AsyncHttpClient: return from requests before body completion

Store the body completion future at the client and wait for it to
complete before issuing additional requests. This allows the body
FutureStream reader to drain the stream and read buffers to be freed
asynchronously.

Fix #8109

lib/pure/httpclient.nim

@@ -807,6 +807,7 @@ type
     lastProgressReport: float
     when SocketType is AsyncSocket:
       bodyStream: FutureStream[string]
+      parseBodyFut: Future[void]
     else:
       bodyStream: Stream
     getBody: bool ## When `false`, the body is never read in requestAux.
@@ -1066,10 +1067,14 @@ proc parseResponse(client: HttpClient | AsyncHttpClient,
   if getBody:
     when client is HttpClient:
       client.bodyStream = newStringStream()
+      result.bodyStream = client.bodyStream
+      parseBody(client, result.headers, result.version)
     else:
       client.bodyStream = newFutureStream[string]("parseResponse")
-    await parseBody(client, result.headers, result.version)
-    result.bodyStream = client.bodyStream
+      result.bodyStream = client.bodyStream
+      assert(client.parseBodyFut.isNil or client.parseBodyFut.finished)
+      client.parseBodyFut = parseBody(client, result.headers, result.version)
+        # do not wait here for the body request to complete
 
 proc newConnection(client: HttpClient | AsyncHttpClient,
                    url: Uri) {.multisync.} =
@@ -1159,6 +1164,12 @@ proc requestAux(client: HttpClient | AsyncHttpClient, url: string,
   # Helper that actually makes the request. Does not handle redirects.
   let requestUrl = parseUri(url)
 
+  when client is AsyncHttpClient:
+    if not client.parseBodyFut.isNil:
+      # let the current operation finish before making another request
+      await client.parseBodyFut
+      client.parseBodyFut = nil
+
   await newConnection(client, requestUrl)
 
   let effectiveHeaders = client.headers.override(headers)