Avoid mutation of TTFont to fix issues with concurrent tests

Although this mutation is reversed, it could still interfere with TTFont
instances being used concurrently by other tests.

Possible fix for #4834

Lib/fontbakery/checks/glyphset.py

@@ -1,3 +1,5 @@
+from copy import deepcopy
+
 from fontbakery.constants import (
     NameID,
     PlatformID,
@@ -370,6 +372,11 @@ def check_soft_hyphen(ttFont):
 def unreachable_glyphs(ttFont, config):
     """Check font contains no unreachable glyphs"""
 
+    # remove_lookup_outputs() mutates the TTF; deep copy to avoid this, and so
+    # avoid issues with concurrent tests that also use ttFont.
+    # See https://github.com/fonttools/fontbakery/issues/4834
+    ttFont = deepcopy(ttFont)
+
     def remove_lookup_outputs(all_glyphs, lookup):
         if lookup.LookupType == 1:  # Single:
             # Replace one glyph with one glyph

Lib/fontbakery/utils.py

@@ -17,6 +17,7 @@
 import subprocess
 import sys
 from typing import Text, Optional
+from copy import deepcopy
 
 from fontTools.pens.basePen import BasePen
 from fontTools.ttLib import TTFont
@@ -537,19 +538,19 @@ def iterate_lookup_list_with_extensions(ttFont, table, callback, *args):
     if table not in ttFont or not ttFont[table].table.LookupList:
         return
 
+    # This function mutates the TTF; deep copy to avoid this, and so avoid
+    # issues with concurrent tests that also use ttFont.
+    # See https://github.com/fonttools/fontbakery/issues/4834
+    ttFont = deepcopy(ttFont)
+
     extension_type = 9 if table == "GPOS" else 7
 
     for lookup in ttFont[table].table.LookupList.Lookup:
         if lookup.LookupType == extension_type:
             for xt in lookup.SubTable:
-                original_LookupType = xt.LookupType
-                try:
-                    xt.SubTable = [xt.ExtSubTable]
-                    xt.LookupType = xt.ExtSubTable.LookupType
-                    callback(xt, *args)
-                finally:
-                    del xt.SubTable
-                    xt.LookupType = original_LookupType
+                xt.SubTable = [xt.ExtSubTable]
+                xt.LookupType = xt.ExtSubTable.LookupType
+                callback(xt, *args)
         else:
             callback(lookup, *args)