gh-145968: fix base64.b64decode altchars translation in specific cases

[mayeut]

When altchars overlaps with the standard ones, the translation does not always yield to the expected outcome.
This updates bytes.maketrans arguments to take those overlap cases into account.

• Issue: base64.b64decode error raised on valid input #145968

[python-cla-bot]

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[mayeut]

cc @serhiy-storchaka

[serhiy-storchaka]

It can be simply:

Suggested change

	altchars_out = (
	altchars[0] if altchars[0] not in b'+/' else altchars[1],
	altchars[1] if altchars[1] not in b'+/' else altchars[0],
	)
	trans_in = bytearray(altchars)
	trans_out = bytearray(b'+/')
	for b, b_out in zip(b'+/', altchars_out):
	if b not in altchars:
	trans_in.append(b)
	trans_out.append(b_out)
	trans = bytes.maketrans(trans_in, trans_out)
	trans = bytes.maketrans(altchars + bytes(set(b'+/') - set(altchars)),
	b'+/' + bytes(set(altchars) - set(b'+/')))

[mayeut]

I commited the suggestion but after further reflection, the sets are unordered and thus the translation might not be correct when altchars and standard ones do not overlap (although this might be hard to catch in tests ?).
I'll fix that later

[serhiy-storchaka]

You are right. What about the following?

trans_in = altchars
trans_out = b'+/'
if b'+' not in altchars and b'-' not in altchars:
    trans_in += b'+/'
    trans_out += altchars
elif b'+' not in altchars or b'-' not in altchars:
    trans_in += bytes(set(b'+/') - set(altchars))
    trans_out += bytes(set(altchars) - set(b'+/'))

You can also return your original code. #145981 also fixes this issue, so this code can be replaced, but your tests will remain.

[serhiy-storchaka]

LGTM. 👍