Fix for nim-lang#18161 Too many open files.

I've been checking the code in asynchttpserver file and found that in the processClient function the connection is never closed after the "while" loop. So even if the keepalive header is off the number of open files is always increasing. With the keepalive off and if the connection is closed after the "while" loop the server never dies.

If the header keepalive is on and if it exceeds the pre-defined number of file descriptors it automatically closes the connection! I also added a timeout for keepalive connections.

lib/pure/asynchttpserver.nim

@@ -44,6 +44,7 @@
 
 import asyncnet, asyncdispatch, parseutils, uri, strutils
 import httpcore
+from times import DateTime, now, `-`, inSeconds
 
 export httpcore except parseHeader
 
@@ -289,6 +290,9 @@ proc processRequest(
     request.client.close()
     return false
 
+const
+  keepaliveTimeout* {.intdefine.} = 60 ### default value for keepaliveTimeout in seconds
+
 proc processClient(server: AsyncHttpServer, client: AsyncSocket, address: string,
                    callback: proc (request: Request):
                       Future[void] {.closure, gcsafe.}) {.async.} =
@@ -298,12 +302,20 @@ proc processClient(server: AsyncHttpServer, client: AsyncSocket, address: string
   var lineFut = newFutureVar[string]("asynchttpserver.processClient")
   lineFut.mget() = newStringOfCap(80)
 
+  let startTimeout = now()
   while not client.isClosed:
+    let fds = activeDescriptors()
+    # The maxFDs should be replaced by the keepaliveConn?
+    if (fds > server.maxFDs) or ((now() - startTimeout).inSeconds > keepaliveTimeout):
+      break # Connection timeout
+
     let retry = await processRequest(
       server, request, client, address, lineFut, callback
     )
     if not retry: break
 
+  client.close() # Close the connection to not increase the number of open files. 
+
 const
   nimMaxDescriptorsFallback* {.intdefine.} = 16_000 ## fallback value for \
     ## when `maxDescriptors` is not available.
@@ -339,6 +351,7 @@ proc acceptRequest*(server: AsyncHttpServer,
   var (address, client) = await server.socket.acceptAddr()
   asyncCheck processClient(server, client, address, callback)
 
+
 proc serve*(server: AsyncHttpServer, port: Port,
             callback: proc (request: Request): Future[void] {.closure, gcsafe.},
             address = "";
@@ -365,6 +378,7 @@ proc serve*(server: AsyncHttpServer, port: Port,
     #echo(f.isNil)
     #echo(f.repr)
 
+
 proc close*(server: AsyncHttpServer) =
   ## Terminates the async http server instance.
   server.socket.close()