UI Freezes and the Dangers of Non-Cancellable Read Actions in Background Threads

**UI Freezes and the Dangers of Non-Cancellable Read Actions in Background Threads**

In JetBrains IDEs, UI freezes are often blamed on “heavy work on the EDT,” but our recent investigations show another common culprit in plugins: long, non-cancellable read actions running in background threads. We receive a lot of freeze reports via our automated exceptions reporting system, and many reports actually show problems in plugins that contain that single erroneous pattern. Let’s try to highlight this problem arising from non-cancellable code and figure out how to fix it. A real-world example Let’s look at the following stack traces that come from a Package Checker plugin freeze. Note how a background thread “DefaultDispatcher-worker-27” ends up executing ReadAction.compute: "AWT-EventQueue-0" prio=0 tid=0x0 nid=0x0 waiting on condition java.lang.Thread.State: TIMED_WAITING on com.intellij.openapi.progress.util.EternalEventStealer@1356e599 at java.base@21.0.8/java.lang.Object.wait0(Native Method) at java.base@21.0.8/java.lang.Object.wait(Object.java:366) at com.intellij.openapi.progress.util.EternalEventStealer.dispatchAllEventsForTimeout(SuvorovProgress.kt:261) at com.intellij.openapi.progress.util.SuvorovProgress.processInvocationEventsWithoutDialog(SuvorovProgress.kt:125) at com.intellij.openapi.progress.util.SuvorovProgress.dispatchEventsUntilComputationCompletes(SuvorovProgress.kt:73) at com.intellij.openapi.application.impl.ApplicationImpl.lambda$postInit$14(ApplicationImpl.java:1434) at com.intellij.openapi.application.impl.ApplicationImpl$$Lambda/0x000001fafa58b530.invoke(Unknown Source) at com.intellij.platform.locking.impl.RunSuspend.await(NestedLocksThreadingSupport.kt:1517) .....

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