better logging in srv when fake Mailer is used

Makefile

@@ -1,4 +1,3 @@
-
 all:
 	nix-build -A entrypoint --arg baseConfig '(import ./config.nix) // { staticProxyURL = ""; }'
 

srv/cmd/mediocre-blog/main.go

@@ -74,8 +74,8 @@ func main() {
 
 	var mailer mailinglist.Mailer
 	if mailerParams.SMTPAddr == "" {
-		logger.Info(ctx, "-smtp-addr not given, using NullMailer")
-		mailer = mailinglist.NullMailer
+		logger.Info(ctx, "-smtp-addr not given, using a fake Mailer")
+		mailer = mailinglist.NewLogMailer(logger.WithNamespace("fake-mailer"))
 	} else {
 		mailer = mailinglist.NewMailer(mailerParams)
 	}

srv/mailinglist/mailer.go

@@ -9,6 +9,7 @@ import (
 	"github.com/emersion/go-smtp"
 	"github.com/mediocregopher/blog.mediocregopher.com/srv/cfg"
 	"github.com/mediocregopher/mediocre-go-lib/v2/mctx"
+	"github.com/mediocregopher/mediocre-go-lib/v2/mlog"
 )
 
 // Mailer is used to deliver emails to arbitrary recipients.
@@ -16,6 +17,25 @@ type Mailer interface {
 	Send(to, subject, body string) error
 }
 
+type logMailer struct {
+	logger *mlog.Logger
+}
+
+// NewLogMailer returns a Mailer instance which will not actually send any
+// emails, it will only log to the given Logger when Send is called.
+func NewLogMailer(logger *mlog.Logger) Mailer {
+	return &logMailer{logger: logger}
+}
+
+func (l *logMailer) Send(to, subject, body string) error {
+	ctx := mctx.Annotate(context.Background(),
+		"to", to,
+		"subject", subject,
+	)
+	l.logger.Info(ctx, "would have sent email")
+	return nil
+}
+
 // NullMailer acts as a Mailer but actually just does nothing.
 var NullMailer = nullMailer{}