markFeatureWriter: Catch and skip contextual anchors with no context

Instead of raising KeyError trying to access non existing context.

Lib/ufo2ft/featureWriters/markFeatureWriter.py

@@ -744,7 +744,14 @@ def _makeContextualAttachments(self, glyphClass, liga=False):
                 # If we are building the mark2base lookup, skip anchors with a number
                 if not liga and anchor.number is not None:
                     continue
-                anchor_context = anchor.libData["GPOS_Context"].strip()
+                anchor_context = anchor.libData.get("GPOS_Context", "").strip()
+                if not anchor_context:
+                    self.log.warning(
+                        "contextual anchor '%s' in glyph '%s' has no context data; skipped",
+                        anchor.name,
+                        glyphName,
+                    )
+                    continue
                 result[anchor_context].append((glyphName, anchor))
         return result
 

tests/featureWriters/markFeatureWriter_test.py

@@ -1968,6 +1968,55 @@ def test_contextual_liga_anchors(self, testufo):
             """
         )
 
+    def test_contextual_anchor_no_context(self, testufo, caplog):
+        a = testufo["a"]
+        a.appendAnchor({"name": "*top", "x": 200, "y": 200, "identifier": "*top"})
+        a.lib[OBJECT_LIBS_KEY] = {"*top": {"foo": "bar"}}
+
+        writer = MarkFeatureWriter()
+        feaFile = ast.FeatureFile()
+        assert str(feaFile) == ""
+
+        logger = "ufo2ft.featureWriters.markFeatureWriter.MarkFeatureWriter"
+        with caplog.at_level(logging.WARNING, logger=logger):
+            assert writer.write(testufo, feaFile)
+        assert len(caplog.records) == 1
+        assert (
+            "contextual anchor '*top' in glyph 'a' has no context data; skipped"
+            in caplog.text
+        )
+        assert str(feaFile) == dedent(
+            """\
+            markClass acutecomb <anchor 100 200> @MC_top;
+            markClass tildecomb <anchor 100 200> @MC_top;
+
+            feature mark {
+                lookup mark2base {
+                    pos base a
+                        <anchor 100 200> mark @MC_top;
+                } mark2base;
+
+                lookup mark2liga {
+                    pos ligature f_i
+                            <anchor 100 500> mark @MC_top
+                        ligComponent
+                            <anchor 600 500> mark @MC_top;
+                } mark2liga;
+
+            } mark;
+
+            feature mkmk {
+                lookup mark2mark_top {
+                    @MFS_mark2mark_top = [acutecomb tildecomb];
+                    lookupflag UseMarkFilteringSet @MFS_mark2mark_top;
+                    pos mark tildecomb
+                        <anchor 100 300> mark @MC_top;
+                } mark2mark_top;
+
+            } mkmk;
+            """
+        )
+
     def test_ignorable_anchors(self, FontClass):
         dirname = os.path.dirname(os.path.dirname(__file__))
         fontPath = os.path.join(dirname, "data", "IgnoreAnchorsTest-Thin.ufo")