Fix for nim-lang#18161 AsyncHttpServer Issue: Too many open files, against devel branch.

The client header "connection" if equal to "keep-alive" is disabled whenever the number of file descriptors exceeds the specified maxFDs or exceeds the keep-alive timeout.
If the value of header "connection" equal "close", the connection is closed, if it is "keep-alive" it remains open and the number of file descriptors increases and the server die.
To avoid this issue the "keep-alive" connection is disabled. It is temporary until the number of files descriptors goes down, when the value goes down the connection can remain open. It also solves the issue when the "connection" is equal to "update".

Author: mrhdias

lib/pure/asynchttpserver.nim

@@ -43,6 +43,7 @@ import asyncnet, asyncdispatch, parseutils, uri, strutils
 import httpcore
 from nativesockets import getLocalAddr, AF_INET
 import std/private/since
+from times import DateTime, now, `-`, inSeconds
 
 export httpcore except parseHeader
 
@@ -63,13 +64,16 @@ type
     url*: Uri
     hostname*: string    ## The hostname of the client that made the request.
     body*: string
+    disableKeepalive: bool
 
   AsyncHttpServer* = ref object
     socket: AsyncSocket
     reuseAddr: bool
     reusePort: bool
     maxBody: int ## The maximum content-length that will be read for the body.
     maxFDs: int
+    keepaliveTimeout: int ## The value for keepalive timeout in seconds and \
+      ## after which the connection will be closed.
 
 proc getPort*(self: AsyncHttpServer): Port {.since: (1, 5, 1).} =
   ## Returns the port `self` was bound to.
@@ -85,9 +89,10 @@ proc getPort*(self: AsyncHttpServer): Port {.since: (1, 5, 1).} =
   result = getLocalAddr(self.socket.getFd, AF_INET)[1]
 
 proc newAsyncHttpServer*(reuseAddr = true, reusePort = false,
-                         maxBody = 8388608): AsyncHttpServer =
+                         maxBody = 8388608, keepaliveTimeout = 3600): AsyncHttpServer =
   ## Creates a new `AsyncHttpServer` instance.
-  result = AsyncHttpServer(reuseAddr: reuseAddr, reusePort: reusePort, maxBody: maxBody)
+  result = AsyncHttpServer(reuseAddr: reuseAddr, reusePort: reusePort,
+    maxBody: maxBody, keepaliveTimeout: keepaliveTimeout)
 
 proc addHeaders(msg: var string, headers: HttpHeaders) =
   for k, v in headers:
@@ -336,7 +341,8 @@ proc processRequest(
   # connection will not be closed and will be kept in the connection pool.
 
   # Persistent connections
-  if (request.protocol == HttpVer11 and
+  if (not request.disableKeepalive and
+      request.protocol == HttpVer11 and
       cmpIgnoreCase(request.headers.getOrDefault("connection"), "close") != 0) or
      (request.protocol == HttpVer10 and
       cmpIgnoreCase(request.headers.getOrDefault("connection"), "keep-alive") == 0):
@@ -355,15 +361,24 @@ proc processClient(server: AsyncHttpServer, client: AsyncSocket, address: string
   var request = newFutureVar[Request]("asynchttpserver.processClient")
   request.mget().url = initUri()
   request.mget().headers = newHttpHeaders()
+  request.mget().disableKeepalive = false
   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 > server.keepaliveTimeout):
+      request.mget().disableKeepalive = true
+
     let retry = await processRequest(
       server, request, client, address, lineFut, callback
     )
     if not retry: break
 
+  client.close() # When in doubt, close the connection to not increase the number of open files. 
+
 const
   nimMaxDescriptorsFallback* {.intdefine.} = 16_000 ## fallback value for \
     ## when `maxDescriptors` is not available.