All BSP actions (builds, runs) fail due to closed thread pool

[nk2IsHere]

Hello, we've been using mill in our project. There is another issue we experience with mill and running actions on the project. After starting BSP server and running the action the first time, any subsequent runs crash the BSP server.

Current workaround we found is to stop all BSP connections in idea and rerun action after that.

OS: macos 15.6
Java: azul 24
Mill version: 1.0.6
Logs included:

[15-compile] Entered buildTargetCompile
[15-compile] Evaluating 2 tasks
[15-compile] buildTargetCompile took 24 msec
[15-compile] buildTargetCompile caught exception: java.util.concurrent.RejectedExecutionException: Task mill.exec.ExecutionContexts$ThreadPool$PriorityRunnable@2f0c87f3 rejected from java.util.concurrent.ThreadPoolExecutor@fd07cbb[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 19202]
[15-compile] java.util.concurrent.RejectedExecutionException: Task mill.exec.ExecutionContexts$ThreadPool$PriorityRunnable@2f0c87f3 rejected from java.util.concurrent.ThreadPoolExecutor@fd07cbb[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 19202]
[15-compile] 	at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2032)
[15-compile] 	at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:787)
[15-compile] 	at java.base/java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1328)
[15-compile] 	at mill.exec.ExecutionContexts$ThreadPool.execute(ExecutionContexts.scala:63)
[15-compile] 	at scala.concurrent.impl.Promise$Transformation.submitWithValue(Promise.scala:429)
[15-compile] 	at scala.concurrent.impl.Promise$DefaultPromise.submitWithValue(Promise.scala:338)
[15-compile] 	at scala.concurrent.impl.Promise$DefaultPromise.dispatchOrAddCallbacks(Promise.scala:312)
[15-compile] 	at scala.concurrent.impl.Promise$DefaultPromise.map(Promise.scala:182)
[15-compile] 	at mill.exec.Execution.evaluateTerminals$1$$anonfun$1(Execution.scala:271)
[15-compile] 	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[15-compile] 	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[15-compile] 	at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:619)
[15-compile] 	at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:617)
[15-compile] 	at scala.collection.AbstractIterable.foreach(Iterable.scala:935)
[15-compile] 	at mill.exec.Execution.evaluateTerminals$1(Execution.scala:160)
[15-compile] 	at mill.exec.Execution.execute0$$anonfun$1(Execution.scala:293)
[15-compile] 	at mill.api.daemon.Logger.withChromeProfile(Logger.scala:50)
[15-compile] 	at mill.api.daemon.Logger.withChromeProfile$(Logger.scala:12)
[15-compile] 	at mill.internal.PrefixLogger.withChromeProfile(PrefixLogger.scala:21)
[15-compile] 	at mill.exec.Execution.execute0(Execution.scala:334)
[15-compile] 	at mill.exec.Execution.executeTasks$$anonfun$1$$anonfun$1(Execution.scala:97)
[15-compile] 	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
[15-compile] 	at mill.exec.Execution.executeTasks$$anonfun$1(Execution.scala:98)
[15-compile] 	at mill.api.daemon.Logger$Prompt$NoOp.withPromptUnpaused(Logger.scala:191)
[15-compile] 	at mill.exec.Execution.executeTasks(Execution.scala:99)
[15-compile] 	at mill.eval.EvaluatorImpl.execute(EvaluatorImpl.scala:194)
[15-compile] 	at mill.api.Evaluator.executeApi$$anonfun$2(Evaluator.scala:128)
[15-compile] 	at mill.api.BuildCtx$.withFilesystemCheckerDisabled$$anonfun$1(BuildCtx.scala:31)
[15-compile] 	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
[15-compile] 	at mill.api.BuildCtx$.withFilesystemCheckerDisabled(BuildCtx.scala:31)
[15-compile] 	at mill.api.Evaluator.executeApi(Evaluator.scala:130)
[15-compile] 	at mill.api.Evaluator.executeApi$(Evaluator.scala:22)
[15-compile] 	at mill.eval.EvaluatorImpl.executeApi(EvaluatorImpl.scala:24)
[15-compile] 	at mill.bsp.worker.MillBuildServer.evaluate(MillBuildServer.scala:1012)
[15-compile] 	at mill.bsp.worker.MillBuildServer.$anonfun$23(MillBuildServer.scala:537)
[15-compile] 	at scala.collection.StrictOptimizedIterableOps.map(StrictOptimizedIterableOps.scala:100)
[15-compile] 	at scala.collection.StrictOptimizedIterableOps.map$(StrictOptimizedIterableOps.scala:87)
[15-compile] 	at scala.collection.immutable.Map$Map1.map(Map.scala:259)
[15-compile] 	at mill.bsp.worker.MillBuildServer.buildTargetCompile$$anonfun$1(MillBuildServer.scala:529)
[15-compile] 	at mill.bsp.worker.MillBuildServer.$anonfun$41(MillBuildServer.scala:922)
[15-compile] 	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[15-compile] 	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[15-compile] 	at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1$$anonfun$1$$anonfun$1(MillBuildServer.scala:876)
[15-compile] 	at scala.Option.foreach(Option.scala:437)
[15-compile] 	at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1$$anonfun$1(MillBuildServer.scala:873)
[15-compile] 	at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1$$anonfun$adapted$1(MillBuildServer.scala:886)
