`testAllBase64Features_librt` fails with Python 3.14.4

[mr-c]

Bug Report

Yes, I know that Python 3.14.4 is not yet released, but the Python3.14 package in Debian's development distribution (sid a.k.a. unstable) is built from the "3.14 branch 2026-03-27."

which includes Do not ignore excess Base64 data after the first padded quad

and thus test

_________________________ testAllBase64Features_librt __________________________
[gw0] linux -- Python 3.14.3 /usr/bin/python3.14
data: /build/reproducible-path/package/mypyc/test-data/run-base64.test:1:
Failed: Invalid output (/build/reproducible-path/package/mypyc/test-data/run-base64.test, line 1)
----------------------------- Captured stdout call -----------------------------

*** Exit status: 1
----------------------------- Captured stderr call -----------------------------

Generated files: /build/reproducible-path/package/.mypyc_test_output (for first failure only)

Expected:
Actual:
  << test_decode_with_extra_data_after_padding >> (diff)
  Traceback (most recent call last): (diff)
    File "driver.py", line 57, in <module> (diff)
      raise failures[-1][1][1] (diff)
    File "driver.py", line 28, in <module> (diff)
      test_func() (diff)
      ~~~~~~~~~^^ (diff)
    File "run-base64.test", line 149, in test_decode_with_extra_data_after_padding (diff)
      check_decode(b"eA==x", encoded=True) (diff)
    File "run-base64.test", line 70, in check_decode (diff)
      assert b64decode(enc) == getattr(base64, "b64decode")(enc) (diff)
    File "/usr/lib/python3.14/base64.py", line 85, in b64decode (diff)
      return binascii.a2b_base64(s, strict_mode=validate) (diff)
             ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^ (diff)
  binascii.Error: Incorrect padding (diff)

Update the test output using --update-data (implies -n0; you can additionally use the -k selector to update only specific tests)
------------------------------ Captured log call -------------------------------
INFO     root:dist.py:1018 running build_ext
INFO     root:build_ext.py:538 building 'native' extension
INFO     root:dir_util.py:58 creating build/temp.linux-x86_64-cpython-314/build
INFO     root:spawn.py:77 x86_64-linux-gnu-gcc -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O2 -Wall -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/build/reproducible-path/package/.pybuild/cpython3_3.14_mypy/build/mypyc/lib-rt -I/usr/include/python3.14 -c build/__native.c -o build/temp.linux-x86_64-cpython-314/build/__native.o -O0 -g1 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable -Wno-ignored-optimization-argument -Wno-array-bounds -Wno-stringop-overread -Wno-stringop-overflow -Wno-cpp -UNDEBUG
INFO     root:dir_util.py:58 creating build/lib.linux-x86_64-cpython-314
INFO     root:spawn.py:77 x86_64-linux-gnu-gcc -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-z,relro -g -fwrapv -O2 -Wl,-z,relro -Wl,-z,now -Wdate-time -D_FORTIFY_SOURCE=2 build/temp.linux-x86_64-cpython-314/build/__native.o -L/usr/lib/x86_64-linux-gnu -o build/lib.linux-x86_64-cpython-314/native.cpython-314-x86_64-linux-gnu.so
INFO     root:file_util.py:130 copying build/lib.linux-x86_64-cpython-314/native.cpython-314-x86_64-linux-gnu.so ->

Is this important, or can we freely ignore this test and publish mypy 1.20.0 to Debian? Or should we wait until a fix is agreed upon?

[ilevkivskyi]

cc @JukkaL, but I guess it is safe to ignore for now.

[cdce8p]

On a related note, the same test case will start to emit FutureWarnings for Python 3.15 caused by python/cpython#141128 here:

mypy/mypyc/test-data/run-base64.test

Lines 186 to 195 in a25c331

	def test_urlsafe_b64decode() -> None:
	# Don't test everything, since the implementation is mostly shared with b64decode.
	check_urlsafe_decode(b"")
	check_urlsafe_decode(b"a")
	check_urlsafe_decode(b"\xf8")
	check_urlsafe_decode(b"\xfc")
	check_urlsafe_decode(b"\xfcx")
	check_urlsafe_decode(b"\xfcxy")
	check_urlsafe_decode(b"\xfcxyz")
	check_urlsafe_decode(bytes([x for x in range(256)]))

[out]
driver.py:28: FutureWarning: invalid character '+' in URL-safe Base64 data will be discarded in future Python versions
  test_func()
driver.py:28: FutureWarning: invalid character '/' in URL-safe Base64 data will be discarded in future Python versions
  test_func()

[ilevkivskyi]

OK, it looks like this started causing troubles for real: see test failures in #21175 I guess we can simply update the test. Or do we actually need to update our base64 implementation to be more strict as well?

[mr-c]

Yeah, Python 3.14.4 is officially out: https://www.python.org/downloads/release/python-3144/

For Python 3.15 I would recommend to match the stricter behavior for sure.

Since you can detect the micro version at run time, it makes sense to also be stricter for Python 3.14.4+, to prevent interoperability issues

[ilevkivskyi]

For now I am skipping the tests in #21193 (to unblock multiple pending PRs), we can apply an actual fix later.