WebviewWidget: avoid hangs with webengine

Waiting for execution to complete was handled in an unsafe way. As all
js executions set the same flag when they were done, it was possible
that multiple executions set the flag before the thread waiting for them
could have noticed it. As a result, if two or more executions ended at
the same time the first waiting thread to notice that would clear the
flag and remaining threads would keep waiting indefinetly.

Should fix ocasionally stalled PyQt5 builds on travis.

Orange/widgets/utils/webview.py

@@ -5,6 +5,7 @@
 (extends QWebView), as available.
 """
 import os
+import threading
 from os.path import join, dirname, abspath
 import warnings
 from random import random
@@ -444,6 +445,36 @@ def exposeObject(self, name, obj):
 
 
 elif HAVE_WEBENGINE:
+    class IdStore:
+        """Generates and stores unique ids.
+
+        Used in WebviewWidget._evalJS below to match scheduled js executions
+        and returned results. WebEngine operations are async, so locking is
+        used to guard against problems that could occur if multiple executions
+        ended at exactly the same time.
+        """
+
+        def __init__(self):
+            self.id = 0
+            self.lock = threading.Lock()
+            self.ids = set()
+
+        def create(self):
+            with self.lock:
+                self.id += 1
+                return self.id
+
+        def store(self, id):
+            with self.lock:
+                self.ids.add(id)
+
+        def __contains__(self, id):
+            return id in self.ids
+
+        def remove(self, id):
+            with self.lock:
+                self.ids.remove(id)
+
 
     class _JSObjectChannel(QObject):
         """ This class hopefully prevent options data from being
@@ -487,7 +518,6 @@ def is_all_exposed(self):
     @inherit_docstrings
     class WebviewWidget(_WebViewBase, WebEngineView):
         _html = _NOTSET
-        _result = _NOTSET
 
         def __init__(self, parent=None, bridge=None, *, debug=False, **kwargs):
             WebEngineView.__init__(self, parent, bridge, debug=debug, **kwargs)
@@ -502,18 +532,18 @@ def __init__(self, parent=None, bridge=None, *, debug=False, **kwargs):
             self._jsobject_channel = jsobj = _JSObjectChannel(self)
             self.page().webChannel().registerObject(
                 '__js_object_channel', jsobj)
+            self._results = IdStore()
 
         def _evalJS(self, code):
             while not self._jsobject_channel.is_all_exposed():
                 qApp.processEvents(QEventLoop.ExcludeUserInputEvents)
             if sip.isdeleted(self):
                 return
-            self.runJavaScript(code,
-                               lambda result: setattr(self, '_result', result))
-            while self._result is _NOTSET:
+            result = self._results.create()
+            self.runJavaScript(code, lambda x: self._results.store(result))
+            while result not in self._results:
                 qApp.processEvents(QEventLoop.ExcludeUserInputEvents)
-            result, self._result = self._result, _NOTSET
-            return result
+            self._results.remove(result)
 
         def onloadJS(self, code):
             self._onloadJS(code, injection_point=QWebEngineScript.Deferred)

Orange/widgets/visualize/owmap.py

@@ -370,7 +370,7 @@ def _update_js_markers(self, visible, in_subset):
                                    raw_values=self._raw_sizes[visible]))
         self.evalJS('''
             window.latlon_data = latlon_data.data;
-            window.selected_markers = selected_markers.data
+            window.selected_markers = selected_markers.data;
             add_markers(latlon_data);
         ''')