attr: add flag `--source` to work with tree-ish

The contents of the .gitattributes files may evolve over time, but "git
check-attr" always checks attributes against them in the working tree
and/or in the index. It may be beneficial to optionally allow the users
to check attributes taken from a commit other than HEAD against paths.

Add a new flag `--source` which will allow users to check the
attributes against a commit (actually any tree-ish would do). When the
user uses this flag, we go through the stack of .gitattributes files but
instead of checking the current working tree and/or in the index, we
check the blobs from the provided tree-ish object. This allows the
command to also be used in bare repositories.

Since we use a tree-ish object, the user can pass "--source
HEAD:subdirectory" and all the attributes will be looked up as if
subdirectory was the root directory of the repository.

We cannot simply use the `<rev>:<path>` syntax without the `--source`
flag, similar to how it is used in `git show` because any non-flag
parameter before `--` is treated as an attribute and any parameter after
`--` is treated as a pathname.

The change involves creating a new function `read_attr_from_blob`, which
given the path reads the blob for the path against the provided source and
parses the attributes line by line. This function is plugged into
`read_attr()` function wherein we go through the stack of attributes
files.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Toon Claes <toon@iotcl.com>
Co-authored-by: toon@iotcl.com
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 23 insertions, 14 deletions

diff --git a/builtin/check-attr.c b/builtin/check-attr.c
index 0fef10e..d7a40e6 100644
--- a/builtin/check-attr.c
+++ b/builtin/check-attr.c
@@ -9,9 +9,10 @@
 static int all_attrs;
 static int cached_attrs;
 static int stdin_paths;
+static char *source;
 static const char * const check_attr_usage[] = {
-N_("git check-attr [-a | --all | <attr>...] [--] <pathname>..."),
-N_("git check-attr --stdin [-z] [-a | --all | <attr>...]"),
+N_("git check-attr [--source <tree-ish>] [-a | --all | <attr>...] [--] <pathname>..."),
+N_("git check-attr --stdin [-z] [--source <tree-ish>] [-a | --all | <attr>...]"),
 NULL
 };
 
@@ -23,6 +24,7 @@ static const struct option check_attr_options[] = {
 	OPT_BOOL(0 , "stdin", &stdin_paths, N_("read file names from stdin")),
 	OPT_BOOL('z', NULL, &nul_term_line,
 		 N_("terminate input and output records by a NUL character")),
+	OPT_STRING(0, "source", &source, N_("<tree-ish>"), N_("which tree-ish to check attributes at")),
 	OPT_END()
 };
 
@@ -55,27 +57,26 @@ static void output_attr(struct attr_check *check, const char *file)
 	}
 }
 
-static void check_attr(const char *prefix,
-		       struct attr_check *check,
-		       int collect_all,
+static void check_attr(const char *prefix, struct attr_check *check,
+		       const struct object_id *tree_oid, int collect_all,
 		       const char *file)
+
 {
 	char *full_path =
 		prefix_path(prefix, prefix ? strlen(prefix) : 0, file);
 
 	if (collect_all) {
-		git_all_attrs(&the_index, full_path, check);
+		git_all_attrs(&the_index, tree_oid, full_path, check);
 	} else {
-		git_check_attr(&the_index, full_path, check);
+		git_check_attr(&the_index, tree_oid, full_path, check);
 	}
 	output_attr(check, file);
 
 	free(full_path);
 }
 
-static void check_attr_stdin_paths(const char *prefix,
-				   struct attr_check *check,
-				   int collect_all)
+static void check_attr_stdin_paths(const char *prefix, struct attr_check *check,
+				   const struct object_id *tree_oid, int collect_all)
 {
 	struct strbuf buf = STRBUF_INIT;
 	struct strbuf unquoted = STRBUF_INIT;
@@ -89,7 +90,7 @@ static void check_attr_stdin_paths(const char *prefix,
 				die("line is badly quoted");
 			strbuf_swap(&buf, &unquoted);
 		}
-		check_attr(prefix, check, collect_all, buf.buf);
+		check_attr(prefix, check, tree_oid, collect_all, buf.buf);
 		maybe_flush_or_die(stdout, "attribute to stdout");
 	}
 	strbuf_release(&buf);
@@ -105,6 +106,8 @@ static NORETURN void error_with_usage(const char *msg)
 int cmd_check_attr(int argc, const char **argv, const char *prefix)
 {
 	struct attr_check *check;
+	struct object_id *tree_oid = NULL;
+	struct object_id initialized_oid;
 	int cnt, i, doubledash, filei;
 
 	if (!is_bare_repository())
@@ -176,11 +179,17 @@ int cmd_check_attr(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	if (source) {
+		if (repo_get_oid_tree(the_repository, source, &initialized_oid))
+			die("%s: not a valid tree-ish source", source);
+		tree_oid = &initialized_oid;
+	}
+
 	if (stdin_paths)
-		check_attr_stdin_paths(prefix, check, all_attrs);
+		check_attr_stdin_paths(prefix, check, tree_oid, all_attrs);
 	else {
 		for (i = filei; i < argc; i++)
-			check_attr(prefix, check, all_attrs, argv[i]);
+			check_attr(prefix, check, tree_oid, all_attrs, argv[i]);
 		maybe_flush_or_die(stdout, "attribute to stdout");
 	}
 
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 2193f80..cabace4 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -1318,7 +1318,7 @@ static int no_try_delta(const char *path)
 
 	if (!check)
 		check = attr_check_initl("delta", NULL);
-	git_check_attr(the_repository->index, path, check);
+	git_check_attr(the_repository->index, NULL, path, check);
 	if (ATTR_FALSE(check->items[0].value))
 		return 1;
 	return 0;