Speed up Local

[akx]

While profiling a Django app with viztracer, I noticed asgiref Local access/locking being actually visible in the trace, and figured there could be something to fix perf-wise :)

@contextlib.contextmanager can be slow due to the fact that the interpreter needs to juggle a generator frame, etc. This PR replaces it with a small real context manager (that is stored on the instance for reuse instead, in the first commit.

The second commit further specializes Local (using __new__ magic, so this is transparent to consumers) to 2 classes depending on if thread_critical is set, so accesses do not require branches at all – and indeed, in the thread-critical case, nullcontext isn't required anymore. (Further, the guards in the setattr case become unnecessary since we can just use object.__setattr__ to sidestep the classes' impls.)

EDIT: the third commit gets rid of the context manager altogether in favor of just inlining the same things as a try:finally: block, as per comments here, for even more performance. 😄

I pytest-benchmarked this to be pleasantly faster (and given that these are often very hot paths in Django, this is a nice win for not an awful lot of additional code):

Real context manager

Benchmark	Kops/s before	Kops/s after	Change
test_getattr_thread_critical	1,041.50	3,715.52	+257%
test_setattr_thread_critical	1,040.11	3,575.43	+244%
test_getattr_default	1,034.84	2,493.43	+141%
test_setattr_default	913.03	1,833.74	+101%
test_delattr_thread_critical	504.15	1,747.49	+247%
test_getattr_missing_default	544.32	968.90	+78%
test_delattr_default	444.68	1,030.41	+132%
test_getattr_thread_critical_async	31.40	32.50	+4%
test_setattr_thread_critical_async	32.69	33.34	+2%

No context manager (only affects default)

Benchmark	Kops/s before	Kops/s after	Change
test_getattr_thread_critical	1,047.4632	3,649.6306	+248.4%
test_setattr_thread_critical	1,013.8543	3,500.2482	+245.2%
test_getattr_default	986.0904	3,586.0984	+263.7%
test_setattr_default	859.1037	2,673.6265	+211.2%
test_getattr_missing_default	520.0822	1,068.1260	+105.4%
test_delattr_thread_critical	502.1324	1,712.0688	+241.0%
test_delattr_default	423.2087	1,347.7917	+218.5%
test_setattr_thread_critical_async	31.4873	32.1533	+2.1%
test_getattr_thread_critical_async	30.7843	31.7018	+3.0%

[bluetech]

I have seen Local show up in profiles as well, and I don't even use async, so it'd be great to have its performance improved.

I wonder given that Local is performance sensitive if it won't be better to just use try/finally for the lock instead of the _LockContext helper? Sure it's not as nice and a bit more error prone, but I think this is a case where efficiency beats beauty (of course as long as avoiding _LockContext is actually faster).

[akx]

I wonder given that Local is performance sensitive if it won't be better to just use try/finally for the lock instead of the _LockContext helper?

Sure. Triplicating the logic into the functions would probably make a lot of sense for performance, but those functions should probably be comment-marked "there be dragons here, be very careful" 😄

[akx]

@bluetech Great news: according to my measurements it is indeed a whole lot faster, another +100% on top of the context-manager version 😄

Name (time in ns)                      0001_baselin(*) OPS (Kops/s)  0002_real-co OPS (Kops/s)  ΔOPS (Kops/s)  0003_real-co OPS (Kops/s)  ΔOPS (Kops/s)
-------------------------------------------------------------------------------------------------------------------------------------------------------
test_getattr_default                                       986.0904                 2,318.5320        +135.1%                 3,586.0984        +263.7%
test_setattr_default                                       859.1037                 1,839.8386        +114.2%                 2,673.6265        +211.2%
test_getattr_missing_default                               520.0822                   941.0258         +80.9%                 1,068.1260        +105.4%
test_delattr_default                                       423.2087                   988.1233        +133.5%                 1,347.7917        +218.5%

[akx]

Gentle review nudge, @carltongibson? 😄 (Ditto for #552)

[carltongibson]

@akx Nice 😃 — I'm just working my way back to asgiref now, as it happens. 🎁