Merge pull request #4528 from guardian/an/treat-501-as-400

add custom error handler to suppress 501 alarms

Author: andrew-nowak

common-lib/src/main/resources/application.conf

@@ -17,6 +17,8 @@ play {
   assets.defaultCache="public, max-age=60"
 }
 
+pekko.http.server.parsing.error-handler = "com.gu.mediaservice.lib.play.UnknownMethodParsingErrorHandler$"
+
 es {
   cluster: media-service
   port: 9300

rest-lib/src/main/scala/com/gu/mediaservice/lib/play/UnknownMethodParsingErrorHandler.scala

@@ -0,0 +1,32 @@
+package com.gu.mediaservice.lib.play
+
+import com.gu.mediaservice.lib.logging.GridLogging
+import org.apache.pekko.event.LoggingAdapter
+import org.apache.pekko.http.{DefaultParsingErrorHandler, ParsingErrorHandler}
+import org.apache.pekko.http.scaladsl.model.{ErrorInfo, HttpResponse, StatusCode, StatusCodes}
+import org.apache.pekko.http.scaladsl.settings.ServerSettings
+
+import scala.annotation.unused
+
+/*
+  Coerces responses from Pekko in response to unknown HTTP methods from 501 to 400.
+  Technically HTTP spec requires 501s in this case, but this allows external parties
+  to spam our 5xx alarms with silly requests. We're comfortable treating these as "Bad
+  requests" instead.
+ */
+@unused
+object UnknownMethodParsingErrorHandler extends ParsingErrorHandler with GridLogging {
+  override def handle(
+    status: StatusCode,
+    error: ErrorInfo,
+    log: LoggingAdapter,
+    settings: ServerSettings
+  ): HttpResponse = {
+    if (status == StatusCodes.NotImplemented && error.summary == "Unsupported HTTP method") {
+      logger.warn(s"Client attempted to use unknown HTTP method '${error.detail}'. Returning 400 instead of 501.")
+      HttpResponse(StatusCodes.BadRequest)
+    } else {
+      DefaultParsingErrorHandler.handle(status, error, log, settings)
+    }
+  }
+}