core: add exec_output_is_file() helper

The set of file based output types is open-coded in three places (and a
fourth one is about to be added), let's define it once, next to the
other ExecInput/ExecOutput predicates.

src/core/exec-invoke.c

@@ -459,7 +459,7 @@ static bool can_inherit_stderr_from_stdout(const ExecContext *context) {
         if (e == EXEC_OUTPUT_NAMED_FD)
                 return false;
 
-        if (IN_SET(e, EXEC_OUTPUT_FILE, EXEC_OUTPUT_FILE_APPEND, EXEC_OUTPUT_FILE_TRUNCATE))
+        if (exec_output_is_file(e))
                 return path_equal(context->stdio_file[STDOUT_FILENO], context->stdio_file[STDERR_FILENO]);
 
         return true;

src/core/execute.h

@@ -513,6 +513,12 @@ static inline bool exec_input_is_inheritable(ExecInput i) {
         return exec_input_is_terminal(i) || IN_SET(i, EXEC_INPUT_SOCKET, EXEC_INPUT_NAMED_FD);
 }
 
+static inline bool exec_output_is_file(ExecOutput o) {
+        /* The output types that refer to a regular file (StandardOutput=file:…, append:…, truncate:…), i.e.
+         * that require a companion path in ExecContext.stdio_file[] */
+        return IN_SET(o, EXEC_OUTPUT_FILE, EXEC_OUTPUT_FILE_APPEND, EXEC_OUTPUT_FILE_TRUNCATE);
+}
+
 int exec_spawn(
                 Unit *unit,
                 ExecCommand *command,

src/core/load-fragment.c

@@ -6541,7 +6541,7 @@ int config_parse_output_restricted(
                         return 0;
                 }
 
-                if (IN_SET(t, EXEC_OUTPUT_SOCKET, EXEC_OUTPUT_NAMED_FD, EXEC_OUTPUT_FILE, EXEC_OUTPUT_FILE_APPEND, EXEC_OUTPUT_FILE_TRUNCATE)) {
+                if (IN_SET(t, EXEC_OUTPUT_SOCKET, EXEC_OUTPUT_NAMED_FD) || exec_output_is_file(t)) {
                         log_syntax(unit, LOG_WARNING, filename, line, 0, "Standard output types socket, fd:, file:, append:, truncate: are not supported as defaults, ignoring: %s", rvalue);
                         return 0;
                 }