Flush until it's all down the drain

VSLQ_Flush() vtx_force()s all incomplete transactions to be ready and calls
vslq_process_ready().

vslq_process_ready() retires vtxes until the callback returns with non zero,
so if our dispatch callback does return with an error, flushing will stop,
so we may end up with a non-empty ready list in vslq after calling flush
just once.

To ensure that we flushed everything and vtx_retire()d all vtxes
(which may contain refs to chunk_t_bufs living in malloc()ed memory),
we need to keep on flushing until we know the bowl is empty.

src/child.c

@@ -893,7 +893,8 @@ CHILD_Main(int readconfig)
         if (flush && !term) {
             LOG_Log0(LOG_NOTICE, "Flushing transactions");
             take_free();
-            VSLQ_Flush(vslq, dispatch, NULL);
+            do {}
+            while (VSLQ_Flush(vslq, dispatch, NULL) != DISPATCH_RETURN_OK);
             flush = 0;
             if (EMPTY(config.varnish_bindump) && status != DISPATCH_CLOSED
                 && status != DISPATCH_OVERRUN && status != DISPATCH_IOERR)
@@ -929,7 +930,8 @@ CHILD_Main(int readconfig)
     if (term && status != DISPATCH_EOF && flush && vslq != NULL) {
         LOG_Log0(LOG_NOTICE, "Flushing transactions");
         take_free();
-        VSLQ_Flush(vslq, dispatch, NULL);
+        do {}
+        while (VSLQ_Flush(vslq, dispatch, NULL) != DISPATCH_RETURN_OK);
     }
 
     WRK_Halt();