Capture fontTools logs when subsetting fonts

Fix #2169.

tests/testing_utils.py

@@ -1,8 +1,6 @@
 """Helpers for tests."""
 
-import contextlib
 import functools
-import logging
 import sys
 from pathlib import Path
 
@@ -12,7 +10,7 @@ from weasyprint.css.counters import CounterStyle
 from weasyprint.css.targets import TargetCollector
 from weasyprint.formatting_structure import boxes, build
 from weasyprint.html import HTML5_UA_STYLESHEET
-from weasyprint.logger import LOGGER
+from weasyprint.logger import capture_logs
 from weasyprint.text.fonts import FontConfiguration
 from weasyprint.urls import path2url
 
@@ -73,35 +71,6 @@ BASE_URL = path2url(resource_path('<test>'))
 TEST_UA_FONT_CONFIG = FontConfiguration()
 TEST_UA_STYLESHEET = CSS(resource_path('tests_ua.css'), font_config=TEST_UA_FONT_CONFIG)
 
-class CallbackHandler(logging.Handler):
-    """A logging handler that calls a function for every message."""
-    def __init__(self, callback):
-        logging.Handler.__init__(self)
-        self.emit = callback
-
-
-@contextlib.contextmanager
-def capture_logs():
-    """Return a context manager that captures all logged messages."""
-    logger = LOGGER
-    messages = []
-
-    def emit(record):
-        if record.name == 'weasyprint.progress':
-            return
-        messages.append(f'{record.levelname.upper()}: {record.getMessage()}')
-
-    previous_handlers = logger.handlers
-    previous_level = logger.level
-    logger.handlers = []
-    logger.addHandler(CallbackHandler(emit))
-    logger.setLevel(logging.DEBUG)
-    try:
-        yield messages
-    finally:
-        logger.handlers = previous_handlers
-        logger.level = previous_level
-
 
 def assert_no_logs(function):
     """Decorator that asserts that nothing is logged in a function."""

weasyprint/logger.py

@@ -13,6 +13,7 @@ Logging levels are used for specific purposes:
 
 """
 
+import contextlib
 import logging
 
 LOGGER = logging.getLogger('weasyprint')
@@ -21,3 +22,35 @@ if not LOGGER.handlers:  # pragma: no cover
     LOGGER.addHandler(logging.NullHandler())
 
 PROGRESS_LOGGER = logging.getLogger('weasyprint.progress')
+
+
+class CallbackHandler(logging.Handler):
+    """A logging handler that calls a function for every message."""
+    def __init__(self, callback):
+        logging.Handler.__init__(self)
+        self.emit = callback
+
+
+@contextlib.contextmanager
+def capture_logs(logger='weasyprint', level=None):
+    """Return a context manager that captures all logged messages."""
+    logger = logging.getLogger(logger)
+    messages = []
+
+    def emit(record):
+        if record.name == 'weasyprint.progress':
+            return
+        if level is not None and record.levelno < level:
+            return
+        messages.append(f'{record.levelname.upper()}: {record.getMessage()}')
+
+    previous_handlers = logger.handlers
+    previous_level = logger.level
+    logger.handlers = []
+    logger.addHandler(CallbackHandler(emit))
+    logger.setLevel(logging.DEBUG)
+    try:
+        yield messages
+    finally:
+        logger.handlers = previous_handlers
+        logger.level = previous_level

weasyprint/pdf/fonts.py

@@ -2,6 +2,7 @@
 
 import io
 from hashlib import md5
+from logging import WARNING
 from math import ceil
 
 import pydyf
@@ -9,7 +10,7 @@ from fontTools import subset
 from fontTools.ttLib import TTFont, TTLibError, ttFont
 from fontTools.varLib.mutator import instantiateVariableFont
 
-from ..logger import LOGGER
+from ..logger import LOGGER, capture_logs
 from ..text.constants import PANGO_STRETCH_PERCENT
 from ..text.ffi import ffi, harfbuzz, harfbuzz_subset, pango, units_to_double
 from ..text.fonts import get_hb_object_data, get_pango_font_hb_face
@@ -257,7 +258,12 @@ class Font:
         # Subset font.
         try:
             ttfont = TTFont(full_font, fontNumber=self.index)
-            subsetter.subset(ttfont)
+            with capture_logs('fontTools', level=WARNING) as logs:
+                subsetter.subset(ttfont)
+            for log in logs:
+                LOGGER.warning(
+                    'fontTools warning when subsetting "%s": %s',
+                    self.family.decode(), log)
         except TTLibError:
             LOGGER.warning('Unable to subset font with fontTools')
         else: