Identify the culprit glif in read errors with a loose backport of PEP678

This commit annotates errors from GlyphSet.readGlyph() with the details
of the glyph that originated them (e.g. name, path to glif). This is
implemented with a loose backport of PEP678, to avoid adding a wrapper
error that would be less specific and would break API compatibility.

In addition, this commit adds a test to ensure that the new details are
present (specifically, in the case of parsing invalid XML).

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

@@ -416,6 +416,7 @@ class GlyphSet(_UFOBaseIO):
         if validate is None:
             validate = self._validateRead
         text = self.getGLIF(glyphName)
+        try:
             tree = _glifTreeFromString(text)
             formatVersions = GLIFFormatVersion.supported_versions(
                 self.ufoFormatVersionTuple
@@ -427,6 +428,21 @@ class GlyphSet(_UFOBaseIO):
                 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,

Tests/ufoLib/glifLib_test.py

@@ -138,7 +138,8 @@ class GlyphSetTests(unittest.TestCase):
     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."""
+        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)
@@ -150,10 +151,16 @@ class GlyphSetTests(unittest.TestCase):
         invalid_xml = b"<abc></def>"
         Path(self.dstDir, glyph_set.contents["c"]).write_bytes(invalid_xml)
 
-        # Confirm that reading /c raises an appropriate error.
+        # Confirm that reading /a and /b is fine...
         glyph_set.readGlyph("a", _Glyph())
         glyph_set.readGlyph("b", _Glyph())
-        with pytest.raises(GlifLibError, match="GLIF contains invalid XML"):
+
+        # ...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())