Merge pull request #2404 from simoncozens/fix-contextual-overflow

[otlLib] Fix chained contextual builder overflow

Lib/fontTools/otlLib/builder.py

@@ -388,7 +388,20 @@ class ChainContextualBuilder(LookupBuilder):
             if not ruleset.hasAnyGlyphClasses:
                 candidates[1] = [self.buildFormat1Subtable(ruleset, chaining)]
 
+            for i in [1, 2, 3]:
+                try:
+                    self.getCompiledSize_(candidates[i])
+                except Exception as e:
+                    log.warning(
+                        "Contextual format %i at %s overflowed (%s)"
+                        % (i, str(self.location), e)
+                    )
+                    candidates[i] = None
+
             candidates = [x for x in candidates if x is not None]
+            if not candidates:
+                raise OpenTypeLibError("All candidates overflowed", self.location)
+
             winner = min(candidates, key=self.getCompiledSize_)
             subtables.extend(winner)