Merge pull request #3029 from daltonmaag/wrap-glif-xml-errors

[glifLib] Wrap XML library exceptions with glifLib types when parsing glifs

Lib/fontTools/ufoLib/errors.py

@@ -1,3 +1,6 @@
+from __future__ import annotations
+
+
 class UFOLibError(Exception):
     pass
 
@@ -7,7 +10,12 @@ class UnsupportedUFOFormat(UFOLibError):
 
 
 class GlifLibError(UFOLibError):
-    pass
+    def _add_note(self, note: str) -> None:
+        # Loose backport of PEP 678 until we only support Python 3.11+, used for
+        # adding additional context to errors.
+        # TODO: Replace with https://docs.python.org/3.11/library/exceptions.html#BaseException.add_note
+        (message, *rest) = self.args
+        self.args = ((message + "\n" + note), *rest)
 
 
 class UnsupportedGLIFFormat(GlifLibError):

Lib/fontTools/ufoLib/glifLib.py

@@ -416,17 +416,33 @@ class GlyphSet(_UFOBaseIO):
         if validate is None:
             validate = self._validateRead
         text = self.getGLIF(glyphName)
-        tree = _glifTreeFromString(text)
-        formatVersions = GLIFFormatVersion.supported_versions(
-            self.ufoFormatVersionTuple
-        )
-        _readGlyphFromTree(
-            tree,
-            glyphObject,
-            pointPen,
-            formatVersions=formatVersions,
-            validate=validate,
-        )
+        try:
+            tree = _glifTreeFromString(text)
+            formatVersions = GLIFFormatVersion.supported_versions(
+                self.ufoFormatVersionTuple
+            )
+            _readGlyphFromTree(
+                tree,
+                glyphObject,
+                pointPen,
+                formatVersions=formatVersions,
+                validate=validate,
+            )
+        except GlifLibError as glifLibError:
+            # Re-raise with a note that gives extra context, describing where
+            # the error occurred.
+            fileName = self.contents[glyphName]
+            try:
+                glifLocation = f"'{self.fs.getsyspath(fileName)}'"
+            except fs.errors.NoSysPath:
+                # Network or in-memory FS may not map to a local path, so use
+                # the best string representation we have.
+                glifLocation = f"'{fileName}' from '{str(self.fs)}'"
+
+            glifLibError._add_note(
+                f"The issue is in glyph '{glyphName}', located in {glifLocation}."
+            )
+            raise
 
     def writeGlyph(
         self,
@@ -1082,10 +1098,14 @@ 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 os
 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 +135,34 @@ class GlyphSetTests(unittest.TestCase):
             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. In addition, check that the raised exception describes
+        the glyph by name and gives the location of the broken .glif file."""
+
+        # 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 /a and /b is fine...
+        glyph_set.readGlyph("a", _Glyph())
+        glyph_set.readGlyph("b", _Glyph())
+
+        # ...but that reading /c raises a descriptive library error.
+        expected_message = (
+            r"GLIF contains invalid XML\.\n"
+            r"The issue is in glyph 'c', located in '.*c\.glif.*\."
+        )
+        with pytest.raises(GlifLibError, match=expected_message):
+            glyph_set.readGlyph("c", _Glyph())
+
 
 class FileNameTest:
     def test_default_file_name_scheme(self):
@@ -228,6 +257,17 @@ class ReadWriteFuncTest:
         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">