maillog: Introduce a special-purpose mail logging package

The default INFO logs are more oriented towards debugging and can be
a bit too verbose when looking for high-level information.

This patch introduces a new "maillog" package, used to log messages of
particular relevance to mail transmission at a higher level.

internal/maillog/maillog_test.go

@@ -0,0 +1,116 @@
+package maillog
+
+import (
+	"bytes"
+	"fmt"
+	"net"
+	"strings"
+	"testing"
+	"time"
+)
+
+var netAddr = &net.TCPAddr{
+	IP:   net.ParseIP("1.2.3.4"),
+	Port: 4321,
+}
+
+func expect(t *testing.T, buf *bytes.Buffer, s string) {
+	if strings.Contains(buf.String(), s) {
+		return
+	}
+	t.Errorf("buffer mismatch:")
+	t.Errorf("  expected to contain: %q", s)
+	t.Errorf("  got: %q", buf.String())
+}
+
+func TestLogger(t *testing.T) {
+	buf := &bytes.Buffer{}
+	l := New(buf)
+
+	l.Listening("1.2.3.4:4321")
+	expect(t, buf, "daemon listening on 1.2.3.4:4321")
+	buf.Reset()
+
+	l.Auth(netAddr, "user@domain", false)
+	expect(t, buf, "1.2.3.4:4321 authentication failed for user@domain")
+	buf.Reset()
+
+	l.Auth(netAddr, "user@domain", true)
+	expect(t, buf, "1.2.3.4:4321 authentication successful for user@domain")
+	buf.Reset()
+
+	l.Rejected(netAddr, "from", []string{"to1", "to2"}, "error")
+	expect(t, buf, "1.2.3.4:4321 rejected from=from to=[to1 to2] - error")
+	buf.Reset()
+
+	l.Queued(netAddr, "from", []string{"to1", "to2"}, "qid")
+	expect(t, buf, "qid from=from queued ip=1.2.3.4:4321 to=[to1 to2]")
+	buf.Reset()
+
+	l.SendAttempt("qid", "from", "to", nil, false)
+	expect(t, buf, "qid from=from to=to sent successfully")
+	buf.Reset()
+
+	l.SendAttempt("qid", "from", "to", fmt.Errorf("error"), false)
+	expect(t, buf, "qid from=from to=to sent failed (temporary): error")
+	buf.Reset()
+
+	l.SendAttempt("qid", "from", "to", fmt.Errorf("error"), true)
+	expect(t, buf, "qid from=from to=to sent failed (permanent): error")
+	buf.Reset()
+
+	l.QueueLoop("qid", 17*time.Second)
+	expect(t, buf, "qid completed loop, next in 17s")
+	buf.Reset()
+
+	l.QueueLoop("qid", 0)
+	expect(t, buf, "qid all done")
+	buf.Reset()
+}
+
+// Test that the default actions go reasonably to the default logger.
+// Unfortunately this is almost the same as TestLogger.
+func TestDefault(t *testing.T) {
+	buf := &bytes.Buffer{}
+	Default = New(buf)
+
+	Listening("1.2.3.4:4321")
+	expect(t, buf, "daemon listening on 1.2.3.4:4321")
+	buf.Reset()
+
+	Auth(netAddr, "user@domain", false)
+	expect(t, buf, "1.2.3.4:4321 authentication failed for user@domain")
+	buf.Reset()
+
+	Auth(netAddr, "user@domain", true)
+	expect(t, buf, "1.2.3.4:4321 authentication successful for user@domain")
+	buf.Reset()
+
+	Rejected(netAddr, "from", []string{"to1", "to2"}, "error")
+	expect(t, buf, "1.2.3.4:4321 rejected from=from to=[to1 to2] - error")
+	buf.Reset()
+
+	Queued(netAddr, "from", []string{"to1", "to2"}, "qid")
+	expect(t, buf, "qid from=from queued ip=1.2.3.4:4321 to=[to1 to2]")
+	buf.Reset()
+
+	SendAttempt("qid", "from", "to", nil, false)
+	expect(t, buf, "qid from=from to=to sent successfully")
+	buf.Reset()
+
+	SendAttempt("qid", "from", "to", fmt.Errorf("error"), false)
+	expect(t, buf, "qid from=from to=to sent failed (temporary): error")
+	buf.Reset()
+
+	SendAttempt("qid", "from", "to", fmt.Errorf("error"), true)
+	expect(t, buf, "qid from=from to=to sent failed (permanent): error")
+	buf.Reset()
+
+	QueueLoop("qid", 17*time.Second)
+	expect(t, buf, "qid completed loop, next in 17s")
+	buf.Reset()
+
+	QueueLoop("qid", 0)
+	expect(t, buf, "qid all done")
+	buf.Reset()
+}