Fix OverlayStream read-to-EOF without a known size

dissect/util/stream.py

@@ -521,6 +521,15 @@ def _read(self, offset: int, length: int) -> bytes:
         overlays = self.overlays
         lookup = self._lookup
 
+        if length < 0:
+            # Only happens when reading to EOF on a stream with no size, so determine the end
+            # from the source file-like object and any overlays that reach past it
+            fh.seek(0, io.SEEK_END)
+            end = fh.tell()
+            for overlay_offset in lookup:
+                end = max(end, overlay_offset + overlays[overlay_offset][0])
+            length = max(0, end - offset)
+
         overlay_len = len(overlays)
         overlay_idx = bisect_left(lookup, offset)
 

tests/test_stream.py

@@ -194,6 +194,24 @@ def test_overlay_stream() -> None:
     assert fh.read(100) == b"\x04" * 4
 
 
+def test_overlay_stream_no_size() -> None:
+    data = b"1234567890"
+    # Wrap in a BufferedReader, which raises OverflowError on absurdly large reads
+    overlay = stream.OverlayStream(io.BufferedReader(io.BytesIO(data)))
+
+    assert overlay.read(10) == data
+
+    overlay.seek(0)
+    assert overlay.read() == data
+
+
+def test_overlay_stream_no_size_overlay_past_end() -> None:
+    overlay = stream.OverlayStream(io.BufferedReader(io.BytesIO(b"1234567890")))
+    overlay.add(8, b"abcd")
+
+    assert overlay.read() == b"12345678abcd"
+
+
 def test_overlay_stream_add_at_offset_zero() -> None:
     buf = io.BytesIO(b"\x00" * 100)
     fh = stream.OverlayStream(buf, size=100)