Catch KeyError when checking mode from PNG IHDR chunk (#9604)

Author: radarhere

Tests/test_file_png.py

@@ -124,6 +124,14 @@ def test_broken(self) -> None:
             with Image.open(test_file):
                 pass
 
+    def test_ihdr_unknown_mode(self) -> None:
+        fp = BytesIO(chunk(b"IHDR", b"\x00" * 13))
+        with PngImagePlugin.PngStream(fp) as png:
+            cid, pos, length = png.read()
+            png.call(cid, pos, length)
+
+            assert png.im_mode == ""
+
     def test_bad_text(self) -> None:
         # Make sure PIL can read malformed tEXt chunks (@PIL152)
 

src/PIL/PngImagePlugin.py

@@ -400,6 +400,9 @@ def __init__(self, fp: IO[bytes]) -> None:
 
         self.text_memory = 0
 
+    def __enter__(self) -> PngStream:
+        return self
+
     def check_text_memory(self, chunklen: int) -> None:
         self.text_memory += chunklen
         if self.text_memory > MAX_TEXT_MEMORY:
@@ -461,7 +464,7 @@ def chunk_IHDR(self, pos: int, length: int) -> bytes:
         self.im_size = i32(s, 0), i32(s, 4)
         try:
             self.im_mode, self.im_rawmode = _MODES[(s[8], s[9])]
-        except Exception:
+        except KeyError:
             pass
         if s[12]:
             self.im_info["interlace"] = 1