Subject: [PATCH 2/2] diff: add --attr-source to read gitattributes from a commit

From: "John Cai via GitGitGadget" <[email-masked]>
Date: Tue, 14 Mar 2023 01:53:38 +0000

From: John Cai <[email-masked]>

The previous commit allows the diff machinery to read gitattributes from
HEAD when a bare repository is being used.

Some users may want to specify which commit to read gitattributes from.
Teach diff a new --attr-source that can be used to pass in a commit.

Signed-off-by: John Cai <[email-masked]>
---
 Documentation/diff-options.txt  |  4 ++++
 Documentation/gitattributes.txt |  8 ++++++--
 diff.c                          | 16 ++++++++++-----
 diff.h                          |  1 +
 t/lib-diff-alternative.sh       | 35 ++++++++++++++++++++++-----------
 t/t4018-diff-funcname.sh        | 18 +++++++++++++++++
 6 files changed, 64 insertions(+), 18 deletions(-)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 7d73e976d99..d3a04937dde 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -173,6 +173,10 @@ endif::git-log[]
 
 --anchored=<text>::
 	Generate a diff using the "anchored diff" algorithm.
+
+--attr-source=<commit>::
+	Specify a commit to read gitattributes from when using a bare
+	repository when defining external drivers, or internal algorithms.
 +
 This option may be specified more than once.
 +
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 15488bd92b2..7f9451b56fa 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -758,7 +758,9 @@ 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.
+When using a bare repository, the gitattributes from HEAD will be used, unless
+the --attr-source option is used to specify a commit from which the
+gitattributes will be read.
 
 Setting the internal diff algorithm
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -787,7 +789,9 @@ 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.
+When using a bare repository, the gitattributes from HEAD will be used, unless
+the --attr-source option is used to specify a commit from which the
+gitattributes will be read.
 
 NOTE: If `diff.<name>.command` is defined for path with the
 `diff=<name>` attribute, it is executed as an external diff driver
diff --git a/diff.c b/diff.c
index 51baf893bb0..e86cf33b7a9 100644
--- a/diff.c
+++ b/diff.c
@@ -4448,10 +4448,13 @@ static void get_userdiff(struct diff_options *o,
 			     struct userdiff_driver **drv,
 			     const char *attr_path)
 {
-	const char *commit = "HEAD";
+	const char *commit = o->attr_source;
 	struct object_id *tree_oid = NULL;
 
-	if (is_bare_repository() && o->repo->gitdir) {
+	if (!commit)
+		commit = "HEAD";
+
+	if ((o->attr_source || is_bare_repository()) && o->repo->gitdir) {
 		struct object_id oid;
 
 		if (!get_oid(commit, &oid)) {
@@ -4459,6 +4462,8 @@ static void get_userdiff(struct diff_options *o,
 
 			if (t)
 				tree_oid = &t->object.oid;
+		} else if (o->attr_source) {
+			die(_("%s is not a valid object"), commit);
 		}
 	}
 
@@ -4480,10 +4485,8 @@ 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) {
-
+	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;
@@ -5699,6 +5702,9 @@ struct option *add_diff_options(const struct option *opts,
 			 N_("disable all output of the program")),
 		OPT_BOOL(0, "ext-diff", &options->flags.allow_external,
 			 N_("allow an external diff helper to be executed")),
+		OPT_STRING(0, "attr-source", &options->attr_source,
+			 N_("attributes-source"),
+			 N_("the commit to read attributes from")),
 		OPT_CALLBACK_F(0, "textconv", options, NULL,
 			       N_("run external text conversion filters when comparing binary files"),
 			       PARSE_OPT_NOARG, diff_opt_textconv),
diff --git a/diff.h b/diff.h
index 8d770b1d579..59db40f66be 100644
--- a/diff.h
+++ b/diff.h
@@ -334,6 +334,7 @@ struct diff_options {
 	const char *stat_sep;
 	int xdl_opts;
 	int ignore_driver_algorithm;
+	const char *attr_source;
 
 	/* see Documentation/diff-options.txt */
 	char **anchors;
diff --git a/t/lib-diff-alternative.sh b/t/lib-diff-alternative.sh
index 0d99af83dd2..a7cd4966749 100644
--- a/t/lib-diff-alternative.sh
+++ b/t/lib-diff-alternative.sh
@@ -112,25 +112,38 @@ EOF
 
 	STRATEGY=$1
 
-	test_expect_success "$STRATEGY diff from attributes" '
+	test_expect_success "setup attributes files for tests with $STRATEGY" '
+		git checkout -b master &&
 		echo "file* diff=driver" >.gitattributes &&
-		git config diff.driver.algorithm "$STRATEGY" &&
-		test_must_fail git diff --no-index file1 file2 > output &&
-		cat expect &&
-		cat output &&
+		git add file1 file2 .gitattributes &&
+		git commit -m "adding files" &&
+		git checkout -b branchA &&
+		echo "file* diff=driverA" >.gitattributes &&
+		git add .gitattributes &&
+		git commit -m "adding driverA as diff driver" &&
+		git checkout master &&
+		git clone --bare --no-local . bare.git
+	'
+
+	test_expect_success "$STRATEGY diff from attributes" '
+		test_must_fail git -c diff.driver.algorithm=$STRATEGY diff --no-index file1 file2 > output &&
 		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 &&
+		git -C bare.git -c diff.driver.algorithm=$STRATEGY diff HEAD:file1 HEAD:file2 >output &&
+		test_cmp expect output
+	'
+
+	test_expect_success "diff from attributes with bare repo when branch different than HEAD" '
+		git -C bare.git -c diff.driver.algorithm=myers -c diff.driverA.algorithm=$STRATEGY diff --attr-source branchA HEAD:file1 HEAD:file2 >output &&
 		test_cmp expect output
 	'
 
+	test_expect_success "diff from attributes with bare repo with invalid source" '
+		test_must_fail git -C bare.git diff --attr-source invalid-branch HEAD:file1 HEAD:file2
+	'
+
 	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 451af08c611..30babcfae91 100755
--- a/t/t4018-diff-funcname.sh
+++ b/t/t4018-diff-funcname.sh
@@ -63,6 +63,7 @@ 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 &&
@@ -74,6 +75,23 @@ do
 		test_i18ngrep ! fatal msg &&
 		test_i18ngrep ! error msg
 	'
+	test_expect_success "builtin $p pattern compiles on bare repo with --attr-source" '
+		test_when_finished "rm -rf bare.git" &&
+		git checkout -B master &&
+		echo "*.java diff=notexist" > .gitattributes &&
+		git add .gitattributes &&
+		git commit -am "changing gitattributes" &&
+		git checkout -B branchA &&
+		echo "*.java diff=$p" >.gitattributes &&
+		git add .gitattributes &&
+		git commit -am "changing gitattributes" &&
+		git clone --bare --no-local . bare.git &&
+		git -C bare.git symbolic-ref HEAD refs/heads/master &&
+		test_expect_code 1 git -C bare.git diff --exit-code \
+			--attr-source branchA 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' '
-- 
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