real_path: remove unsafe API

Returning a shared buffer invites very subtle bugs due to reentrancy or
multi-threading, as demonstrated by the previous patch.

There was an unfinished effort to abolish this [1].

Let's finally rid of `real_path()`, using `strbuf_realpath()` instead.

This patch uses a local `strbuf` for most places where `real_path()` was
previously called.

However, two places return the value of `real_path()` to the caller. For
them, a `static` local `strbuf` was added, effectively pushing the
problem one level higher:
    read_gitfile_gently()
    get_superproject_working_tree()

[1] https://lore.kernel.org/git/1480964316-99305-1-git-send-email-bmwill@google.com/

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 19 insertions, 6 deletions

diff --git a/builtin/clone.c b/builtin/clone.c
index 1ad26f4..488bdb0 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -420,6 +420,7 @@ static void copy_or_link_directory(struct strbuf *src, struct strbuf *dest,
 	struct dir_iterator *iter;
 	int iter_status;
 	unsigned int flags;
+	struct strbuf realpath = STRBUF_INIT;
 
 	mkdir_if_missing(dest->buf, 0777);
 
@@ -454,7 +455,8 @@ static void copy_or_link_directory(struct strbuf *src, struct strbuf *dest,
 		if (unlink(dest->buf) && errno != ENOENT)
 			die_errno(_("failed to unlink '%s'"), dest->buf);
 		if (!option_no_hardlinks) {
-			if (!link(real_path(src->buf), dest->buf))
+			strbuf_realpath(&realpath, src->buf, 1);
+			if (!link(realpath.buf, dest->buf))
 				continue;
 			if (option_local > 0)
 				die_errno(_("failed to create link '%s'"), dest->buf);
@@ -468,6 +470,8 @@ static void copy_or_link_directory(struct strbuf *src, struct strbuf *dest,
 		strbuf_setlen(src, src_len);
 		die(_("failed to iterate over '%s'"), src->buf);
 	}
+
+	strbuf_release(&realpath);
 }
 
 static void clone_local(const char *src_repo, const char *dest_repo)
diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
index 4a70b33..d1ab662 100644
--- a/builtin/commit-graph.c
+++ b/builtin/commit-graph.c
@@ -39,14 +39,17 @@ static struct object_directory *find_odb(struct repository *r,
 {
 	struct object_directory *odb;
 	char *obj_dir_real = real_pathdup(obj_dir, 1);
+	struct strbuf odb_path_real = STRBUF_INIT;
 
 	prepare_alt_odb(r);
 	for (odb = r->objects->odb; odb; odb = odb->next) {
-		if (!strcmp(obj_dir_real, real_path(odb->path)))
+		strbuf_realpath(&odb_path_real, odb->path, 1);
+		if (!strcmp(obj_dir_real, odb_path_real.buf))
 			break;
 	}
 
 	free(obj_dir_real);
+	strbuf_release(&odb_path_real);
 
 	if (!odb)
 		die(_("could not find object directory matching %s"), obj_dir);
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index 7a00da8..06ca717 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -857,7 +857,10 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 					if (!gitdir && !prefix)
 						gitdir = ".git";
 					if (gitdir) {
-						puts(real_path(gitdir));
+						struct strbuf realpath = STRBUF_INIT;
+						strbuf_realpath(&realpath, gitdir, 1);
+						puts(realpath.buf);
+						strbuf_release(&realpath);
 						continue;
 					}
 				}
diff --git a/builtin/worktree.c b/builtin/worktree.c
index 24f2280..d99db35 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -258,7 +258,7 @@ static int add_worktree(const char *path, const char *refname,
 			const struct add_opts *opts)
 {
 	struct strbuf sb_git = STRBUF_INIT, sb_repo = STRBUF_INIT;
-	struct strbuf sb = STRBUF_INIT;
+	struct strbuf sb = STRBUF_INIT, realpath = STRBUF_INIT;
 	const char *name;
 	struct child_process cp = CHILD_PROCESS_INIT;
 	struct argv_array child_env = ARGV_ARRAY_INIT;
@@ -330,9 +330,11 @@ static int add_worktree(const char *path, const char *refname,
 
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/gitdir", sb_repo.buf);
-	write_file(sb.buf, "%s", real_path(sb_git.buf));
+	strbuf_realpath(&realpath, sb_git.buf, 1);
+	write_file(sb.buf, "%s", realpath.buf);
+	strbuf_realpath(&realpath, get_git_common_dir(), 1);
 	write_file(sb_git.buf, "gitdir: %s/worktrees/%s",
-		   real_path(get_git_common_dir()), name);
+		   realpath.buf, name);
 	/*
 	 * This is to keep resolve_ref() happy. We need a valid HEAD
 	 * or is_git_directory() will reject the directory. Any value which
@@ -418,6 +420,7 @@ done:
 	strbuf_release(&sb_repo);
 	strbuf_release(&sb_git);
 	strbuf_release(&sb_name);
+	strbuf_release(&realpath);
 	return ret;
 }