Long runtime classpaths slow down unit test execution

[rsalvador]

Description of the bug:

The classpath pre-processing loop in java_stub_template.txt is inefficient for very long classpaths and slows down unit test execution. The slowdown is in this loop in java_stub_template.txt:

bazel/src/main/java/com/google/devtools/build/lib/bazel/rules/java/java_stub_template.txt

Line 309 in fcfcb92

for path in ${CLASSPATH}; do

for classpaths with ~250,000 and ~700,000 entries the loop takes 28 and 50 seconds, respectively, on an intel MacBook.

Which category does this issue belong to?

Performance

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Run a unit test with a very long runtime classpath.

Which operating system are you running Bazel on?

OSX

What is the output of bazel info release?

release 6.1.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

No response

Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

Performance regression seems to have been introduced in d0ee889

Have you found anything relevant by searching the web?

No

Any other information, logs, or outputs that you want to share?

No response

[cushon]

This is part of the logic that generates a separate jar file as a way to configure classpath that are too long to pass on the command line.

@rsalvador I'm curious which JDK you're using, is it newer than JDK 8? The long term plan here is to replace this logic in the stub template with an @argument file: #6354

(I'm also not sure why the stub template generates this every time it's executed, instead of having Bazel create a separate action that generates the separate jar file once; that came up in #19036.)

[rsalvador]

@rsalvador I'm curious which JDK you're using, is it newer than JDK 8? The long term plan here is to replace this logic in the stub template with an @argument file: #6354

@cushon using JDK 11: OpenJDK 64-Bit Server VM (build 11.0.19+7-LTS, mixed mode) by Azul Systems, Inc.

[iancha1992]

A fix for this issue has been included in Bazel 6.4.0 RC1. Please test out the release candidate and report any issues as soon as possible. Thanks!

[rsalvador]

@iancha1992 verified that the generated test script executables include the fix and that the test execution is faster than with 6.3.2 for a sample test (35 seconds with 6.3.2 vs 12 seconds with 6.4.0rc1)