It's not possible to serve HEAD request with Content-Lenght

[APshenkin]

In HttpMethods

akka-http/akka-http-core/src/main/scala/akka/http/scaladsl/model/HttpMethod.scala

Line 107 in 0fb5d36

val HEAD = register(HttpMethod("HEAD" , isSafe = true , isIdempotent = true , requestEntityAcceptance = Disallowed, contentLengthAllowed = contentLengthAllowedForHead))

For HEAD method contentLengthAllowed = contentLengthAllowedForHead, which return false for all cases.

This actually doesn't allow to specify Content-Lenght in responses, that may cause unexpected behaviours. For example if you serve Windows appInstaller, it tries to fetch HEAD to understand Content-Lenght to fetch later with Range Header. And without Content-Lenght it fails to install it.

[APshenkin]

Found a way to workaround this.

It's possible to create custom Header class

class HeadContentLength(override val value: String) extends CustomHeader {

  override def name(): String =
    "Content-Length"

  override def renderInRequests(): Boolean =
    true

  override def renderInResponses(): Boolean =
    true
}

and then add it to HttpResponse

HttpResponse(
  status  = StatusCodes.OK,
  headers = List(
    new HeadContentLength(contentLength.toString)
  ),
  entity  = HttpEntity.Default(
    contentType   = ContentType(MediaType.customBinary(
      "application",
      "appinstaller",
      MediaType.NotCompressible
    )),
    contentLength = contentLength,
    data          = Source.empty
  )
)

[johanandren]

Hmm, trying to remember from #4213: RFC 9110 says there must be no content in the response but it may send content length, and that:

Such a response to GET might contain Content-Length and Vary fields, for example, that are not generated within a HEAD response. These minor inconsistencies are considered preferable to generating and discarding the content for a HEAD request, since HEAD is usually requested for the sake of efficiency

The spec also says:

A server MUST NOT send a Content-Length header field in any response with a status code of 1xx (Informational) or 204 (No Content). A server MUST NOT send a Content-Length header field in any 2xx (Successful) response to a CONNECT request (Section 9.3.6).

It looks like there was an ambition to cover that but then conditional logic on the status code is missing here:

akka-http/akka-http-core/src/main/scala/akka/http/scaladsl/model/HttpMethod.scala

Line 94 in ccf4373

private def contentLengthAllowedForHead(forStatus: StatusCode): Boolean = false