Recommend completely removing the 'DSIG' table.

We may make this a FAIL by November 2023 when the EOL date for MS Office 2013 is reached.

OpenType profile: com.google.fonts/check/dsig
(issues fonttools#3398, fonttools#1845
and googlefonts/fontmake#431)

Author: felipesanches

CHANGELOG.md

@@ -6,8 +6,10 @@ A more detailed list of changes is available in the corresponding milestones for
   - Fix crash on is_OFL condition when a font project lacks a license. (issue #3393)
 
 ### Changes to existing checks
+#### OpenType Profile
+  - **[com.google.fonts/check/dsig]:** We now recommend (with a WARN) completely removing the 'DSIG' table. We may make this a FAIL by November 2023 when the EOL date for MS Office 2013 is reached. (issue #3398)
 #### Universal Profile
-  - **[com.google.fonts/check/required_tables]:** remove 'DSIG' from list of optional tables (issue #3398)
+  - **[com.google.fonts/check/required_tables]:** remove 'DSIG' from list of optional tables and improve wording on the check rationale. (issue #3398)
 
 
 ## 0.8.0 (2021-Jul-21)

Lib/fontbakery/profiles/dsig.py

@@ -1,5 +1,5 @@
 from fontbakery.callable import check
-from fontbakery.status import FAIL, PASS
+from fontbakery.status import WARN, PASS
 from fontbakery.message import Message
 # used to inform get_module_profile whether and how to create a profile
 from fontbakery.fonts_profile import profile_factory # NOQA pylint: disable=unused-import
@@ -9,23 +9,25 @@
     rationale = """
         Microsoft Office 2013 and below products expect fonts to have a digital signature declared in a DSIG table in order to implement OpenType features. The EOL date for Microsoft Office 2013 products is 4/11/2023. This issue does not impact Microsoft Office 2016 and above products.
 
-        This checks verifies that this signature is available in the font.
+        As we approach the EOL date, it is now considered better to completely remove the table.
 
-        A fake signature is enough to address this issue. If needed, a dummy table can be added to the font with the `gftools fix-dsig` script available at https://github.com/googlefonts/gftools
+        But if you still want your font to support OpenType features on Office 2013, then you may find it handy to add a fake signature on a dummy DSIG table by running one of the helper scripts provided at https://github.com/googlefonts/gftools
 
         Reference: https://github.com/googlefonts/fontbakery/issues/1845
     """,
-    proposal = 'legacy:check/045'
+    proposal = ['legacy:check/045',
+                'https://github.com/googlefonts/fontbakery/issues/3398']
 )
 def com_google_fonts_check_dsig(ttFont):
     """Does the font have a DSIG table?"""
-    if "DSIG" in ttFont:
-        yield PASS, "Digital Signature (DSIG) exists."
+    if "DSIG" not in ttFont:
+        yield PASS, "ok"
     else:
-        yield FAIL,\
-              Message("lacks-signature",
-                      "This font lacks a digital signature (DSIG table)."
-                      " Some applications may require one (even if only a"
-                      " dummy placeholder) in order to work properly. You"
-                      " can add a DSIG table by running the"
-                      " `gftools fix-dsig` script.")
+        yield WARN,\
+              Message("found-DSIG",
+                      "This font has a digital signature (DSIG table) which"
+                      " is only required - even if only a dummy placeholder"
+                      " - on old programs like MS Office 2013 in order to"
+                      " work properly.\n"
+                      "The current recommendation is to completely"
+                      " remove the DSIG table.")

tests/profiles/dsig_test.py

@@ -2,7 +2,7 @@
                                     assert_results_contain,
                                     CheckTester,
                                     TEST_FILE)
-from fontbakery.checkrunner import FAIL
+from fontbakery.checkrunner import WARN
 from fontbakery.profiles import opentype as opentype_profile
 
 from fontTools.ttLib import TTFont
@@ -13,15 +13,13 @@ def test_check_dsig():
     check = CheckTester(opentype_profile,
                         "com.google.fonts/check/dsig")
 
-    # Our reference Cabin Regular font is good (theres a DSIG table declared):
+    # Our reference Cabin Regular font is bad (theres a DSIG table declared):
     ttFont = TTFont(TEST_FILE("cabin/Cabin-Regular.ttf"))
+    assert_results_contain(check(ttFont),
+                           WARN, 'found-DSIG',
+                           'with a font containing a DSIG table...')
 
-    # So it must PASS the check:
+    # Then we remove the DSIG table and it should now PASS the check:
+    del ttFont['DSIG']
     assert_PASS(check(ttFont),
                 'with a good font...')
-
-    # Then we remove the DSIG table so that we get a FAIL:
-    del ttFont['DSIG']
-    assert_results_contain(check(ttFont),
-                           FAIL, 'lacks-signature',
-                           'with a font lacking a DSIG table...')