+
+/* Check transport for the given concurrency limit. Return TRUE if over
+the limit (or an expansion failure), else FALSE and if there was a limit,
+the key for the hints database used for the concurrency count. */
+
+static BOOL
+tpt_parallel_check(transport_instance * tp, address_item * addr, uschar ** key)
+{
+unsigned max_parallel;
+
+if (!tp->max_parallel) return FALSE;
+
+max_parallel = (unsigned) expand_string_integer(tp->max_parallel, TRUE);
+if (expand_string_message)
+ {
+ log_write(0, LOG_MAIN|LOG_PANIC, "Failed to expand max_parallel option "
+ "in %s transport (%s): %s", tp->name, addr->address,
+ expand_string_message);
+ return TRUE;
+ }
+
+if (max_parallel > 0)
+ {
+ uschar * serialize_key = string_sprintf("tpt-serialize-%s", tp->name);
+ if (!enq_start(serialize_key, max_parallel))
+ {
+ address_item * next;
+ DEBUG(D_transport)
+ debug_printf("skipping tpt %s because concurrency limit %u reached\n",
+ tp->name, max_parallel);
+ do
+ {
+ next = addr->next;
+ addr->message = US"concurrency limit reached for transport";
+ addr->basic_errno = ERRNO_TRETRY;
+ post_process_one(addr, DEFER, LOG_MAIN, DTYPE_TRANSPORT, 0);
+ } while ((addr = next));
+ return TRUE;
+ }
+ *key = serialize_key;
+ }
+return FALSE;
+}
+
+
+