[MLA-1306] avoid copying png data (#4430)

ml-agents-envs/mlagents_envs/rpc_utils.py

     return BehaviorSpec(observation_shape, action_type, action_shape)
 
 
+class OffsetBytesIO:
+    """
+    Simple file-like class that wraps a bytes, and allows moving its "start"
+    position in the bytes. This is only used for reading concatenated PNGs,
+    because Pillow always calls seek(0) at the start of reading.
+    """
+
+    __slots__ = ["fp", "offset"]
+
+    def __init__(self, data: bytes):
+        self.fp = io.BytesIO(data)
+        self.offset = 0
+
+    def seek(self, offset: int, whence: int = io.SEEK_SET) -> int:
+        if whence == io.SEEK_SET:
+            res = self.fp.seek(offset + self.offset)
+            return res - self.offset
+        raise NotImplementedError()
+
+    def tell(self) -> int:
+        return self.fp.tell() - self.offset
+
+    def read(self, size: int = -1) -> bytes:
+        return self.fp.read(size)
+
+    def original_tell(self) -> int:
+        """
+        Returns the offset into the original byte array
+        """
+        return self.fp.tell()
+
+
 @timed
 def process_pixels(image_bytes: bytes, expected_channels: int) -> np.ndarray:
     """
     :param expected_channels: Expected output channels
     :return: processed numpy array of observation from environment
     """
-    image_bytearray = bytearray(image_bytes)
+    image_fp = OffsetBytesIO(image_bytes)
-            image = Image.open(io.BytesIO(image_bytearray))
+            image = Image.open(image_fp)
             # Normally Image loads lazily, load() forces it to do loading in the timer scope.
             image.load()
         s = np.array(image, dtype=np.float32) / 255.0
 
     image_arrays = []
 
-    bytes_read = 0
-        # TODO avoid creating a new array here. Unfortunately, Pillow doesn't respect the current state of the buffer
-        # and always starts with seek(0), but we should be able to wrap BytesIO with something that lets us adjust
-        # the "start" offset.
-        buffer = io.BytesIO(image_bytearray[bytes_read:])
-            image = Image.open(buffer)
+            image = Image.open(image_fp)
-            offset = buffer.getvalue().index(PNG_HEADER, buffer.tell())
-            bytes_read += offset
+            new_offset = image_bytes.index(PNG_HEADER, image_fp.original_tell())
+            image_fp.offset = new_offset
         except ValueError:
             # Didn't find the header, so must be at the end.
             break