Fix to asynchttpserver form data/body broken with nim-lang#13147

This fix add a new server option that allow the request body to be processed outside the asynchttpserver library to break big files into chunks of data. This change does not break anything and works with the jester and the rosencrantz.

The new server option "stream" when it is true uses the Future Streams otherwise all body is readed at once. The "stream" option by default is set to false.

The idea of this option I took from the python requests module (https://requests.readthedocs.io/en/v1.2.3/api/).
"When stream=True is set on the request, this avoids reading the content at once into memory for large responses. The chunk size is the number of bytes it should read into memory. This is not necessarily the length of each item returned as decoding can take place."

Example:
let server = newAsyncHttpServer(stream = true) # if true use Future Streams
waitFor server.serve(Port(8080), cb)

lib/pure/asynchttpserver.nim

@@ -75,7 +75,7 @@
 ##            bodyLength += data[1].len
 ##      await req.respond(Http200, htmlpage(contentLength, bodyLength))
 ##
-##    let server = newAsyncHttpServer(maxBody = 10485760) # 10 MB
+##    let server = newAsyncHttpServer(maxBody = 10485760, stream = true) # 10 MB
 ##    waitFor server.serve(Port(8080), cb)
 
 import tables, asyncnet, asyncdispatch, parseutils, uri, strutils
@@ -91,46 +91,34 @@ export httpcore except parseHeader
 
 const
   maxLine = 8*1024
-
-when (NimMajor, NimMinor) >= (1, 1):
-  const
-    chunkSize = 8*1024 ## This seems perfectly reasonable for default chunkSize.
-
-  type
-    Request* = object
-      client*: AsyncSocket # TODO: Separate this into a Response object?
-      reqMethod*: HttpMethod
-      headers*: HttpHeaders
-      protocol*: tuple[orig: string, major, minor: int]
-      url*: Uri
-      hostname*: string    ## The hostname of the client that made the request.
-      body*: string # For future removal
-      bodyStream*: FutureStream[string]
-else:
-  type
-    Request* = object
-      client*: AsyncSocket # TODO: Separate this into a Response object?
-      reqMethod*: HttpMethod
-      headers*: HttpHeaders
-      protocol*: tuple[orig: string, major, minor: int]
-      url*: Uri
-      hostname*: string    ## The hostname of the client that made the request.
-      body*: string
+  chunkSize = 8*1024 ## This seems perfectly reasonable for default chunkSize.
 
 type
+  Request* = object
+    client*: AsyncSocket # TODO: Separate this into a Response object?
+    reqMethod*: HttpMethod
+    headers*: HttpHeaders
+    protocol*: tuple[orig: string, major, minor: int]
+    url*: Uri
+    hostname*: string    ## The hostname of the client that made the request.
+    body*: string
+    bodyStream*: FutureStream[string]
+
   AsyncHttpServer* = ref object
     socket: AsyncSocket
     reuseAddr: bool
     reusePort: bool
     maxBody: int ## The maximum content-length that will be read for the body.
+    stream: bool ## By default, the body of the response is readed immediately
 
 proc newAsyncHttpServer*(reuseAddr = true, reusePort = false,
-                         maxBody = 8388608): AsyncHttpServer =
+                         maxBody = 8388608, stream = false): AsyncHttpServer =
   ## Creates a new ``AsyncHttpServer`` instance.
   new result
   result.reuseAddr = reuseAddr
   result.reusePort = reusePort
   result.maxBody = maxBody
+  result.stream = stream
 
 proc addHeaders(msg: var string, headers: HttpHeaders) =
   for k, v in headers:
@@ -213,13 +201,10 @@ proc processRequest(
   request.hostname.shallowCopy(address)
   assert client != nil
   request.client = client
-  when (NimMajor, NimMinor) >= (1, 1):
+  if server.stream:
     request.bodyStream = newFutureStream[string]()
-  # To uncomment in the future after compatibility issues
-  # with third parties are solved
-  # else:
-  #   request.body = ""
-  request.body = "" # Temporary fix for future removal
+  else:
+    request.body = ""
 
   # We should skip at least one empty line before the request
   # https://tools.ietf.org/html/rfc7230#section-3.5
@@ -315,7 +300,7 @@ proc processRequest(
         await request.respondError(Http413)
         return false
 
-      when (NimMajor, NimMinor) >= (1, 1):
+      if server.stream:
         var remainder = contentLength
         while remainder > 0:
           let readSize = min(remainder, chunkSize)