core: initialize priority_set when parsing swap unit files

Fixes: #14524

src/core/load-fragment-gperf.gperf.m4

@@ -435,7 +435,7 @@ Automount.DirectoryMode,         config_parse_mode,                  0,
 Automount.TimeoutIdleSec,        config_parse_sec_fix_0,             0,                             offsetof(Automount, timeout_idle_usec)
 m4_dnl
 Swap.What,                       config_parse_unit_path_printf,      0,                             offsetof(Swap, parameters_fragment.what)
-Swap.Priority,                   config_parse_int,                   0,                             offsetof(Swap, parameters_fragment.priority)
+Swap.Priority,                   config_parse_swap_priority,         0,                             0
 Swap.Options,                    config_parse_unit_string_printf,    0,                             offsetof(Swap, parameters_fragment.options)
 Swap.TimeoutSec,                 config_parse_sec_fix_0,             0,                             offsetof(Swap, timeout_usec)
 EXEC_CONTEXT_CONFIG_ITEMS(Swap)m4_dnl

src/core/load-fragment.c

@@ -5002,3 +5002,51 @@ int config_parse_crash_chvt(
 
         return 0;
 }
+
+int config_parse_swap_priority(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        Swap *s = userdata;
+        int r, priority;
+
+        assert(s);
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+        assert(data);
+
+        if (isempty(rvalue)) {
+                s->parameters_fragment.priority = -1;
+                s->parameters_fragment.priority_set = false;
+                return 0;
+        }
+
+        r = safe_atoi(rvalue, &priority);
+        if (r < 0) {
+                log_syntax(unit, LOG_ERR, filename, line, r, "Invalid swap pririty '%s', ignoring.", rvalue);
+                return 0;
+        }
+
+        if (priority < -1) {
+                log_syntax(unit, LOG_ERR, filename, line, 0, "Sorry, swap priorities smaller than -1 may only be assigned by the kernel itself, ignoring: %s", rvalue);
+                return 0;
+        }
+
+        if (priority > 32767) {
+                log_syntax(unit, LOG_ERR, filename, line, 0, "Swap priority out of range, ignoring: %s", rvalue);
+                return 0;
+        }
+
+        s->parameters_fragment.priority = priority;
+        s->parameters_fragment.priority_set = true;
+        return 0;
+}

src/core/load-fragment.h

@@ -121,6 +121,7 @@ CONFIG_PARSER_PROTOTYPE(config_parse_status_unit_format);
 CONFIG_PARSER_PROTOTYPE(config_parse_output_restricted);
 CONFIG_PARSER_PROTOTYPE(config_parse_crash_chvt);
 CONFIG_PARSER_PROTOTYPE(config_parse_timeout_abort);
+CONFIG_PARSER_PROTOTYPE(config_parse_swap_priority);
 
 /* gperf prototypes */
 const struct ConfigPerfItem* load_fragment_gperf_lookup(const char *key, GPERF_LEN_TYPE length);