Fix: load: incomplete error handling for load_session_from_file

This fix is adapted from a fix against the stable-2.11 branch.  The
commit message of the stable-2.11 branch follows.

An equivalent fix was already in place in `load_session_from_path()`,
but the same problem as the stable-2.11 branch is present in
`load_session_from_file()`.

Original message:

Observed issue
==============

lttng-ivc test fails to fail.

test_save_load_blocking_timeout[lttng-tools-2.12-lttng-tools-2.11-False]

Here we load a xml created by lttng-tools-2.12 and try to load it using
lttng-tools 2.11. We expect this to fail on the load.

The command report an error on the stderr but the command return code
value is zero.

From lttng-ivc test runtime.log:
Command #0
  Return value: 0
  Command: lttng load --input-path=/home/joraj/lttng/lttng-ivc/.tox/py3/tmp/test_save_load_blocking_timeou0/save_load saved_trace
  STDOUT:
      Session saved_trace has been loaded successfully
  STDERR:
      XML Error: Element 'process_attr_trackers': This element is not expected.
      Error: Session configuration file validation failed

Cause
=====

The error coming from load_session_from_file is not handled correctly.

Solution
========

Rework error handling in load_session_from_path and
load_session_from_file.

LTTNG_ERR_LOAD_SESSION_NOENT is NOT an error when session_name is
specified in load_session_from_path. In this scenario, we are actively
looking for the configuration of the session.

Signed-off-by: Jérémie Galarneau <[email protected]>
Signed-off-by: Jonathan Rajotte <[email protected]>
Change-Id: Ic68c253aa194bf8ab72c3c271f10d443118bdeee

Author: jgalar

src/common/config/session-config.c

@@ -3767,20 +3767,31 @@ int load_session_from_file(const char *path, const char *session_name,
 			xmlNextElementSibling(session_node)) {
 		ret = process_session_node(session_node,
 			session_name, overwrite, overrides);
-		if (session_name && ret == 0) {
-			/* Target session found and loaded */
-			session_found = 1;
-			break;
+		if (!session_name && ret) {
+			/* Loading error occurred. */
+			goto end;
+		} else if (session_name) {
+			if (ret == 0) {
+				/* Target session found and loaded */
+				session_found = 1;
+				break;
+			} else if (ret == -LTTNG_ERR_NO_SESSION) {
+				/*
+				 * Ignore this error, we are looking for a
+				 * specific session.
+				 */
+				ret = 0;
+			} else {
+				/* Loading error occurred. */
+				goto end;
+			}
 		}
 	}
 end:
 	xmlFreeDoc(doc);
 	if (!ret) {
 		ret = session_found ? 0 : -LTTNG_ERR_LOAD_SESSION_NOENT;
 	}
-	if (ret == -LTTNG_ERR_NO_SESSION) {
-		ret = -LTTNG_ERR_LOAD_SESSION_NOENT;
-	}
 	return ret;
 }