Wrap XML library exceptions with glifLib types when parsing glifs

This allows dependent projects to catch errors parsing glifs without
requiring logic to account for which XML library fonttools is using
internally (e.g. for implementing fonttools/ufoLib2#264).

This commit also adds tests to ensure that the exception we expose when
glifs have invalid syntax remains stable across future releases.

Lib/fontTools/ufoLib/glifLib.py

@@ -1082,10 +1082,14 @@ def _glifTreeFromFile(aFile):
 
 def _glifTreeFromString(aString):
     data = tobytes(aString, encoding="utf-8")
-    if etree._have_lxml:
-        root = etree.fromstring(data, parser=etree.XMLParser(remove_comments=True))
-    else:
-        root = etree.fromstring(data)
+    try:
+        if etree._have_lxml:
+            root = etree.fromstring(data, parser=etree.XMLParser(remove_comments=True))
+        else:
+            root = etree.fromstring(data)
+    except Exception as etree_exception:
+        raise GlifLibError("GLIF contains invalid XML.") from etree_exception
+
     if root.tag != "glyph":
         raise GlifLibError("The GLIF is not properly formatted.")
     if root.text and root.text.strip() != "":

Tests/ufoLib/glifLib_test.py

@@ -3,6 +3,7 @@
 import tempfile
 import shutil
 import unittest
+from pathlib import Path
 from io import open
 from .testSupport import getDemoFontGlyphSetPath
 from fontTools.ufoLib.glifLib import (
@@ -134,6 +135,27 @@ def testGetUnicodes(self):
             else:
                 self.assertEqual(g.unicodes, unicodes[glyphName])
 
+    def testReadGlyphInvalidXml(self):
+        """Test that calling readGlyph() to read a .glif with invalid XML raises
+        a library error, instead of an exception from the XML dependency that is
+        used internally."""
+
+        # Create a glyph set with three empty glyphs.
+        glyph_set = GlyphSet(self.dstDir)
+        glyph_set.writeGlyph("a", _Glyph())
+        glyph_set.writeGlyph("b", _Glyph())
+        glyph_set.writeGlyph("c", _Glyph())
+
+        # Corrupt the XML of /c.
+        invalid_xml = b"<abc></def>"
+        Path(self.dstDir, glyph_set.contents["c"]).write_bytes(invalid_xml)
+
+        # Confirm that reading /c raises an appropriate error.
+        glyph_set.readGlyph("a", _Glyph())
+        glyph_set.readGlyph("b", _Glyph())
+        with pytest.raises(GlifLibError, match="GLIF contains invalid XML"):
+            glyph_set.readGlyph("c", _Glyph())
+
 
 class FileNameTest:
     def test_default_file_name_scheme(self):
@@ -228,6 +250,17 @@ def test_parse_xml_remove_comments(self):
         assert g.width == 1290
         assert g.unicodes == [0x0041]
 
+    def test_read_invalid_xml(self):
+        """Test that calling readGlyphFromString() with invalid XML raises a
+        library error, instead of an exception from the XML dependency that is
+        used internally."""
+
+        invalid_xml = b"<abc></def>"
+        empty_glyph = _Glyph()
+
+        with pytest.raises(GlifLibError, match="GLIF contains invalid XML"):
+            readGlyphFromString(invalid_xml, empty_glyph)
+
     def test_read_unsupported_format_version(self, caplog):
         s = """<?xml version='1.0' encoding='utf-8'?>
 		<glyph name="A" format="0" formatMinor="0">