prepare gradual transition between smtp configs

app/controllers/Dev.scala

@@ -10,6 +10,7 @@ final class Dev(env: Env) extends LilaController(env) {
   private lazy val settingsList = List[lila.memo.SettingStore[_]](
     env.security.ugcArmedSetting,
     env.security.spamKeywordsSetting,
+    env.security.mailerSecondaryPercentageSetting,
     env.irwin.irwinThresholdsSetting,
     env.explorer.indexFlowSetting,
     env.report.scoreThresholdsSetting,

conf/base.conf

@@ -193,6 +193,7 @@ security {
       user = "noreply@lichess.org"
       password = "???"
     }
+    secondary = ${security.mailer.primary}
     sender = "lichess.org <noreply@lichess.org>"
   }
   email_confirm {

modules/security/src/main/Env.scala

@@ -79,7 +79,16 @@ final class Env(
     mk(ugcArmedSetting.get _)
   }
 
-  private lazy val mailer: Mailer = wire[Mailer]
+  lazy val mailerSecondaryPercentageSetting = settingStore[Int](
+    "mailerSecondaryPercentage",
+    default = 0,
+    text = "Percentage of mails to send using secondary SMTP configuration".some
+  )
+
+  private lazy val mailer = new Mailer(
+    config.mailer,
+    getSecondaryPercentage = () => mailerSecondaryPercentageSetting.get()
+  )
 
   lazy val emailConfirm: EmailConfirm =
     if (config.emailConfirm.enabled)

modules/security/src/main/Mailer.scala

@@ -9,12 +9,13 @@ import scala.concurrent.{ blocking, Future }
 import scalatags.Text.all._
 
 import lila.common.config.Secret
-import lila.common.{ Chronometer, EmailAddress }
+import lila.common.{ Chronometer, EmailAddress, ThreadLocalRandom }
 import lila.common.String.html.{ escapeHtml, nl2brUnsafe }
 import lila.i18n.I18nKeys.{ emails => trans }
 
 final class Mailer(
-    config: Mailer.Config
+    config: Mailer.Config,
+    getSecondaryPercentage: () => Int
 )(implicit
     ec: scala.concurrent.ExecutionContext,
     system: ActorSystem
@@ -23,7 +24,12 @@ final class Mailer(
   private val workQueue =
     new lila.hub.DuctSequencer(maxSize = 512, timeout = Mailer.timeout * 2, name = "mailer")
 
-  private val client = new SMTPMailer(config.primary.toClientConfig)
+  private val primaryClient   = new SMTPMailer(config.primary.toClientConfig)
+  private val secondaryClient = new SMTPMailer(config.secondary.toClientConfig)
+
+  private def randomClient(): SMTPMailer =
+    if (ThreadLocalRandom.nextInt(100) < getSecondaryPercentage()) secondaryClient
+    else primaryClient
 
   def send(msg: Mailer.Message): Funit =
     if (msg.to.isNoReply) {
@@ -34,7 +40,7 @@ final class Mailer(
         Future {
           Chronometer.syncMon(_.email.send.time) {
             blocking {
-              client
+              randomClient()
                 .send(
                   Email(
                     subject = msg.subject,
@@ -78,6 +84,7 @@ object Mailer {
 
   case class Config(
       primary: Smtp,
+      secondary: Smtp,
       sender: String
   )
   implicit val configLoader = AutoConfig.loader[Config]