Bugfix: If the user's app exits (or crashes) before calling CloseHandle()

on the WinDivert handle, the driver's cleanup function cannot make FWPM
calls. The cleanup function detects this FWPM error and returns
without ever calling the Fwps cleanup functions (which are independent
of fwpm).

As a result, the driver will not unregister the callouts, which leaves
the Windows kernel confused. You can reproduce the problem by having
a user app close uncleanly, then "sc stop windivert1.2", then try to
re-run the app. You'll get "file not found" when StartService() is called.

sys/windivert.c

@@ -1133,8 +1133,12 @@ static void windivert_uninstall_callouts(context_t context)
     status = FwpmTransactionBegin0(context->engine_handle, 0);
     if (!NT_SUCCESS(status))
     {
+        // If the userspace app closes without closing the handle to
+        // WinDivert, any actions on engine_handle fail because the
+        // RPC handle was closed first. So, this path is "normal" if
+        // the user's app crashed or never closed the WinDivert handle.
         DEBUG_ERROR("failed to begin WFP transaction", status);
-        return;
+        goto unregister_callouts;
     }
     for (i = 0; i < WINDIVERT_CONTEXT_MAXLAYERS; i++)
     {
@@ -1162,15 +1166,15 @@ static void windivert_uninstall_callouts(context_t context)
     if (!NT_SUCCESS(status))
     {
         FwpmTransactionAbort0(context->engine_handle);
-        return;
+        goto unregister_callouts;
     }
     status = FwpmTransactionCommit0(context->engine_handle);
     if (!NT_SUCCESS(status))
     {
         DEBUG_ERROR("failed to commit WFP transaction", status);
-        return;
+        //fallthrough
     }
-
+unregister_callouts:
     for (i = 0; i < WINDIVERT_CONTEXT_MAXLAYERS; i++)
     {
         FwpsCalloutUnregisterByKey0(&context->callout_guid[i]);