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Nick Pope b47f2f5b90 Fixed #33865 -- Optimized LimitedStream wrapper.
The current implementation of LimitedStream is slow because .read()
performs an extra copy into a buffer and .readline() performs two
extra copies. The stream being wrapped is already typically a BytesIO
object so this is unnecessary.

This implementation has largely been untouched for 12 years and,
inspired by a simpler implementation in werkzeug, it was possible to
achieve the following performance improvement:

LimitedStream.read() (single line):
  Mean +- std dev: [bench_limitedstream-main] 286 ns +- 6 ns
  -> [bench_limitedstream-patch] 227 ns +- 6 ns: 1.26x faster
LimitedStream.readline() (single line):
  Mean +- std dev: [bench_limitedstream-main] 507 ns +- 11 ns
  -> [bench_limitedstream-patch] 232 ns +- 8 ns: 2.18x faster
LimitedStream.read(8192) (single line):
  Mean +- std dev: [bench_limitedstream-main] 360 ns +- 8 ns
  -> [bench_limitedstream-patch] 297 ns +- 6 ns: 1.21x faster
LimitedStream.readline(8192) (single line):
  Mean +- std dev: [bench_limitedstream-main] 602 ns +- 10 ns
  -> [bench_limitedstream-patch] 305 ns +- 10 ns: 1.98x faster
LimitedStream.read() (multiple lines):
  Mean +- std dev: [bench_limitedstream-main] 290 ns +- 5 ns
  -> [bench_limitedstream-patch] 236 ns +- 6 ns: 1.23x faster
LimitedStream.readline() (multiple lines):
  Mean +- std dev: [bench_limitedstream-main] 517 ns +- 19 ns
  -> [bench_limitedstream-patch] 239 ns +- 7 ns: 2.16x faster
LimitedStream.read(8192) (multiple lines):
  Mean +- std dev: [bench_limitedstream-main] 363 ns +- 8 ns
  -> [bench_limitedstream-patch] 311 ns +- 11 ns: 1.17x faster
LimitedStream.readline(8192) (multiple lines):
  Mean +- std dev: [bench_limitedstream-main] 601 ns +- 12 ns
  -> [bench_limitedstream-patch] 308 ns +- 7 ns: 1.95x faster

Geometric mean: 1.59x faster
2023-01-05 19:26:56 +01:00
..
2022-03-08 14:50:06 +01:00

To run the test suite, first, create and activate a virtual environment. Then
install some requirements and run the tests::

    $ cd tests
    $ python -m pip install -e ..
    $ python -m pip install -r requirements/py3.txt
    $ ./runtests.py

For more information about the test suite, see
https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/unit-tests/.