Prevent sticky Pypdl._kwargs['headers']['range'] reference bug when reusing Pypdl object with user-supplied headers and multiple multi-segment downloads

A simple workaround for a bug with a complicated chain of events, due an oversight in the usage of the `Pypdl._kwargs` attribute.

This PR fixes #23.

When creating a Pypdl object, one has the ability to pass in arbitrary headers which will then be forwarded later to an aiohttp.ClientSession() object. These headers are collected within **kwargs and then stored in the self._kwargs attribute:

https://github.com/mjishnu/pypdl/blob/12a20ad6285fb0f4fddcb2ff81742b646c142c6a/pypdl/pypdl_manager.py#L44-L48

This dictionary is passed to multiple objects during normal procedure, such as during the Pypdl._get_header() method...

https://github.com/mjishnu/pypdl/blob/12a20ad6285fb0f4fddcb2ff81742b646c142c6a/pypdl/pypdl_manager.py#L247-L252

...and the Pypdl._multi_segment() method:

https://github.com/mjishnu/pypdl/blob/12a20ad6285fb0f4fddcb2ff81742b646c142c6a/pypdl/pypdl_manager.py#L259-L270

In order to request ranges of bytes to perform multi-segment downloads, Multidown.worker() collects the user-supplied arguments in **kwargs and then later adds a range header, as this needs to be passed later to another aiohttp.ClientSession() object:

https://github.com/mjishnu/pypdl/blob/12a20ad6285fb0f4fddcb2ff81742b646c142c6a/pypdl/downloader.py#L46-L48 Line 65 updates the kwargs['headers'] dictionary. https://github.com/mjishnu/pypdl/blob/12a20ad6285fb0f4fddcb2ff81742b646c142c6a/pypdl/downloader.py#L63-L66

This has an unintended side-effect that:

If a user has passed custom headers through Pypdl(headers=headers), then Line 65 permanently alters Pypdl._kwargs due to Python passing Pypdl._kwargs by reference instead of by value.
When a subsequent request is made later by Pypdl._get_header() (see Line 249 above), the range header is still present and will be included in a request that is not meant to include a range header.
Pypdl._get_header() only expects to receive status code 200 in response, however the MDN web docs suggest that servers should respond with 206 if a range header is supplied.
If a server does respond with 206 then the Pypdl._get_header() method falls off and returns None. This has dire consequences when used by Pypdl._get_info(), as it sets the local attribute header to None...

https://github.com/mjishnu/pypdl/blob/12a20ad6285fb0f4fddcb2ff81742b646c142c6a/pypdl/pypdl_manager.py#L228-L230

...which when passed to get_filepath() will result in a None reference error.

https://github.com/mjishnu/pypdl/blob/12a20ad6285fb0f4fddcb2ff81742b646c142c6a/pypdl/utls.py#L31-L32

Stacktrace:

(Pypdl)  21-07-24 15:21:29 - ERROR: (AttributeError) ['NoneType' object has no attribute 'get']
Traceback (most recent call last):
  File "C:\Users\Speyedr\Documents\PyCharm Projects\pypdl-sticky-header-investigation\pypdl\pypdl_manager.py", line 103, in download
    result = self._execute(
             ^^^^^^^^^^^^^^
  File "C:\Users\Speyedr\Documents\PyCharm Projects\pypdl-sticky-header-investigation\pypdl\pypdl_manager.py", line 175, in _execute
    file_path, multisegment, etag = self._get_info(
                                    ^^^^^^^^^^^^^^^
  File "C:\Users\Speyedr\Documents\PyCharm Projects\pypdl-sticky-header-investigation\pypdl\pypdl_manager.py", line 230, in _get_info
    file_path = get_filepath(url, header, file_path)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Speyedr\Documents\PyCharm Projects\pypdl-sticky-header-investigation\pypdl\utls.py", line 32, in get_filepath
    content_disposition = headers.get("Content-Disposition", None)
                          ^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get'

This bug has likely gone unsighted because it requires:

A user downloading multiple files using the same Pypdl object
The user passing in custom headers
The server supporting multi-segment downloads
The server returning 206 because the range header was included, instead of ignoring it and simply returning 200.

This PR includes a simple addition to the Multidown.worker() method that creates a shallow copy of kwargs['headers'] if it already exists, which disconnects it from the original reference and allows the range header to be added without affecting Pypdl._kwargs['header'].

This fix is faster than alternative patches which may require the use of deepcopy to properly disconnect or reset the Pypdl._kwargs attribute, however as a consequence this PR does not prevent the same issue from occurring again (i.e. unintentionally modifying attributes of Pypdl that have been passed by reference to callee functions).

mjishnu / pypdl