Fix 1983

[oberstet]

fixes #1983 - with proper two-phase, red-green TDD

[oberstet]

TDD phase 1: red

This PR fixes #1983 test-first: first the red commits, which only add tests and must fail on
the current code, then the green commit with the fix, which must turn the same tests green
without changing them.

Red commit 1: 23c94af1, the regression tests

New src/autobahn/test/test_import_websocket_only.py. Each test imports in a fresh
subprocess (so modules other tests already loaded don't hide anything), with
USE_TWISTED / USE_ASYNCIO removed from the environment, and with a fake numpy package put
first on PYTHONPATH:

• TestWebSocketOnlyImports.test_no_flatbuffers_no_numpy: import autobahn,
  import autobahn.twisted.websocket and import autobahn.asyncio.websocket (one sub-test each)
  must load neither the FlatBuffers runtime (autobahn.flatbuffers.builder) nor numpy
  (goal 3 in autobahn/__init__.py eagerly imports flatbuffers → numpy is loaded in every process (for v25.12.1 and later) #1983).
• TestWebSocketOnlyImports.test_broken_numpy_does_not_break_import: the same imports must succeed
  when numpy is installed but fails to import (impact 2 in autobahn/__init__.py eagerly imports flatbuffers → numpy is loaded in every process (for v25.12.1 and later) #1983).
• TestFlatBuffersStillAvailable.test_flatbuffers_serializer_roundtrip: FlatBuffers
  serialization still works and does load the runtime, so the fix must not just remove it (goal 1).

The tests are plain unittest test cases, so both trial (Twisted) and pytest (asyncio)
collect them.

CI run 38387068318:
red as expected. Code test coverage (Combined Twisted + asyncio) failed in
check-coverage-twisted with FAILED (skips=32, failures=2, successes=326). The 2 failures are
exactly the two tests above:

builtins.AssertionError: True is not false : import autobahn loads the FlatBuffers runtime
autobahn.test.test_import_websocket_only.TestWebSocketOnlyImports.test_no_flatbuffers_no_numpy

  File ".../src/autobahn/__init__.py", line 35, in <module>
    from autobahn import flatbuffers
  ...
  File ".../src/autobahn/flatbuffers/number_types.py", line 21, in <module>
    np = import_numpy()
  File ".../src/autobahn/flatbuffers/compat.py", line 75, in import_numpy
    import numpy as np
  File "/tmp/tmpek9e0oqp/numpy/__init__.py", line 1, in <module>
    raise ImportError('simulating a broken numpy install')
ImportError: simulating a broken numpy install
...
autobahn.test.test_import_websocket_only.TestWebSocketOnlyImports.test_broken_numpy_does_not_break_import

The roundtrip test passed, as it should on the current code.

Red commit 2: 9ca0d0bb, run the tests on every interpreter

The main test suite runs on CPython 3.14 only. To cover the other interpreters, and PyPy in
particular (where we use importlib.util.LazyLoader for the fix), the just test-imports recipe
now also runs test_import_websocket_only.py. That recipe is the "Import Smoke Test" job, which
runs on cpy311, cpy312, cpy313, cpy314 and pypy311.

CI run 38396172114: red as
expected.

• Import Smoke Test (cpy314), CPython 3.14.8: 6 failed, 35 passed. The 6 failures are the two
  tests above, one per import, and the rest of the import smoke test still passes:

  SUBFAILED(modname='autobahn') ...::test_broken_numpy_does_not_break_import - AssertionError: import autobahn failed:
  SUBFAILED(modname='autobahn.twisted.websocket') ...::test_broken_numpy_does_not_break_import - AssertionError: import autobahn.twisted.websocket failed:
  SUBFAILED(modname='autobahn.asyncio.websocket') ...::test_broken_numpy_does_not_break_import - AssertionError: import autobahn.asyncio.websocket failed:
  SUBFAILED(modname='autobahn') ...::test_no_flatbuffers_no_numpy - AssertionError: True is not false : import autobahn loads the FlatBuffers runtime
  SUBFAILED(modname='autobahn.twisted.websocket') ...::test_no_flatbuffers_no_numpy - AssertionError: True is not false : import autobahn.twisted.websocket loads the FlatBuffers runtime
  SUBFAILED(modname='autobahn.asyncio.websocket') ...::test_no_flatbuffers_no_numpy - AssertionError: True is not false : import autobahn.asyncio.websocket loads the FlatBuffers runtime
  ========================= 6 failed, 35 passed in 4.75s =========================
  

  So autobahn.asyncio.websocket and autobahn.twisted.websocket load the FlatBuffers runtime
  too, not just import autobahn. They do it through autobahn.wamp (see autobahn/__init__.py eagerly imports flatbuffers → numpy is loaded in every process (for v25.12.1 and later) #1983).

