Don’t crash when the font size is really small

Fix #1499.

tests/test_text.py

@@ -99,6 +99,23 @@ def test_text_font_size_zero():
     assert paragraph.height == 0
 
 
+@assert_no_logs
+def test_text_font_size_very_small():
+    # Test regression: https://github.com/Kozea/WeasyPrint/issues/1499
+    page, = render_pages('''
+      <style>
+        p { font-size: 0.00000001px }
+      </style>
+      <p>test font size zero</p>
+    ''')
+    html, = page.children
+    body, = html.children
+    paragraph, = body.children
+    line, = paragraph.children
+    assert line.height < 0.001
+    assert paragraph.height < 0.001
+
+
 @assert_no_logs
 def test_text_spaced_inlines():
     page, = render_pages('''

weasyprint/document.py

@@ -91,12 +91,15 @@ class Font:
             b'/' + self.hash.encode('ascii') + b'+' +
             self.family.replace(b' ', b''))
         self.italic_angle = 0  # TODO: this should be different
-        self.ascent = int(
-            pango.pango_font_metrics_get_ascent(pango_metrics) /
-            font_size * 1000)
-        self.descent = -int(
-            pango.pango_font_metrics_get_descent(pango_metrics) /
-            font_size * 1000)
+        if font_size:
+            self.ascent = int(
+                pango.pango_font_metrics_get_ascent(pango_metrics) /
+                font_size * 1000)
+            self.descent = -int(
+                pango.pango_font_metrics_get_descent(pango_metrics) /
+                font_size * 1000)
+        else:
+            self.ascent = self.descent = 0
         self.upem = harfbuzz.hb_face_get_upem(hb_face)
         self.png = harfbuzz.hb_ot_color_has_png(hb_face)
         self.svg = harfbuzz.hb_ot_color_has_svg(hb_face)