smtpsrv: Disable SPF lookups in tests

When testing, we don't want the server to do SPF lookups, as those cause
real DNS queries which can be problematic and add a dependency on the
environment.

This patch adds an internal boolean to disable the SPF lookups, which is
only set from the tests.

internal/smtpsrv/conn.go

@@ -48,6 +48,9 @@ var (
 var (
 	maxReceivedHeaders = flag.Int("testing__max_received_headers", 50,
 		"max Received headers, for loop detection; ONLY FOR TESTING")
+
+	// Some go tests disable SPF, to avoid leaking DNS lookups.
+	disableSPFForTesting = false
 )
 
 // Mode for a socket (listening or connection).
@@ -385,6 +388,10 @@ func (c *Conn) checkSPF(addr string) (spf.Result, error) {
 		return "", nil
 	}
 
+	if disableSPFForTesting {
+		return "", nil
+	}
+
 	if tcp, ok := c.conn.RemoteAddr().(*net.TCPAddr); ok {
 		res, err := spf.CheckHost(
 			tcp.IP, envelope.DomainOf(addr))

internal/smtpsrv/server_test.go

@@ -453,6 +453,9 @@ func realMain(m *testing.M) int {
 		s.AddDomain("localhost")
 		s.AddUserDB("localhost", udb)
 
+		// Disable SPF lookups, to avoid leaking DNS queries.
+		disableSPFForTesting = true
+
 		go s.ListenAndServe()
 	}