tr2: leave the parent list empty upon failure & don't leak memory

In a subsequent commit I'll be replacing most of this code to log N
parents, but let's first fix bugs introduced in the recent
2f732bf15e (tr2: log parent process name, 2021-07-21).

It was using the strbuf_read_file() in the wrong way, its return value
is either a length or a negative value on error. If we didn't have a
procfs, or otherwise couldn't access it we'd end up pushing an empty
string to the trace2 ancestry array.

It was also using the strvec_push() API the wrong way. That API always
does an xstrdup(), so by detaching the strbuf here we'd leak
memory. Let's instead pass in our pointer for strvec_push() to
xstrdup(), and then free our own strbuf. I do have some WIP changes to
make strvec_push_nodup() non-static, which makes this and some other
callsites nicer, but let's just follow the prevailing pattern of using
strvec_push() for now.

We'll also need to free that "procfs_path" strbuf whether or not
strbuf_read_file() succeeds, which was another source of memory leaks
in 2f732bf15e, i.e. we'd leak that memory as well if we weren't on a
system where we could read the file from procfs.

Let's move all the freeing of the memory to the end of the
function. If we're still at STRBUF_INIT with "name" due to not having
taken the branch where the strbuf_read_file() succeeds freeing it is
redundant. So we could move it into the body of the "if", but just
handling freeing the same way for all branches of the function makes
it more readable.

In combination with the preceding commit this makes all of
t[0-9]*trace2*.sh pass under SANITIZE=leak on Linux.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/linux/procinfo.c

@@ -18,12 +18,14 @@ static void get_ancestry_names(struct strvec *names)
 
 	/* try to use procfs if it's present. */
 	strbuf_addf(&procfs_path, "/proc/%d/comm", getppid());
-	if (strbuf_read_file(&name, procfs_path.buf, 0)) {
-		strbuf_release(&procfs_path);
+	if (strbuf_read_file(&name, procfs_path.buf, 0) > 0) {
 		strbuf_trim_trailing_newline(&name);
-		strvec_push(names, strbuf_detach(&name, NULL));
+		strvec_push(names, name.buf);
 	}
 
+	strbuf_release(&procfs_path);
+	strbuf_release(&name);
+
 	return;
 }