Add middlewares to forward header and ensure the existence of a header

zio-http/jvm/src/test/scala/zio/http/ForwardHeaderSpec.scala

@@ -0,0 +1,30 @@
+package zio.http
+
+import zio._
+import zio.test._
+
+object ForwardHeaderSpec extends ZIOSpecDefault {
+  override def spec: Spec[TestEnvironment with Scope, Any] =
+    suite("ForwardHeaderSpec")(
+      test("should forward headers") {
+        val routes = Routes(
+          Method.GET / "get"   -> handler((_: Request) =>
+            for {
+              client   <- ZIO.service[Client]
+              response <- (client @@ ZClientAspect.forwardHeaders)
+                .batched(Request.post(url"http://localhost:8080/post", Body.empty))
+            } yield response,
+          ),
+          Method.POST / "post" -> handler((req: Request) => Response.ok.addHeader(req.header(Header.Accept).get)),
+        ).sandbox @@ Middleware.forwardHeaders(Header.Accept)
+
+        for {
+          _        <- Server.install(routes)
+          response <- Client.batched(
+            Request.get(url"http://localhost:8080/get").addHeader(Header.Accept(MediaType.application.json)),
+          )
+        } yield assertTrue(response.headers(Header.Accept).contains(Header.Accept(MediaType.application.json)))
+      },
+    ).provide(Client.default, Server.default) @@ TestAspect.withLiveClock
+
+}

zio-http/shared/src/main/scala/zio/http/Header.scala

@@ -51,7 +51,7 @@ sealed trait Header {
 object Header {
 
   sealed trait HeaderType {
-    type HeaderValue
+    type HeaderValue <: Header
 
     def name: String
 

zio-http/shared/src/main/scala/zio/http/Middleware.scala

@@ -174,6 +174,86 @@ object Middleware extends HandlerAspects {
     }
   }
 
+  def ensureHeader(header: Header.HeaderType)(make: => header.HeaderValue): Middleware[Any] =
+    new Middleware[Any] {
+      def apply[Env1 <: Any, Err](routes: Routes[Env1, Err]): Routes[Env1, Err] =
+        routes.transform[Env1] { h =>
+          handler { (req: Request) =>
+            if (req.headers.contains(header.name)) h(req)
+            else h(req.addHeader(make))
+          }
+        }
+    }
+
+  def ensureHeader(headerName: String)(make: => String): Middleware[Any] =
+    new Middleware[Any] {
+      def apply[Env1 <: Any, Err](routes: Routes[Env1, Err]): Routes[Env1, Err] =
+        routes.transform[Env1] { h =>
+          handler { (req: Request) =>
+            if (req.headers.contains(headerName)) h(req)
+            else h(req.addHeader(headerName, make))
+          }
+        }
+    }
+
+  private[http] case class ForwardedHeaders(headers: Headers)
+
+  def forwardHeaders(header: Header.HeaderType, headers: Header.HeaderType*)(implicit
+    trace: Trace,
+  ): Middleware[Any] = {
+    val allHeaders = header +: headers
+    new Middleware[Any] {
+      def apply[Env1 <: Any, Err](routes: Routes[Env1, Err]): Routes[Env1, Err] =
+        routes.transform[Env1] { h =>
+          handler { (req: Request) =>
+            val headerValues = ChunkBuilder.make[Header]()
+            headerValues.sizeHint(allHeaders.length)
+            var i            = 0
+            while (i < allHeaders.length) {
+              val name = allHeaders(i)
+              req.headers.get(name).foreach { value =>
+                headerValues += value
+              }
+              i += 1
+            }
+            RequestStore.update[ForwardedHeaders] { old =>
+              ForwardedHeaders {
+                old.map(_.headers).getOrElse(Headers.empty) ++
+                  Headers.fromIterable(headerValues.result())
+              }
+            } *> h(req)
+          }
+        }
+    }
+  }
+
+  def forwardHeaders(headerName: String, headerNames: String*)(implicit trace: Trace): Middleware[Any] = {
+    val allHeaders = headerName +: headerNames
+    new Middleware[Any] {
+      def apply[Env1 <: Any, Err](routes: Routes[Env1, Err]): Routes[Env1, Err] =
+        routes.transform[Env1] { h =>
+          handler { (req: Request) =>
+            val headerValues = ChunkBuilder.make[Header]()
+            headerValues.sizeHint(allHeaders.length)
+            var i            = 0
+            while (i < allHeaders.length) {
+              val name = allHeaders(i)
+              req.headers.get(name).foreach { value =>
+                headerValues += Header.Custom(name, value)
+              }
+              i += 1
+            }
+            RequestStore.update[ForwardedHeaders] { old =>
+              ForwardedHeaders {
+                old.map(_.headers).getOrElse(Headers.empty) ++
+                  Headers.fromIterable(headerValues.result())
+              }
+            } *> h(req)
+          }
+        }
+    }
+  }
+
   def logAnnotate(key: => String, value: => String)(implicit trace: Trace): Middleware[Any] =
     logAnnotate(LogAnnotation(key, value))
 

