[PATCH 4/6] tr2: fix memory leak & logic error in 2f732bf15e6

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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
2f732bf15e6 (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.

Furthermore we need to free that "procfs_path" strbuf whether or not
strbuf_read_file() succeeds, which was another source of memory leaks
in 2f732bf15e6, i.e. we'd leak that memory as well if we weren't on a
system where we could read the file from procfs.

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>
---
 compat/linux/procinfo.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/compat/linux/procinfo.c b/compat/linux/procinfo.c
index 62f8aaed4cc..c86daff2b26 100644
--- a/compat/linux/procinfo.c
+++ b/compat/linux/procinfo.c
@@ -18,12 +18,13 @@ 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(&name);
 	}
 
+	strbuf_release(&procfs_path);
 	return;
 }
 
-- 
2.33.0.733.ga72a4f1c2e1