Fix connections management in http client. (#246)

* Proper connection lifetime handling mechanism.
HttpClientResponse do not close connection anymore.

* Add unique HTTP client connection identifier.

chronos/apps/http/httpclient.nim

@@ -89,6 +89,7 @@ type
     data: seq[byte]
 
   HttpClientConnection* = object of RootObj
+    id*: uint64
     case kind*: HttpClientScheme
     of HttpClientScheme.NonSecure:
       discard
@@ -107,6 +108,7 @@ type
 
   HttpSessionRef* = ref object
     connections*: Table[string, seq[HttpClientConnectionRef]]
+    counter*: uint64
     maxRedirections*: int
     connectTimeout*: Duration
     headersTimeout*: Duration
@@ -452,11 +454,16 @@ proc redirect*(session: HttpSessionRef,
       addresses: srcaddr.addresses
     ))
 
+proc getUniqueConnectionId(session: HttpSessionRef): uint64 =
+  inc(session.counter)
+  session.counter
+
 proc new(t: typedesc[HttpClientConnectionRef], session: HttpSessionRef,
          ha: HttpAddress, transp: StreamTransport): HttpClientConnectionRef =
   case ha.scheme
   of HttpClientScheme.NonSecure:
     let res = HttpClientConnectionRef(
+      id: session.getUniqueConnectionId(),
       kind: HttpClientScheme.NonSecure,
       transp: transp,
       reader: newAsyncStreamReader(transp),
@@ -472,6 +479,7 @@ proc new(t: typedesc[HttpClientConnectionRef], session: HttpSessionRef,
     let tls = newTLSClientAsyncStream(treader, twriter, ha.hostname,
                                       flags = session.flags.getTLSFlags())
     let res = HttpClientConnectionRef(
+      id: session.getUniqueConnectionId(),
       kind: HttpClientScheme.Secure,
       transp: transp,
       treader: treader,
@@ -639,14 +647,39 @@ proc removeConnection(session: HttpSessionRef,
   await conn.closeWait()
 
 proc releaseConnection(session: HttpSessionRef,
-                       conn: HttpClientConnectionRef) {.async.} =
+                       conn: HttpClientConnectionRef,
+                       forceRemove = false) {.async.} =
   ## Return connection back to the ``session``.
   ##
   ## If connection not in ``Ready`` state it will be closed and removed from
   ## the ``session``.
+  if forceRemove:
+    await session.removeConnection(conn)
+  else:
     if conn.state != HttpClientConnectionState.Ready:
       await session.removeConnection(conn)
 
+proc needKeepConnection(request: HttpClientRequestRef): bool =
+  ## Returns ``true`` if the request's corresponding connection should be kept
+  ## alive, and ``false`` if this connection should be dropped.
+  case request.state
+  of HttpClientRequestState.ResponseReceived:
+    case request.version
+    of HttpVersion11, HttpVersion20:
+      let connection = toLowerAscii(request.headers.getString(ConnectionHeader))
+      if connection == "keep-alive":
+        true
+      else:
+        # If `Connection` header is missing or its value not equal to
+        # `keep-alive`.
+        false
+    else:
+      # Versions prior HTTP 1.1 do not support request pipelining.
+      false
+  else:
+    # Request not in proper state.
+    false
+
 proc closeWait*(session: HttpSessionRef) {.async.} =
   ## Closes HTTP session object.
   ##
@@ -665,9 +698,11 @@ proc closeWait*(request: HttpClientRequestRef) {.async.} =
       if not(request.writer.closed()):
         await request.writer.closeWait()
       request.writer = nil
-    if request.state != HttpClientRequestState.ResponseReceived:
     if not(isNil(request.connection)):
-        await request.session.releaseConnection(request.connection)
+      if request.needKeepConnection():
+        await request.session.releaseConnection(request.connection, false)
+      else:
+        await request.session.releaseConnection(request.connection, true)
       request.connection = nil
     request.session = nil
     request.error = nil
@@ -682,9 +717,6 @@ proc closeWait*(response: HttpClientResponseRef) {.async.} =
       if not(response.reader.closed()):
         await response.reader.closeWait()
       response.reader = nil
-    if not(isNil(response.connection)):
-      await response.session.releaseConnection(response.connection)
-      response.connection = nil
     response.session = nil
     response.error = nil
     response.setState(HttpClientResponseState.Closed)