Crash in IntelliJ plugin

[mikehearn]

java.lang.IndexOutOfBoundsException: Index: 0
	at java.base/java.util.Collections$EmptyList.get(Collections.java:4808)
	at org.pkl.intellij.PklBreadcrumbsProvider$special$$inlined$handler$6.getElementInfo(PklBreadcrumbsProvider.kt:215)
	at org.pkl.intellij.PklBreadcrumbsProvider.getElementInfo(PklBreadcrumbsProvider.kt:30)
	at com.intellij.xml.breadcrumbs.PsiCrumb.<init>(PsiCrumb.java:24)
	at com.intellij.xml.breadcrumbs.PsiFileBreadcrumbsCollector.computeCrumbs(PsiFileBreadcrumbsCollector.java:99)
	at com.intellij.xml.breadcrumbs.BreadcrumbsXmlWrapper.computeCrumbs(BreadcrumbsXmlWrapper.java:51)
	at com.intellij.xml.breadcrumbs.BreadcrumbsPanel.lambda$updateCrumbsAsync$3(BreadcrumbsPanel.java:197)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$OTelMonitor.callWrapped(NonBlockingReadActionImpl.java:876)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$OTelMonitor$MonitoredComputation.call(NonBlockingReadActionImpl.java:908)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$Submission.insideReadAction(NonBlockingReadActionImpl.java:636)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$Submission.lambda$attemptComputation$3(NonBlockingReadActionImpl.java:599)
	at com.intellij.platform.locking.impl.NestedLocksThreadingSupport.tryRunReadAction(NestedLocksThreadingSupport.kt:826)
	at com.intellij.openapi.application.impl.ApplicationImpl.tryRunReadAction(ApplicationImpl.java:1221)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.lambda$runInReadActionWithWriteActionPriority$0(ProgressIndicatorUtils.java:95)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtilService.runActionAndCancelBeforeWrite(ProgressIndicatorUtilService.java:73)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.runActionAndCancelBeforeWrite(ProgressIndicatorUtils.java:152)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.lambda$runWithWriteActionPriority$1(ProgressIndicatorUtils.java:140)
	at com.intellij.openapi.progress.ProgressManager.lambda$runProcess$0(ProgressManager.java:98)
	at com.intellij.openapi.progress.impl.CoreProgressManager.lambda$runProcess$1(CoreProgressManager.java:229)
	at com.intellij.platform.diagnostic.telemetry.helpers.TraceKt.use(trace.kt:44)
	at com.intellij.openapi.progress.impl.CoreProgressManager.lambda$runProcess$2(CoreProgressManager.java:228)
	at com.intellij.openapi.progress.impl.CoreProgressManager.lambda$executeProcessUnderProgress$14(CoreProgressManager.java:681)
	at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:756)
	at com.intellij.openapi.progress.impl.CoreProgressManager.computeUnderProgress(CoreProgressManager.java:712)
	at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:680)
	at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:78)
	at com.intellij.openapi.progress.impl.CoreProgressManager.runProcess(CoreProgressManager.java:209)
	at com.intellij.openapi.progress.ProgressManager.runProcess(ProgressManager.java:98)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.runWithWriteActionPriority(ProgressIndicatorUtils.java:137)
	at com.intellij.openapi.progress.util.ProgressIndicatorUtils.runInReadActionWithWriteActionPriority(ProgressIndicatorUtils.java:95)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$Submission.attemptComputation(NonBlockingReadActionImpl.java:599)
	at com.intellij.openapi.application.impl.NonBlockingReadActionImpl$Submission.lambda$transferToBgThread$1(NonBlockingReadActionImpl.java:485)
	at com.intellij.util.concurrency.BoundedTaskExecutor.doRun(BoundedTaskExecutor.java:248)
	at com.intellij.util.concurrency.BoundedTaskExecutor.access$200(BoundedTaskExecutor.java:27)
	at com.intellij.util.concurrency.BoundedTaskExecutor$1.executeFirstTaskAndHelpQueue(BoundedTaskExecutor.java:226)
	at com.intellij.util.concurrency.BoundedTaskExecutor$1.run(BoundedTaskExecutor.java:214)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
	at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:735)
	at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:732)
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:400)
	at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1.run(Executors.java:732)
	at java.base/java.lang.Thread.run(Thread.java:1583)

[bioball]

Thanks for the bug report. Can you include steps to reproduce?

[mikehearn]

Unfortunately I don't recall how it happened, I only noticed it after some time. If the stack trace isn't sufficient to understand the nature of the fault then it might be worth closing this issue.

However, maybe we can debug it with just the stack trace? The key frames are these:

java.lang.IndexOutOfBoundsException: Index: 0
	at java.base/java.util.Collections$EmptyList.get(Collections.java:4808)
	at org.pkl.intellij.PklBreadcrumbsProvider$special$$inlined$handler$6.getElementInfo(PklBreadcrumbsProvider.kt:215)
	at org.pkl.intellij.PklBreadcrumbsProvider.getElementInfo(PklBreadcrumbsProvider.kt:30)

The penultimate frame is an inline so the line number is wrong. Fortunately we can work it out another way: the mangled name says it's the 6th handler lexically in the file, which is forGeneratorHandler, and if we look at PklBreadcrumbsProvider.kt we can see that indeed forGeneratorHandler is the only handler that invokes get on a collection via the index operator. So we can be fairly certain the cursor was in a for statement at the time.

The handler code looks like this:

  private val forGeneratorHandler =
    handler<PklForGenerator> { elem ->
      buildString {
        append("for (")
        append(elem.keyValueVars[0].identifier.text)
        if (elem.keyValueVars.size == 2) {
          append(", ")
          append(elem.keyValueVars[1].identifier.text)
        }
        append(" in ")
        append(elem.iterableExpr?.toDisplayText() ?: "…")
        append(") {…}")
      }
    }

We're indexing in two places here, but the top frame indicates that elem.keyValueVars is an EmptyList at this point so we must have crashed on the first line when trying to compute the identifier of the first keyValueVars.

Hypothesis: I positioned the cursor into an invalid/unfinished for loop construct, and the breadcrumb handler couldn't parse it as a result.

Experiment: write a for statement and observe if there's another crash. There is, so this is the cause of the problem.

Proposed solution:

  private val forGeneratorHandler =
    handler<PklForGenerator> { elem ->
      buildString {
        if (elem.keyValueVars.isEmpty) {
            append("for")
        } else {
            append("for (")
            append(elem.keyValueVars[0].identifier.text)
            if (elem.keyValueVars.size == 2) {
              append(", ")
              append(elem.keyValueVars[1].identifier.text)
            }
            append(" in ")
            append(elem.iterableExpr?.toDisplayText() ?: "…")
            append(") {…}")
          }
      }
    }