markFeatureWriter: Ignore contextual anchors

We don’t support contextual anchors in markFeatureWriter, but glyphsLib
subclasses it and adds support for them. However, since we are unaware
of contextual anchors, we end up with positioning statements with
duplicated mark positions like this:

    pos base beh-ar.init
        <anchor 91 -316> mark @MC_top
        <anchor 91 -226> mark @MC_top;

(one is the regular anchor, and the other is the contextual one). Which
makes no sense (feaLib shouldn’t probably allow the same mark class to
be used multiple times in the same statement).

This PR adds isContextual to NamedAnchor and ignores any such anchors
when writing pose statements. It is already set by glyphsLib’s
MarkFeatureWriter, but otherwise should not affect ufo2ft’s as it always
sets it to False.

Lib/ufo2ft/featureWriters/markFeatureWriter.py

@@ -162,7 +162,16 @@ def parseAnchorName(
 class NamedAnchor:
     """A position with a name, and an associated markClass."""
 
-    __slots__ = ("name", "x", "y", "isMark", "key", "number", "markClass")
+    __slots__ = (
+        "name",
+        "x",
+        "y",
+        "isMark",
+        "key",
+        "number",
+        "markClass",
+        "isContextual",
+    )
 
     # subclasses can customize these to use different anchor naming schemes
     markPrefix = MARK_PREFIX
@@ -190,6 +199,7 @@ def __init__(self, name, x, y, markClass=None):
         self.key = key
         self.number = number
         self.markClass = markClass
+        self.isContextual = False
 
     @property
     def markAnchorName(self):
@@ -620,6 +630,11 @@ def _makeMarkToBaseAttachments(self):
                     # skip '_1', '_2', etc. suffixed anchors for this lookup
                     # type; these will be are added in the mark2liga lookup
                     continue
+                if anchor.isContextual:
+                    # Skip contextual anchors. We don't support them in ufo2ft
+                    # and isContextual is always False, but subclasses may
+                    # handle them (e.g. glyphsLib’s MarkFeatureWriter).
+                    continue
                 assert not anchor.isMark
                 baseMarks.append(anchor)
             if not baseMarks:
@@ -640,6 +655,11 @@ def _makeMarkToMarkAttachments(self):
                 # skip anchors for which no mark class is defined
                 if anchor.markClass is None or anchor.isMark:
                     continue
+                if anchor.isContextual:
+                    # Skip contextual anchors. We don't support them in ufo2ft
+                    # and isContextual is always False, but subclasses may
+                    # handle them (e.g. glyphsLib’s MarkFeatureWriter).
+                    continue
                 if anchor.number is not None:
                     self.log.warning(
                         "invalid ligature anchor '%s' in mark glyph '%s'; " "skipped",
@@ -671,6 +691,11 @@ def _makeMarkToLigaAttachments(self):
                 if number is None:
                     # we handled these in the mark2base lookup
                     continue
+                if anchor.isContextual:
+                    # Skip contextual anchors. We don't support them in ufo2ft
+                    # and isContextual is always False, but subclasses may
+                    # handle them (e.g. glyphsLib’s MarkFeatureWriter).
+                    continue
                 # unnamed anchors with only a number suffix "_1", "_2", etc.
                 # are understood as the ligature component having <anchor NULL>
                 if not anchor.key: