Implement protection on callback environment.

lib/pure/asyncdispatch.nim

@@ -434,6 +434,9 @@ when defined(windows) or defined(nimdoc):
       fd*: AsyncFD # TODO: Rename this.
       cb*: proc (fd: AsyncFD, bytesTransferred: Dword,
                 errcode: OSErrorCode) {.closure,gcsafe.}
+      cell*: ForeignCell # we need this `cell` to protect our `cb` environment,
+                         # when using RegisterWaitForSingleObject, because
+                         # waiting is done in different thread.
 
     PDispatcher* = ref object of PDispatcherBase
       ioPort: Handle
@@ -517,6 +520,13 @@ when defined(windows) or defined(nimdoc):
 
       customOverlapped.data.cb(customOverlapped.data.fd,
           lpNumberOfBytesTransferred, OSErrorCode(-1))
+
+      # If cell.data != nil, then system.protect(rawEnv(cb)) was called,
+      # so we need to dispose our `cb` environment, because it is not needed
+      # anymore.
+      if customOverlapped.data.cell.data != nil:
+        system.dispose(customOverlapped.data.cell)
+
       GC_unref(customOverlapped)
     else:
       let errCode = osLastError()
@@ -1026,6 +1036,10 @@ when defined(windows) or defined(nimdoc):
               # poll()
               GC_ref(pcd.ovl)
     )
+    # We need to protect our callback environment value, so GC will not free it
+    # accidentally.
+    ol.data.cell = system.protect(rawEnv(ol.data.cb))
+
     # This is main part of `hacky way` is using WSAEventSelect, so `hEvent`
     # will be signaled when appropriate `mask` events will be triggered.
     if wsaEventSelect(fd.SocketHandle, hEvent, mask) != 0: