Fix fragmented message sending (#116)

We previously crashed when sending things over 1mb

tests/testwebsockets.nim

@@ -783,11 +783,11 @@ suite "Test Payload":
 
   asyncTest "Send text message with multiple frames":
     const FrameSize = 3000
-    let testData = rndStr(FrameSize * 3)
+    let testData = rndStr(FrameSize * 3 + 100)
     proc handle(request: HttpRequest) {.async.} =
       check request.uri.path == WSPath
 
-      let server = WSServer.new(protos = ["proto"])
+      let server = WSServer.new(protos = ["proto"], frameSize = FrameSize)
       let ws = await server.handleRequest(request)
       let res = await ws.recvMsg()
 

websock/session.nim

@@ -37,21 +37,21 @@ proc writeMessage*(ws: WSSession,
   let maxSize = ws.frameSize
   var i = 0
   while ws.readyState notin {ReadyState.Closing}:
-    let len = min(data.len, maxSize)
+    let canSend = min(data.len - i, maxSize)
     let frame = Frame(
-        fin: if (len + i >= data.len): true else: false,
+        fin: if (canSend + i >= data.len): true else: false,
         rsv1: false,
         rsv2: false,
         rsv3: false,
         opcode: if i > 0: Opcode.Cont else: opcode, # fragments have to be `Continuation` frames
         mask: ws.masked,
-        data: data[i ..< len + i],
+        data: data[i ..< canSend + i],
         maskKey: maskKey)
 
     let encoded = await frame.encode(extensions)
     await ws.stream.writer.write(encoded)
 
-    i += len
+    i += canSend
     if i >= data.len:
       break