[systemd-sonic-generator] Fix handling service files with additional fields under [Install] section (#17764)

If encountered a line without RequiredBy or WantedBy the code passes uninitialized pointer to get_install_targets_from_line(). Where it can fail with segfault or silently pass randomly.

- Why I did it
Uninitialized target_suffix is passed to get_install_targets_from_line() when other fields are present in [Install] section, like this:

root@sonic:/home/admin# systemctl cat ntpsec
...
[Install]
Alias=ntp.service
Alias=ntpd.service
WantedBy=multi-user.target

- How I did it
Initialize target_suffix with NULL, put an assert in get_install_targets_from_line(). Edited test to cover this scenario.

- How to verify it
UT and on the switch.

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>

src/systemd-sonic-generator/systemd-sonic-generator.c

@@ -141,6 +141,9 @@ static int get_install_targets_from_line(char* target_string, char* install_type
     char final_target[PATH_MAX];
     int num_targets = 0;
 
+    assert(target_string);
+    assert(install_type);
+
     while ((token = strtok_r(target_string, " ", &target_string))) {
         if (num_targets + existing_targets >= MAX_NUM_TARGETS) {
             fputs("Number of targets found exceeds MAX_NUM_TARGETS\n", stderr);
@@ -269,7 +272,7 @@ int get_install_targets(char* unit_file, char* targets[]) {
     char* token;
     char* line = NULL;
     bool first;
-    char* target_suffix;
+    char* target_suffix = NULL;
     char *instance_name;
     char *dot_ptr;
 
@@ -306,6 +309,8 @@ int get_install_targets(char* unit_file, char* targets[]) {
                 }
                 else if (strstr(token, "WantedBy") != NULL) {
                     target_suffix = ".wants";
+                } else {
+                    break;
                 }
             }
             else {

src/systemd-sonic-generator/tests/testfiles/test.service

@@ -7,4 +7,5 @@ Type=oneshot
 RemainAfterExit=yes
 ExecStart=/usr/bin/test.sh start
 [Install]
+Alias=test.service
 WantedBy=multi-user.target