Bazel NPE when setting --java_classpath=bazel

[arunkumar9t2]

Description of the bug:

Setting --java_classpath=bazel on the command line and building a simple project crashes bazel with following stacktrace

java.lang.RuntimeException: Unrecoverable error while evaluating node 'ActionLookupData{actionLookupKey=ConfiguredTargetKey{label=//classpath:B_Java, config=BuildConfigurationKey[62ff62eac06d5b2a3021fd76848185e8623f28c27a49346ecc5997f2c301b7b9]}, actionIndex=0}' (requested by nodes 'TargetCompletionKey{topLevelArtifactContext=com.google.devtools.build.lib.analysis.TopLevelArtifactContext@90904c3b, actionLookupKey=ConfiguredTargetKey{label=//classpath:A_Java, config=BuildConfigurationKey[62ff62eac06d5b2a3021fd76848185e8623f28c27a49346ecc5997f2c301b7b9]}, willTest=false}')
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:642)
	at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:382)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.base/java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
	at com.google.devtools.build.lib.rules.java.JavaCompileAction.getReducedClasspath(JavaCompileAction.java:262)
	at com.google.devtools.build.lib.rules.java.JavaCompileAction.beginExecution(JavaCompileAction.java:404)
	at com.google.devtools.build.lib.actions.Action.execute(Action.java:133)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$5.execute(SkyframeActionExecutor.java:936)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.continueAction(SkyframeActionExecutor.java:1103)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.run(SkyframeActionExecutor.java:1061)
	at com.google.devtools.build.lib.skyframe.ActionExecutionState.runStateMachine(ActionExecutionState.java:160)
	at com.google.devtools.build.lib.skyframe.ActionExecutionState.getResultOrDependOnFuture(ActionExecutionState.java:93)
	at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor.executeAction(SkyframeActionExecutor.java:503)
	at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.checkCacheAndExecuteIfNeeded(ActionExecutionFunction.java:826)
	at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.computeInternal(ActionExecutionFunction.java:323)
	at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.compute(ActionExecutionFunction.java:161)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:571)

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

Add a simple java_library and build it with --java_classpath=bazel

Which operating system are you running Bazel on?

MacOS Monterey 12.4

What is the output of bazel info release?

development version

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

Branched off master and built with bazelisk build //src:bazel.

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

https://github.com/bazelbuild/bazel.git
19b8d24531fb728701a85ec050361d434600b5d2
19b8d24531fb728701a85ec050361d434600b5d2

Have you found anything relevant by searching the web?

The error is caused to JavaCompileActionContext not being registered to ActionContextRegistry

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

N/A

[cushon]

There was some additional context/motivation in #16906

See #16426 (comment)

Originally the reduced classpath reduction happened during action execution, so changes in inputs would cause invalidation even if those inputs were pruned off the reduced classpath. But the classpath reduction now happens in Bazel, so in the example if libZ changes the action will still be a cache hit and won't be reexecuted:
https://cs.github.com/bazelbuild/bazel/blob/60ded2b53d4d9180c213e9e44a8c640368e60822/src/main/java/com/google/devtools/build/lib/rules/java/JavaCompileAction.java#L244

That optimization was implemented for Blaze, but parts of it are not wired up in Bazel, and attempting to enable it results in an NPE.

This bug tracks making --experimental_java_classpath=bazel work, to improve performance of incremental Java builds.

[arunkumar9t2]

Thanks @cushon yes that was the motivation for us as well since we wanted to do classpath reduction before execution to leverage compile avoidance.

[raviagarwal7]

@arunkumar9t2 could you share some docs on how does --java_classpath=bazel help in classpath reduction?
how is it different from --experimental_strict_java_deps or --experimental_prune_transitive_deps (being added in #16426)