core: extend ExecStart= varlink support for StartTransientUnit() to accept execution flags

src/shared/exec-util.c

@@ -519,6 +519,16 @@ ExecCommandFlags exec_command_flags_from_string(const char *s) {
         return 1U << idx;
 }
 
+char* exec_command_flags_to_exec_chars(ExecCommandFlags flags) {
+        /* Returns the prefix characters that encode the flags in unit file ExecStart= lines, i.e. the
+         * inverse of the prefix parsing in load-fragment.c */
+        return strjoin(FLAGS_SET(flags, EXEC_COMMAND_IGNORE_FAILURE)   ? "-" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_NO_ENV_EXPAND)    ? ":" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_FULLY_PRIVILEGED) ? "+" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_NO_SETUID)        ? "!" : "",
+                       FLAGS_SET(flags, EXEC_COMMAND_VIA_SHELL)        ? "|" : "");
+}
+
 int fexecve_or_execve(int executable_fd, const char *executable, char *const argv[], char *const envp[]) {
         /* Refuse invalid fds, regardless if fexecve() use is enabled or not */
         if (executable_fd < 0)

src/shared/exec-util.h

@@ -55,6 +55,7 @@ typedef enum ExecCommandFlags {
 
 int exec_command_flags_from_strv(char * const *ex_opts, ExecCommandFlags *ret);
 int exec_command_flags_to_strv(ExecCommandFlags flags, char ***ret);
+char* exec_command_flags_to_exec_chars(ExecCommandFlags flags);
 
 DECLARE_STRING_TABLE_LOOKUP(exec_command_flags, ExecCommandFlags);