Fix #4279: Sphinx crashes with pickling error when run with multiple processes and remote image

CHANGES

@@ -24,6 +24,8 @@ Bugs fixed
 * #2298: automodule fails to document a class attribute
 * #4099: C++: properly link class reference to class from inside constructor
 * #4267: PDF build broken by Unicode U+2116 NUMERO SIGN character
+* #4279: Sphinx crashes with pickling error when run with multiple processes and
+  remote image
 
 Testing
 --------

sphinx/util/logging.py

@@ -82,6 +82,10 @@ def convert_serializable(records):
         r.msg = r.getMessage()
         r.args = ()
 
+        location = getattr(r, 'location', None)
+        if isinstance(location, nodes.Node):
+            r.location = get_node_location(location)
+
 
 class SphinxWarningLogRecord(logging.LogRecord):
     """Log record class supporting location"""
@@ -415,21 +419,26 @@ class WarningLogRecordTranslator(logging.Filter):
             else:
                 record.location = None
         elif isinstance(location, nodes.Node):
-            (source, line) = get_source_line(location)
-            if source and line:
-                record.location = "%s:%s" % (source, line)
-            elif source:
-                record.location = "%s:" % source
-            elif line:
-                record.location = "<unknown>:%s" % line
-            else:
-                record.location = None
+            record.location = get_node_location(location)
         elif location and ':' not in location:
             record.location = '%s' % self.app.env.doc2path(location)
 
         return True
 
 
+def get_node_location(node):
+    # type: (nodes.Node) -> str
+    (source, line) = get_source_line(node)
+    if source and line:
+        return "%s:%s" % (source, line)
+    elif source:
+        return "%s:" % source
+    elif line:
+        return "<unknown>:%s" % line
+    else:
+        return None
+
+
 class ColorizeFormatter(logging.Formatter):
     def format(self, record):
         # type: (logging.LogRecord) -> str