zio-http/shared/src/main/scala/zio/http/RequestStore.scala

@@ -0,0 +1,22 @@
+package zio.http
+
+import zio.{FiberRef, Tag, Unsafe, ZIO}
+
+object RequestStore {
+
+  private[http] val requestStore: FiberRef[Map[Tag[_], Any]] =
+    FiberRef.unsafe.make[Map[Tag[_], Any]](Map.empty)(Unsafe.unsafe)
+
+  def get[A: Tag]: ZIO[Any, Nothing, Option[A]] =
+    requestStore.get.map(_.get(implicitly[Tag[A]]).asInstanceOf[Option[A]])
+
+  def set[A: Tag](a: A): ZIO[Any, Nothing, Unit] =
+    requestStore.update(_.updated(implicitly[Tag[A]], a))
+
+  def update[A: Tag](a: Option[A] => A): ZIO[Any, Nothing, Unit] =
+    for {
+      current <- get[A]
+      _       <- set(a(current))
+    } yield ()
+
+}

zio-http/shared/src/main/scala/zio/http/ZClientAspect.scala

@@ -447,4 +447,49 @@ object ZClientAspect {
       }
     }
   }
+
+  final def forwardHeaders: ZClientAspect[Nothing, Any, Nothing, Body, Nothing, Any, Nothing, Response] =
+    new ZClientAspect[Nothing, Any, Nothing, Body, Nothing, Any, Nothing, Response] {
+      override def apply[
+        ReqEnv,
+        Env >: Nothing <: Any,
+        In >: Nothing <: Body,
+        Err >: Nothing <: Any,
+        Out >: Nothing <: Response,
+      ](
+        client: ZClient[Env, ReqEnv, In, Err, Out],
+      ): ZClient[Env, ReqEnv, In, Err, Out] =
+        client.copy(
+          driver = new ZClient.Driver[Env, ReqEnv, Err] {
+            override def request(
+              version: Version,
+              method: Method,
+              url: URL,
+              headers: Headers,
+              body: Body,
+              sslConfig: Option[ClientSSLConfig],
+              proxy: Option[Proxy],
+            )(implicit trace: Trace): ZIO[Env & ReqEnv, Err, Response] =
+              RequestStore.get[Middleware.ForwardedHeaders].flatMap {
+                case Some(forwardedHeaders) =>
+                  client.driver
+                    .request(version, method, url, headers ++ forwardedHeaders.headers, body, sslConfig, proxy)
+                case None                   =>
+                  client.driver.request(version, method, url, headers, body, sslConfig, proxy)
+              }
+
+            override def socket[Env1 <: Env](version: Version, url: URL, headers: Headers, app: WebSocketApp[Env1])(
+              implicit
+              trace: Trace,
+              ev: ReqEnv =:= Scope,
+            ): ZIO[Env1 & ReqEnv, Err, Response] =
+              RequestStore.get[Middleware.ForwardedHeaders].flatMap {
+                case Some(forwardedHeaders) =>
+                  client.driver.socket(version, url, headers ++ forwardedHeaders.headers, app)
+                case None                   =>
+                  client.driver.socket(version, url, headers, app)
+              }
+          },
+        )
+    }
 }