check that bitmap is actually a PNG file

src/nanoemoji/color_glyph.py

@@ -31,6 +31,7 @@
     PaintRadialGradient,
     PaintSolid,
 )
+from nanoemoji.png import PNG
 from picosvg.geometric_types import Point, Rect
 from picosvg.svg_meta import number_or_percentage
 from picosvg.svg_reuse import normalize, affine_between
@@ -402,7 +403,7 @@ class ColorGlyph(NamedTuple):
     painted_layers: Optional[Tuple[Paint, ...]]  # None for untouched and bitmap formats
     svg: Optional[SVG]  # None for bitmap formats
     user_transform: Affine2D
-    bitmap: Optional[bytes]  # None for vector formats
+    bitmap: Optional[PNG]  # None for vector formats
 
     @staticmethod
     def create(
@@ -413,7 +414,7 @@ def create(
         ufo_glyph_name: str,
         codepoints: Tuple[int, ...],
         svg: Optional[SVG],
-        bitmap: Optional[bytes] = None,
+        bitmap: Optional[PNG] = None,
     ) -> "ColorGlyph":
         logging.debug(" ColorGlyph for %s (%s)", ", ".join(filenames), codepoints)
         base_glyph = ufo.newGlyph(ufo_glyph_name)

src/nanoemoji/png.py

@@ -0,0 +1,28 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+class PNG(bytes):
+
+    # The first eight bytes of a PNG file always contain the following (decimal) values:
+    #   137 80 78 71 13 10 26 10
+    # https://www.w3.org/TR/PNG-Structure.html
+    SIGNATURE = b"\x89PNG\r\n\x1a\n"
+
+    def __new__(cls, *args, **kwargs):
+        self = super().__new__(cls, *args, **kwargs)
+        header = self[:8]
+        if header != cls.SIGNATURE:
+            raise ValueError("Invalid PNG file: bad signature {header!r}")
+        return self

src/nanoemoji/write_font.py

@@ -51,6 +51,7 @@
     PaintGlyph,
     PaintSolid,
 )
+from nanoemoji.png import PNG
 from nanoemoji.svg import make_svg_table
 from nanoemoji.svg_path import draw_svg_path
 from nanoemoji import util
@@ -92,7 +93,7 @@ class InputGlyph(NamedTuple):
     codepoints: Tuple[int, ...]
     glyph_name: str
     svg: Optional[SVG]  # None for bitmap formats
-    bitmap: Optional[bytes]  # None for vector formats
+    bitmap: Optional[PNG]  # None for vector formats
 
 
 # A color font generator.
@@ -784,7 +785,7 @@ def _inputs(
         if font_config.has_bitmaps:
             if png_file is None:
                 raise ValueError(f"No {file_stem}.png among inputs")
-            bitmap = png_file.read_bytes()
+            bitmap = PNG(png_file.read_bytes())
 
         filenames = []
         if svg_file is not None:

tests/png_test.py

@@ -0,0 +1,22 @@
+from nanoemoji.png import PNG
+import pytest
+
+
+def test_good_png_signature():
+    # example from https://en.wikipedia.org/wiki/Portable_Network_Graphics
+    data = bytes(
+        int(b, 16)
+        for b in (
+            "89 50 4E 47 0D 0A 1A 0A 00 00 00 0D 49 48 44 52 "
+            "00 00 00 01 00 00 00 01 08 02 00 00 00 90 77 53 "
+            "DE 00 00 00 0C 49 44 41 54 08 D7 63 F8 CF C0 00 "
+            "00 03 01 01 00 18 DD 8D B0 00 00 00 00 49 45 4E "
+            "44 AE 42 60 82"
+        ).split()
+    )
+    assert PNG(data) == data
+
+
+def test_bad_png_signature():
+    with pytest.raises(ValueError, match="bad signature"):
+        PNG(b"<?xml")

tests/test_helper.py

@@ -26,6 +26,7 @@
 from nanoemoji import features
 from nanoemoji.glyph import glyph_name
 from nanoemoji import write_font
+from nanoemoji.png import PNG
 from pathlib import Path
 from picosvg.svg import SVG
 import pytest
@@ -64,7 +65,7 @@ def rasterize_svg(filename, bitmap_resolution):
                 output_file,
             ]
         )
-        return output_file.read_bytes()
+        return PNG(output_file.read_bytes())
 
 
 def color_font_config(config_overrides, svgs, tmp_dir=None):