Add header echoing server middleware

server/src/main/scala/org/http4s/server/middleware/HeaderEcho.scala

@@ -0,0 +1,24 @@
+package org.http4s.server.middleware
+
+import cats.Functor
+import cats.data.Kleisli
+import org.http4s._
+
+object HeaderEcho {
+
+  private def isOneOf(headerKeys: Seq[HeaderKey])(header: Header): Boolean =
+    headerKeys.exists(_.matchHeader(header).isDefined)
+
+  /**
+    * Simple server middleware that adds selected headers present on the request to the response.
+    *
+    * @param headerKeys the keys corresponding to the headers to echo on the response
+    * @param http [[HttpRoutes]] to transform
+    */
+  def apply[F[_]: Functor](headerKeys: HeaderKey*)(http: HttpRoutes[F]): HttpRoutes[F] =
+    Kleisli { req: Request[F] =>
+      val headersToEcho = req.headers.filter(isOneOf(headerKeys))
+
+      http(req).map(_.putHeaders(headersToEcho.toList: _*))
+    }
+}

server/src/test/scala/org/http4s/server/middleware/HeaderEchoSpec.scala

@@ -0,0 +1,62 @@
+package org.http4s.server.middleware
+
+import cats.effect.IO
+import org.http4s._
+import org.http4s.dsl.io._
+
+class HeaderEchoSpec extends Http4sSpec {
+
+  object someHeaderKey extends HeaderKey.Default
+  object anotherHeaderKey extends HeaderKey.Default
+
+  val testService = HttpRoutes.of[IO] {
+    case GET -> Root / "request" => Ok("request response")
+  }
+
+  "HeaderEcho" should {
+    "echo a single header in addition to the defaults" in {
+      val requestMatchingSingleHeaderKey =
+        Request[IO](
+          uri = uri("/request"),
+          headers = Headers(Header("someheaderkey", "someheadervalue")))
+      val testee = HeaderEcho(someHeaderKey)(testService)
+      val responseHeaders =
+        testee.orNotFound(requestMatchingSingleHeaderKey).unsafeRunSync().headers
+
+      responseHeaders.exists(_.value == "someheadervalue") must_== true
+      (responseHeaders must have).size(3)
+    }
+
+    "echo multiple headers" in {
+      val requestMatchingMultipleHeaderKeys =
+        Request[IO](
+          uri = uri("/request"),
+          headers = Headers(
+            Header("someheaderkey", "someheadervalue"),
+            Header("anotherheaderkey", "anotherheadervalue")))
+      val testee = HeaderEcho(someHeaderKey, anotherHeaderKey)(testService)
+
+      val responseHeaders =
+        testee.orNotFound(requestMatchingMultipleHeaderKeys).unsafeRunSync().headers
+
+      responseHeaders.exists(_.value == "someheadervalue") must_== true
+      responseHeaders.exists(_.value == "anotherheadervalue") must_== true
+      (responseHeaders must have).size(4)
+    }
+
+    "echo only the default headers where none match the key" in {
+      val requestMatchingNotPresentHeaderKey =
+        Request[IO](
+          uri = uri("/request"),
+          headers = Headers(Header("someunmatchedheader", "someunmatchedvalue")))
+
+      val testee = HeaderEcho(someHeaderKey)(testService)
+      val responseHeaders =
+        testee.orNotFound(requestMatchingNotPresentHeaderKey).unsafeRunSync().headers
+
+      responseHeaders.exists(_.value == "someunmatchedvalue") must_== false
+      (responseHeaders must have).size(2)
+    }
+  }
+
+}