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>

1 files changed, 9 insertions, 2 deletions

diff --git a/editor.c b/editor.c
index f079abbf11..91989ee8a1 100644
--- a/editor.c
+++ b/editor.c
@@ -54,7 +54,8 @@ static int launch_specified_editor(const char *editor, const char *path,
 		return error("Terminal is dumb, but EDITOR unset");
 
 	if (strcmp(editor, ":")) {
-		const char *args[] = { editor, real_path(path), NULL };
+		struct strbuf realpath = STRBUF_INIT;
+		const char *args[] = { editor, NULL, NULL };
 		struct child_process p = CHILD_PROCESS_INIT;
 		int ret, sig;
 		int print_waiting_for_editor = advice_waiting_for_editor && isatty(2);
@@ -75,16 +76,22 @@ static int launch_specified_editor(const char *editor, const char *path,
 			fflush(stderr);
 		}
 
+		strbuf_realpath(&realpath, path, 1);
+		args[1] = realpath.buf;
+
 		p.argv = args;
 		p.env = env;
 		p.use_shell = 1;
 		p.trace2_child_class = "editor";
-		if (start_command(&p) < 0)
+		if (start_command(&p) < 0) {
+			strbuf_release(&realpath);
 			return error("unable to start editor '%s'", editor);
+		}
 
 		sigchain_push(SIGINT, SIG_IGN);
 		sigchain_push(SIGQUIT, SIG_IGN);
 		ret = finish_command(&p);
+		strbuf_release(&realpath);
 		sig = ret - 128;
 		sigchain_pop(SIGINT);
 		sigchain_pop(SIGQUIT);