test: Reduce the loop detection threshold from 50 down to 5

The loop test can be quite slow, specially on computers without
cryptography-friendly instructions.

This patch introduces a new flag for testing, so that we can bring the
threshold down to 5. The test is just as useful but now runs in a few
seconds, as opposed to a few minutes.

internal/smtpsrv/conn.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"crypto/tls"
 	"expvar"
+	"flag"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -44,6 +45,11 @@ var (
 	hookResults       = expvar.NewMap("chasquid/smtpIn/hookResults")
 )
 
+var (
+	maxReceivedHeaders = flag.Int("testing__max_received_headers", 50,
+		"max Received headers, for loop detection; ONLY FOR TESTING")
+)
+
 // Mode for a socket (listening or connection).
 // We keep them distinct, as policies can differ between them.
 type SocketMode string
@@ -619,9 +625,10 @@ func checkData(data []byte) error {
 	// This serves as a basic form of loop prevention. It's not infallible but
 	// should catch most instances of accidental looping.
 	// https://tools.ietf.org/html/rfc5321#section-6.3
-	if len(msg.Header["Received"]) > 50 {
+	if len(msg.Header["Received"]) > *maxReceivedHeaders {
 		loopsDetected.Add(1)
-		return fmt.Errorf("email passed through more than 50 MTAs, looping?")
+		return fmt.Errorf("email passed through more than %d MTAs, looping?",
+			*maxReceivedHeaders)
 	}
 
 	return nil

test/t-09-loop/run.sh

@@ -22,6 +22,7 @@ CONFDIR=B generate_certs_for srv-B
 mkdir -p .logs-A .logs-B
 
 chasquid -v=2 --logfile=.logs-A/chasquid.log --config_dir=A \
+	--testing__max_received_headers=5 \
 	--testing__outgoing_smtp_port=2025 &
 chasquid -v=2 --logfile=.logs-B/chasquid.log --config_dir=B \
 	--testing__outgoing_smtp_port=1025 &