Fix decoding base64 chunk for BodyPartReader

[hyzyla]

What do these changes do?

Those changes add the additional reading of few bytes in BodyPartReader.read_chunk if the size of the chunk is not valid for base64 decoding.

Are there changes in behavior for the user?

Yes, because if someone will try to use read_chunk for the body that has transfer encoding equal to base64, can receive more than expected bytes.

Related issue number

#3843

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[asvetlov]

Thanks!

[Dreamsorcerer]

Think a backport got screwed up here...

[patchback]

Backport to 3.11: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply aeb01ce on top of patchback/backports/3.11/aeb01ce3bda7497d7cc6c434ac6806aca291e273/pr-3867

Backporting merged PR #3867 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.11/aeb01ce3bda7497d7cc6c434ac6806aca291e273/pr-3867 upstream/3.11

4. Now, cherry-pick PR Fix decoding base64 chunk for BodyPartReader #3867 contents into that branch:

   $ git cherry-pick -x aeb01ce3bda7497d7cc6c434ac6806aca291e273

   If it'll yell at you with something like fatal: Commit aeb01ce3bda7497d7cc6c434ac6806aca291e273 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x aeb01ce3bda7497d7cc6c434ac6806aca291e273

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix decoding base64 chunk for BodyPartReader #3867 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.11/aeb01ce3bda7497d7cc6c434ac6806aca291e273/pr-3867

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.10: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply aeb01ce on top of patchback/backports/3.10/aeb01ce3bda7497d7cc6c434ac6806aca291e273/pr-3867

Backporting merged PR #3867 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.10/aeb01ce3bda7497d7cc6c434ac6806aca291e273/pr-3867 upstream/3.10

4. Now, cherry-pick PR Fix decoding base64 chunk for BodyPartReader #3867 contents into that branch:

   $ git cherry-pick -x aeb01ce3bda7497d7cc6c434ac6806aca291e273

   If it'll yell at you with something like fatal: Commit aeb01ce3bda7497d7cc6c434ac6806aca291e273 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x aeb01ce3bda7497d7cc6c434ac6806aca291e273

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix decoding base64 chunk for BodyPartReader #3867 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.10/aeb01ce3bda7497d7cc6c434ac6806aca291e273/pr-3867

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.