rev-parse: fix several options when running in a subdirectory

In addition to making git_path() aware of certain file names that need
to be handled differently e.g. when running in worktrees, the commit
557bd833bb (git_path(): be aware of file relocation in $GIT_DIR,
2014-11-30) also snuck in a new option for `git rev-parse`:
`--git-path`.

On the face of it, there is no obvious bug in that commit's diff: it
faithfully calls git_path() on the argument and prints it out, i.e. `git
rev-parse --git-path <filename>` has the same precise behavior as
calling `git_path("<filename>")` in C.

The problem lies deeper, much deeper. In hindsight (which is always
unfair), implementing the .git/ directory discovery in
`setup_git_directory()` by changing the working directory may have
allowed us to avoid passing around a struct that contains information
about the current repository, but it bought us many, many problems.

In this case, when being called in a subdirectory, `git rev-parse`
changes the working directory to the top-level directory before calling
`git_path()`. In the new working directory, the result is correct. But
in the working directory of the calling script, it is incorrect.

Example: when calling `git rev-parse --git-path HEAD` in, say, the
Documentation/ subdirectory of Git's own source code, the string
`.git/HEAD` is printed.

Side note: that bug is hidden when running in a subdirectory of a
worktree that was added by the `git worktree` command: in that case, the
(correct) absolute path of the `HEAD` file is printed.

In the interest of time, this patch does not go the "correct" route to
introduce a struct with repository information (and removing global
state in the process), instead this patch chooses to detect when the
command was called in a subdirectory and forces the result to be an
absolute path.

While at it, we are also fixing the output of --git-common-dir and
--shared-index-path.

Lastly, please note that we reuse the same strbuf for all of the
relative_path() calls; this avoids frequent allocation (and duplicated
code), and it does not risk memory leaks, for two reasons: 1) the
cmd_rev_parse() function does not return anywhere between the use of
the new strbuf instance and its final release, and 2) git-rev-parse is
one of these "one-shot" programs in Git, i.e. it exits after running
for a very short time, meaning that all allocated memory is released
with the exit() call anyway.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rev-parse.c

@@ -545,6 +545,7 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 	unsigned int flags = 0;
 	const char *name = NULL;
 	struct object_context unused;
+	struct strbuf buf = STRBUF_INIT;
 
 	if (argc > 1 && !strcmp("--parseopt", argv[1]))
 		return cmd_parseopt(argc - 1, argv + 1, prefix);
@@ -599,7 +600,9 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 		if (!strcmp(arg, "--git-path")) {
 			if (!argv[i + 1])
 				die("--git-path requires an argument");
-			puts(git_path("%s", argv[i + 1]));
+			strbuf_reset(&buf);
+			puts(relative_path(git_path("%s", argv[i + 1]),
+					   prefix, &buf));
 			i++;
 			continue;
 		}
@@ -821,8 +824,9 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				continue;
 			}
 			if (!strcmp(arg, "--git-common-dir")) {
-				const char *pfx = prefix ? prefix : "";
+				strbuf_reset(&buf);
-				puts(prefix_filename(pfx, strlen(pfx), get_git_common_dir()));
+				puts(relative_path(get_git_common_dir(),
+						   prefix, &buf));
 				continue;
 			}
 			if (!strcmp(arg, "--is-inside-git-dir")) {
@@ -845,7 +849,9 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 					die(_("Could not read the index"));
 				if (the_index.split_index) {
 					const unsigned char *sha1 = the_index.split_index->base_sha1;
-					puts(git_path("sharedindex.%s", sha1_to_hex(sha1)));
+					const char *path = git_path("sharedindex.%s", sha1_to_hex(sha1));
+					strbuf_reset(&buf);
+					puts(relative_path(path, prefix, &buf));
 				}
 				continue;
 			}
@@ -897,6 +903,7 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 			continue;
 		verify_filename(prefix, arg, 1);
 	}
+	strbuf_release(&buf);
 	if (verify) {
 		if (revs_count == 1) {
 			show_rev(type, sha1, name);

t/t1500-rev-parse.sh

@@ -93,7 +93,7 @@ test_expect_success 'git-common-dir from worktree root' '
 	test_cmp expect actual
 '
 
-test_expect_failure 'git-common-dir inside sub-dir' '
+test_expect_success 'git-common-dir inside sub-dir' '
 	mkdir -p path/to/child &&
 	test_when_finished "rm -rf path" &&
 	echo "$(git -C path/to/child rev-parse --show-cdup).git" >expect &&
@@ -107,7 +107,7 @@ test_expect_success 'git-path from worktree root' '
 	test_cmp expect actual
 '
 
-test_expect_failure 'git-path inside sub-dir' '
+test_expect_success 'git-path inside sub-dir' '
 	mkdir -p path/to/child &&
 	test_when_finished "rm -rf path" &&
 	echo "$(git -C path/to/child rev-parse --show-cdup).git/objects" >expect &&

t/t1700-split-index.sh

@@ -200,7 +200,7 @@ EOF
 	test_cmp expect actual
 '
 
-test_expect_failure 'rev-parse --shared-index-path' '
+test_expect_success 'rev-parse --shared-index-path' '
 	test_create_repo split-index &&
 	(
 		cd split-index &&

t/t2027-worktree-list.sh

@@ -8,7 +8,7 @@ test_expect_success 'setup' '
 	test_commit init
 '
 
-test_expect_failure 'rev-parse --git-common-dir on main worktree' '
+test_expect_success 'rev-parse --git-common-dir on main worktree' '
 	git rev-parse --git-common-dir >actual &&
 	echo .git >expected &&
 	test_cmp expected actual &&
@@ -18,7 +18,7 @@ test_expect_failure 'rev-parse --git-common-dir on main worktree' '
 	test_cmp expected2 actual2
 '
 
-test_expect_failure 'rev-parse --git-path objects linked worktree' '
+test_expect_success 'rev-parse --git-path objects linked worktree' '
 	echo "$(git rev-parse --show-toplevel)/.git/objects" >expect &&
 	test_when_finished "rm -rf linked-tree && git worktree prune" &&
 	git worktree add --detach linked-tree master &&