Some ETag refactoring

app/controllers/HomeController.scala

@@ -1,17 +1,15 @@
 package controllers
 
-import javax.inject.*
 import play.api.*
-import play.api.mvc.*
-import authentication.{AuthenticatedAction, AuthenticatedRequest}
-import caching.{Etagable, CheckFresh}
+import _root_.controllers.authentication.{AuthenticatedAction, AuthenticatedRequest}
+import _root_.controllers.caching.{CheckFresh, DigestEtagable}
 import models.User
 import play.api.libs.json.Json
+import play.api.mvc.*
 
-import java.nio.ByteBuffer
+import java.io.DataOutputStream
 import java.nio.charset.StandardCharsets.UTF_8
-import java.security.MessageDigest
-import java.util.Base64
+import javax.inject.*
 
 /**
  * This controller creates an `Action` to handle HTTP requests to the
@@ -37,21 +35,15 @@ class HomeController @Inject()(val controllerComponents: ControllerComponents,
     val user = request.user
     checkFresh(user) {
       // TODO: put the max-age somewhere into config
-      Ok(Json.toJson(user)).withHeaders("Etag" -> user.etag, "Cache-Control" -> "private, max-age=15, must-revalidate")
+      Ok(Json.toJson(user)).withHeaders("Cache-Control" -> "private, max-age=15, must-revalidate")
     }
   }
 
-  private given Etagable[models.User] with {
-    private val base64 = Base64.getUrlEncoder
-
-    extension (a: User)
-      override def etag: String = {
-        val md = MessageDigest.getInstance("SHA-1")
-        md.update(ByteBuffer.allocate(8).putLong(a.id.getOrElse(0)))
-        md.update(a.gid.getBytes(UTF_8))
-        md.update(a.username.getBytes(UTF_8))
-
-        base64.encodeToString(md.digest())
-      }
+  private given DigestEtagable[models.User] with {
+    override protected def writeDigestData(a: User, s: DataOutputStream): Unit = {
+      s.writeLong(a.id.getOrElse(0))
+      s.write(a.gid.getBytes(UTF_8))
+      s.write(a.username.getBytes(UTF_8))
+    }
   }
 }

app/controllers/caching/caching.scala

@@ -3,24 +3,55 @@ package controllers.caching
 import play.api.mvc.Results.NotModified
 import play.api.mvc.{Request, Result}
 
+import java.io.{ByteArrayOutputStream, DataOutputStream, OutputStream}
+import java.security.{DigestOutputStream, MessageDigest}
+import java.util.Base64
 import scala.concurrent.{ExecutionContext, Future}
+import scala.util.Using
 
 trait Etagable[A]:
   extension (a: A) def etag: String
 
 
+trait DigestEtagable[A] extends Etagable[A]:
+  private val base64 = Base64.getUrlEncoder.withoutPadding
+
+  protected def writeDigestData(a: A, s: DataOutputStream): Unit
+
+  extension (a: A) {
+    override def etag: String = {
+      val md = MessageDigest.getInstance("SHA-1")
+      Using(DataOutputStream(DigestOutputStream(OutputStream.nullOutputStream(), md))) { dos =>
+        writeDigestData(a, dos)
+        dos.flush()
+      }
+
+      base64.encodeToString(md.digest())
+    }
+  }
+
+
 trait CheckFresh {
   protected def checkFreshAsync[A, E: Etagable](entity: E)(onStale: => Future[Result])(using request: Request[A], ec: ExecutionContext): Future[Result] = {
-    compareEtag(request, entity).fold(onStale)(
-      etag => Future.successful(NotModified.withHeaders("ETag" -> etag))
+    compareEtag(request, entity).fold(
+      entityEtag => onStale.map(_.withHeaders("etag" -> entityEtag)),
+      requestEtag => Future.successful(notModified(requestEtag))
     )
   }
 
   protected def checkFresh[A, E: Etagable](entity: E)(onStale: => Result)(using request: Request[A]): Result = {
-    compareEtag(request, entity).fold(onStale)(
-      etag => NotModified.withHeaders("ETag" -> etag)
+    compareEtag(request, entity).fold(
+      entityEtag => onStale.withHeaders("etag" -> entityEtag),
+      requestEtag => notModified(requestEtag)
     )
   }
 
-  private def compareEtag[A, E: Etagable](request: Request[A], entity: E) = request.headers.get("If-None-Match").filter(e => e == entity.etag)
+  private def compareEtag[A, E: Etagable](request: Request[A], entity: E) = {
+    val entityEtag = entity.etag
+    request.headers.get("If-None-Match").filter(e => e == entityEtag) match {
+      case Some(etag) => Right(etag)
+      case None => Left(entityEtag)
+    }
+  }
+  private def notModified(etag: String) = NotModified.withHeaders("etag" -> etag)
 }