trace2: refactor to avoid gcc warning under -O3

Refactor tr2_dst_try_uds_connect() to avoid a gcc warning[1] that
appears under -O3 (but not -O2). This makes the build pass under
DEVELOPER=1 without needing a DEVOPTS=no-error.

This can be reproduced with GCC Debian 8.3.0-6, but not e.g. with
clang 7.0.1-8+deb10u2. We've had this warning since
ee4512e (trace2: create new combined trace facility, 2019-02-22).

As noted in [2] this warning happens because the compiler doesn't
assume that errno must be non-zero after a failed syscall.

Let's work around by using the well-established "saved_errno" pattern,
along with returning -1 ourselves instead of "errno". The caller can
thus rely on our "errno" on failure.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61846 for a related
bug report against GCC.

1.

    trace2/tr2_dst.c: In function ‘tr2_dst_get_trace_fd.part.5’:
    trace2/tr2_dst.c:296:10: warning: ‘fd’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      dst->fd = fd;
      ~~~~~~~~^~~~
    trace2/tr2_dst.c:229:6: note: ‘fd’ was declared here
      int fd;
          ^~
2. https://lore.kernel.org/git/[email protected]/

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

trace2/tr2_dst.c

@@ -115,15 +115,16 @@ static int tr2_dst_try_uds_connect(const char *path, int sock_type, int *out_fd)
 
 	fd = socket(AF_UNIX, sock_type, 0);
 	if (fd == -1)
-		return errno;
+		return -1;
 
 	sa.sun_family = AF_UNIX;
 	strlcpy(sa.sun_path, path, sizeof(sa.sun_path));
 
 	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
-		int e = errno;
+		int saved_errno = errno;
 		close(fd);
-		return e;
+		errno = saved_errno;
+		return -1;
 	}
 
 	*out_fd = fd;
@@ -138,7 +139,6 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
 {
 	unsigned int uds_try = 0;
 	int fd;
-	int e;
 	const char *path = NULL;
 
 	/*
@@ -182,23 +182,21 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
 	}
 
 	if (uds_try & TR2_DST_UDS_TRY_STREAM) {
-		e = tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd);
-		if (!e)
+		if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd))
 			goto connected;
-		if (e != EPROTOTYPE)
+		if (errno != EPROTOTYPE)
 			goto error;
 	}
 	if (uds_try & TR2_DST_UDS_TRY_DGRAM) {
-		e = tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd);
-		if (!e)
+		if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd))
 			goto connected;
 	}
 
 error:
 	if (tr2_dst_want_warning())
 		warning("trace2: could not connect to socket '%s' for '%s' tracing: %s",
 			path, tr2_sysenv_display_name(dst->sysenv_var),
-			strerror(e));
+			strerror(errno));
 
 	tr2_dst_trace_disable(dst);
 	return 0;