just like `GIT_EXTERNAL_DIFF` program is called.
 See linkgit:git[1] for details.
 
+When using a bare repository, the gitattributes from HEAD will be used.
+
 Setting the internal diff algorithm
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -785,6 +787,8 @@ This diff algorithm applies to user facing diff output like git-diff(1),
 git-show(1) and is used for the `--stat` output as well. The merge machinery
 will not use the diff algorithm set through this method.
 
+When using a bare repository, the gitattributes from HEAD will be used.
+
 NOTE: If `diff.<name>.command` is defined for path with the
 `diff=<name>` attribute, it is executed as an external diff driver
 (see above), and adding `diff.<name>.algorithm` has no effect, as the
diff --git a/diff.c b/diff.c
index 469e18aed20..51baf893bb0 100644
--- a/diff.c
+++ b/diff.c
@@ -29,6 +29,7 @@
 #include "promisor-remote.h"
 #include "dir.h"
 #include "strmap.h"
+#include "tree.h"
 
 #ifdef NO_FAST_WORKING_DIRECTORY
 #define FAST_WORKING_DIRECTORY 0
@@ -4443,6 +4444,27 @@ static void fill_metainfo(struct strbuf *msg,
 	}
 }
 
+static void get_userdiff(struct diff_options *o,
+			     struct userdiff_driver **drv,
+			     const char *attr_path)
+{
+	const char *commit = "HEAD";
+	struct object_id *tree_oid = NULL;
+
+	if (is_bare_repository() && o->repo->gitdir) {
+		struct object_id oid;
+
+		if (!get_oid(commit, &oid)) {
+			struct tree *t = parse_tree_indirect(&oid);
+
+			if (t)
+				tree_oid = &t->object.oid;
+		}
+	}
+
+	*drv = userdiff_find_by_tree_and_path(o->repo->index, tree_oid, attr_path);
+}
+
 static void run_diff_cmd(const char *pgm,
 			 const char *name,
 			 const char *other,
@@ -4458,8 +4480,10 @@ static void run_diff_cmd(const char *pgm,
 	int must_show_header = 0;
 	struct userdiff_driver *drv = NULL;
 
-	if (o->flags.allow_external || !o->ignore_driver_algorithm)
-		drv = userdiff_find_by_path(o->repo->index, attr_path);
+	if (o->flags.allow_external || !o->ignore_driver_algorithm) {
+
+		get_userdiff(o, &drv, attr_path);
+	}
 
 	if (o->flags.allow_external && drv && drv->external)
 		pgm = drv->external;
@@ -4586,8 +4610,9 @@ static void run_diffstat(struct diff_filepair *p, struct diff_options *o,
 	const char *other;
 
 	if (!o->ignore_driver_algorithm) {
-		struct userdiff_driver *drv = userdiff_find_by_path(o->repo->index,
-								    p->one->path);
+		struct userdiff_driver *drv = NULL;
+
+		get_userdiff(o, &drv, p->one->path);
 
 		if (drv && drv->algorithm)
 			set_diff_algorithm(o, drv->algorithm);
diff --git a/t/lib-diff-alternative.sh b/t/lib-diff-alternative.sh
index a8f5d3274a5..0d99af83dd2 100644
--- a/t/lib-diff-alternative.sh
+++ b/t/lib-diff-alternative.sh
@@ -121,6 +121,16 @@ EOF
 		test_cmp expect output
 	'
 
+	test_expect_success "$STRATEGY diff from attributes with bare repo" '
+		echo "file* diff=driver" >.gitattributes &&
+		git add file1 file2 .gitattributes &&
+		git commit -m "adding files" &&
+		git clone --bare --no-local . bare.git &&
+		git -C bare.git config diff.driver.algorithm "$STRATEGY" &&
+		git -C bare.git diff HEAD:file1 HEAD:file2 > output &&
+		test_cmp expect output
+	'
+
 	test_expect_success "$STRATEGY diff from attributes has valid diffstat" '
 		echo "file* diff=driver" >.gitattributes &&
 		git config diff.driver.algorithm "$STRATEGY" &&
diff --git a/t/t4018-diff-funcname.sh b/t/t4018-diff-funcname.sh
index 42a2b9a13b7..451af08c611 100755
--- a/t/t4018-diff-funcname.sh
+++ b/t/t4018-diff-funcname.sh
@@ -63,6 +63,17 @@ do
 		test_i18ngrep ! fatal msg &&
 		test_i18ngrep ! error msg
 	'
+	test_expect_success "builtin $p pattern compiles on bare repo" '
+		test_when_finished "rm -rf bare.git" &&
+		echo "*.java diff=$p" >.gitattributes &&
+		git add . &&
+		git commit -am "adding files" &&
+		git clone --bare --no-local . bare.git &&
+		test_expect_code 1 git -C bare.git diff --exit-code \
+			HEAD:A.java HEAD:B.java 2>msg &&
+		test_i18ngrep ! fatal msg &&
+		test_i18ngrep ! error msg
+	'
 done
 
 test_expect_success 'last regexp must not be negated' '
diff --git a/userdiff.c b/userdiff.c
index 58a3d59ef8f..2305d363244 100644
--- a/userdiff.c
+++ b/userdiff.c
@@ -408,6 +408,13 @@ struct userdiff_driver *userdiff_find_by_name(const char *name)
 
 struct userdiff_driver *userdiff_find_by_path(struct index_state *istate,
 					      const char *path)
+{
+	return userdiff_find_by_tree_and_path(istate, NULL, path);
+}
+
+struct userdiff_driver *userdiff_find_by_tree_and_path(struct index_state *istate,
+						       const struct object_id *tree_oid,
+						       const char *path)
 {
 	static struct attr_check *check;
 
@@ -415,7 +422,7 @@ struct userdiff_driver *userdiff_find_by_path(struct index_state *istate,
 		check = attr_check_initl("diff", NULL);
 	if (!path)
 		return NULL;
-	git_check_attr(istate, NULL, path, check);
+	git_check_attr(istate, tree_oid, path, check);
 
 	if (ATTR_TRUE(check->items[0].value))
 		return &driver_true;
diff --git a/userdiff.h b/userdiff.h
index 24419db6973..f9cd7c238de 100644
--- a/userdiff.h
+++ b/userdiff.h
@@ -33,6 +33,10 @@ int userdiff_config(const char *k, const char *v);
 struct userdiff_driver *userdiff_find_by_name(const char *name);
 struct userdiff_driver *userdiff_find_by_path(struct index_state *istate,
 					      const char *path);
+struct userdiff_driver *userdiff_find_by_tree_and_path(struct index_state *istate,
+						       const struct object_id *tree_oid,
+						       const char *path);
+
 
 /*
  * Initialize any textconv-related fields in the driver and return it, or NULL
-- 
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