• The other matrix legs (cpy311, cpy312, cpy313, pypy311) were cancelled before they
  finished. The matrix is fail-fast, so the first failing leg stops the others.

• Code Quality Checks: failed in the coverage step with FAILED (skips=32, failures=2, successes=326), the same as red commit 1. The two coverage upload steps failed after it because
  no report was written.

• Everything else passed, including both Import Smoke Test (lowest-direct deps) legs, which run
  only test_import_all.py.

Locally, on PyPy 3.11 (7.3.23) without the fix, just test-imports pypy311 gives the same
result, 6 failed, 35 passed.

Next: phase 2, green

The green commit changes only the two places described in #1983:

1. src/autobahn/__init__.py: from autobahn import flatbuffers moves from module level into
   check_zlmdb_flatbuffers_version_in_sync(), the only function that uses it.
2. src/autobahn/wamp/message.py: flatbuffers and message_fbs become lazy modules
   (importlib.util.LazyLoader), loaded on the first FlatBuffers message built or parsed.

No test changes. Expected: the same CI jobs green, now with all five Import Smoke Test legs
including pypy311 running to the end.

Note: This comment was drafted with AI assistance (Claude Code).

[oberstet]

TDD phase 2: green

Green commit: 8077fcb0, the fix

It changes only the two places described in #1983, and no tests:

1. src/autobahn/__init__.py: from autobahn import flatbuffers moves from module level into
   check_zlmdb_flatbuffers_version_in_sync(), the only code that uses it, next to its existing
   local import zlmdb.flatbuffers.
2. src/autobahn/wamp/message.py: autobahn.flatbuffers and autobahn.wamp.message_fbs are
   imported through importlib.util.LazyLoader (the "Implementing lazy imports" recipe from the
   importlib documentation). They are registered when message.py is imported, but executed only
   on first attribute access, i.e. when the first FlatBuffers message is built or parsed.

+28 / -4 lines in total. Nothing changes in the vendored FlatBuffers runtime, in the
flatc-generated code, or in any public import path.

CI

All workflows for 8077fcb0 passed: main,
wheels,
wheels-docker,
wheels-arm64 and
wstest.

The same jobs that were red before:

job	red (9ca0d0bb)	green (8077fcb0)
Import Smoke Test (cpy314), CPython 3.14.8	6 failed, 35 passed	35 passed
Import Smoke Test (cpy313), CPython 3.13.16	cancelled	35 passed
Import Smoke Test (cpy312), CPython 3.12.3	cancelled	35 passed
Import Smoke Test (cpy311), CPython 3.11.17	cancelled	35 passed
Import Smoke Test (pypy311), PyPy 3.11.15	cancelled (6 failed, 35 passed locally)	35 passed
Code Quality Checks: coverage, Twisted / trial (with NVX)	FAILED (skips=32, failures=2, successes=326)	PASSED (skips=32, successes=328)
Code Quality Checks: coverage, Twisted / trial (without NVX)	not run	PASSED (skips=31, successes=329)
Code Quality Checks: coverage, asyncio / pytest	not run	298 passed, 6 skipped / 299 passed, 5 skipped

All three new tests pass under both trial and pytest, and on all five interpreters, PyPy
included. So with the fix:

• import autobahn, import autobahn.twisted.websocket and import autobahn.asyncio.websocket load
  neither the FlatBuffers runtime nor numpy (goal 3).
• The same imports succeed when numpy is installed but broken.
• FlatBuffers serialization still works, and still loads the runtime when used (goal 1).

check_zlmdb_flatbuffers_version_in_sync() (goal 2) keeps its name and behaviour. Locally, against
zlmdb 26.7.1, it returns 25.12.19 as before.

Memory

Measured locally, as peak RSS (VmHWM) right after the import in a fresh interpreter, median of 5
runs, with numpy installed. † means numpy was loaded.

import	CPython 3.14: 26.7.1 → this PR	PyPy 3.11: 26.7.1 → this PR
import autobahn	28.7 MB † → 11.9 MB	87.9 MB † → 64.4 MB
import autobahn.asyncio.websocket	51.0 MB † → 37.7 MB	117.0 MB † → 99.2 MB
import autobahn.twisted.websocket	63.0 MB † → 50.1 MB	132.3 MB † → 113.7 MB
from daphne.server import Server	64.4 MB † → 51.5 MB	135.9 MB † → 115.5 MB

The full tables, the method, and the scripts to reproduce them follow in a separate docs commit on
this PR (docs/memory-footprint.rst, scripts/memory-footprint/).

Note: This comment was drafted with AI assistance (Claude Code).