test: Implement retries on the load generator

If the load generator is sending emails too fast, chasquid queue might
hit the maximum size and fail the test.

This patch makes it sleep and retry, to give the server some time to
catch up.

Thanks to Max Mazurov (fox.cpp@disroot.org) for reporting this problem.

test/util/loadgen.go

@@ -7,6 +7,7 @@ import (
 	"flag"
 	"net"
 	"net/http"
+	"net/textproto"
 	"runtime"
 	"sync"
 	"time"
@@ -172,6 +173,7 @@ func one() error {
 			return err
 		}
 
+	retry:
 		w, err := client.Data()
 		if err != nil {
 			return err
@@ -182,6 +184,16 @@ func one() error {
 		}
 		err = w.Close()
 		if err != nil {
+			// If we are sending too fast we might hit chasquid's queue size
+			// limit. In that case, wait and try again.
+			// We detect it with error code 451 which is used for this
+			// situation.
+			if terr, ok := err.(*textproto.Error); ok {
+				if terr.Code == 451 {
+					time.Sleep(10 * time.Millisecond)
+					goto retry
+				}
+			}
 			return err
 		}
 	}