Make Response parser more relaxed on reason message akka#981

Proposal for issue akka#981 that will allow to parse a non-standard
Http Response which does not contain a status message.

Example: 'HTTP/1.1 400' instead of 'HTTP/1.1 400 Bad Request'

akka-http-core/src/main/scala/akka/http/impl/engine/parsing/HttpResponseParser.scala

@@ -85,19 +85,31 @@ private[http] class HttpResponseParser(protected val settings: ParserSettings, p
         }
       }
     }
+
+    def isLF(idx: Int) = byteChar(input, idx) == '\n'
+    def isCRLF(idx: Int) = byteChar(input, idx) == '\r' && isLF(idx + 1)
+    def isNewLine(idx: Int) = isLF(idx) || isCRLF(idx)
+
+    def skipNewLine(idx: Int) = {
+      if (isCRLF(idx)) idx + 2
+      else if (isLF(idx)) idx + 1
+      else idx
+    }
+
     if (byteChar(input, cursor + 3) == ' ') {
       parseStatusCode()
       val startIdx = cursor + 4
       @tailrec def skipReason(idx: Int): Int =
         if (idx - startIdx <= maxResponseReasonLength)
-          if (byteChar(input, idx) == '\r' && byteChar(input, idx + 1) == '\n') idx + 2
-          else if (byteChar(input, idx) == '\n') idx + 1
+          if (isNewLine(idx)) skipNewLine(idx)
           else skipReason(idx + 1)
         else throw new ParsingException("Response reason phrase exceeds the configured limit of " +
           maxResponseReasonLength + " characters")
       skipReason(startIdx)
-    } else if (byteChar(input, cursor + 3) == '\n' || byteChar(input, cursor + 3) == '\r' && byteChar(input, cursor + 4) == '\n') {
-      throw new ParsingException("Status code misses trailing space")
+    } else if (isNewLine(cursor + 3)) {
+      // Status format with no reason phrase and no trailing space accepted, diverging from the spec
+      // See https://github.com/akka/akka-http/pull/989
+      skipNewLine(cursor + 3)
     } else badStatusCode
   }
 

akka-http-core/src/test/scala/akka/http/impl/engine/parsing/ResponseParserSpec.scala

@@ -89,6 +89,11 @@ abstract class ResponseParserSpec(mode: String, newLine: String) extends FreeSpe
         closeAfterResponseCompletion shouldEqual Seq(false)
       }
 
+      "a response with no reason phrase and no trailing space" in new Test {
+        s"""HTTP/1.1 200${newLine}Content-Length: 0${newLine}${newLine}""".stripMargin should parseTo(HEAD, HttpResponse())
+        closeAfterResponseCompletion shouldEqual Seq(false)
+      }
+
       "a response funky `Transfer-Encoding` header" in new Test {
         override def parserSettings: ParserSettings =
           super.parserSettings.withCustomStatusCodes(ServerOnTheMove)
@@ -242,11 +247,6 @@ abstract class ResponseParserSpec(mode: String, newLine: String) extends FreeSpe
         Seq("HTTP/1.1 204 12345678", s"90123456789012${newLine}") should generalMultiParseTo(Left(
           MessageStartError(400: StatusCode, ErrorInfo("Response reason phrase exceeds the configured limit of 21 characters"))))
       }
-
-      "with a missing reason phrase and no trailing space" in new Test {
-        Seq(s"HTTP/1.1 200${newLine}Content-Length: 0${newLine}${newLine}") should generalMultiParseTo(Left(MessageStartError(
-          400: StatusCode, ErrorInfo("Status code misses trailing space"))))
-      }
     }
   }