[15-compile] 	at mill.server.MillDaemonServer$.withOutLock$$anonfun$1(MillDaemonServer.scala:188)
[15-compile] 	at scala.util.Using$.resource(Using.scala:296)
[15-compile] 	at mill.server.MillDaemonServer$.withOutLock(MillDaemonServer.scala:190)
[15-compile] 	at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1(MillBuildServer.scala:886)
[15-compile] 	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[15-compile] 	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[15-compile] 	at scala.Option.foreach(Option.scala:437)
[15-compile] 	at mill.bsp.worker.MillBuildServer$$anon$1.run(MillBuildServer.scala:863)
Oct 10, 2025 7:17:49 PM org.eclipse.lsp4j.jsonrpc.RemoteEndpoint fallbackResponseError
SEVERE: Internal error: java.util.concurrent.RejectedExecutionException: Task mill.exec.ExecutionContexts$ThreadPool$PriorityRunnable@2f0c87f3 rejected from java.util.concurrent.ThreadPoolExecutor@fd07cbb[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 19202]
java.util.concurrent.CompletionException: java.util.concurrent.RejectedExecutionException: Task mill.exec.ExecutionContexts$ThreadPool$PriorityRunnable@2f0c87f3 rejected from java.util.concurrent.ThreadPoolExecutor@fd07cbb[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 19202]
at java.base/java.util.concurrent.CompletableFuture.wrapInCompletionException(CompletableFuture.java:323)
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:376)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:391)
at java.base/java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:729)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:531)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2221)
at mill.bsp.worker.MillBuildServer.$anonfun$41(MillBuildServer.scala:936)
at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1$$anonfun$1$$anonfun$1(MillBuildServer.scala:876)
at scala.Option.foreach(Option.scala:437)
at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1$$anonfun$1(MillBuildServer.scala:873)
at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1$$anonfun$adapted$1(MillBuildServer.scala:886)
at mill.server.MillDaemonServer$.withOutLock$$anonfun$1(MillDaemonServer.scala:188)
at scala.util.Using$.resource(Using.scala:296)
at mill.server.MillDaemonServer$.withOutLock(MillDaemonServer.scala:190)
at mill.bsp.worker.MillBuildServer$$anon$1.run$$anonfun$1(MillBuildServer.scala:886)
at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
at scala.Option.foreach(Option.scala:437)
at mill.bsp.worker.MillBuildServer$$anon$1.run(MillBuildServer.scala:863)
Caused by: java.util.concurrent.RejectedExecutionException: Task mill.exec.ExecutionContexts$ThreadPool$PriorityRunnable@2f0c87f3 rejected from java.util.concurrent.ThreadPoolExecutor@fd07cbb[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 19202]
at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2032)
at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:787)
at java.base/java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1328)
at mill.exec.ExecutionContexts$ThreadPool.execute(ExecutionContexts.scala:63)
at scala.concurrent.impl.Promise$Transformation.submitWithValue(Promise.scala:429)
at scala.concurrent.impl.Promise$DefaultPromise.submitWithValue(Promise.scala:338)
at scala.concurrent.impl.Promise$DefaultPromise.dispatchOrAddCallbacks(Promise.scala:312)
at scala.concurrent.impl.Promise$DefaultPromise.map(Promise.scala:182)
at mill.exec.Execution.evaluateTerminals$1$$anonfun$1(Execution.scala:271)
at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:619)
at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:617)
at scala.collection.AbstractIterable.foreach(Iterable.scala:935)
at mill.exec.Execution.evaluateTerminals$1(Execution.scala:160)
at mill.exec.Execution.execute0$$anonfun$1(Execution.scala:293)
at mill.api.daemon.Logger.withChromeProfile(Logger.scala:50)
at mill.api.daemon.Logger.withChromeProfile$(Logger.scala:12)
at mill.internal.PrefixLogger.withChromeProfile(PrefixLogger.scala:21)
at mill.exec.Execution.execute0(Execution.scala:334)
at mill.exec.Execution.executeTasks$$anonfun$1$$anonfun$1(Execution.scala:97)
at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
at mill.exec.Execution.executeTasks$$anonfun$1(Execution.scala:98)
at mill.api.daemon.Logger$Prompt$NoOp.withPromptUnpaused(Logger.scala:191)
at mill.exec.Execution.executeTasks(Execution.scala:99)
at mill.eval.EvaluatorImpl.execute(EvaluatorImpl.scala:194)
at mill.api.Evaluator.executeApi$$anonfun$2(Evaluator.scala:128)
at mill.api.BuildCtx$.withFilesystemCheckerDisabled$$anonfun$1(BuildCtx.scala:31)
at scala.util.DynamicVariable.withValue(DynamicVariable.scala:59)
at mill.api.BuildCtx$.withFilesystemCheckerDisabled(BuildCtx.scala:31)
at mill.api.Evaluator.executeApi(Evaluator.scala:130)
at mill.api.Evaluator.executeApi$(Evaluator.scala:22)
at mill.eval.EvaluatorImpl.executeApi(EvaluatorImpl.scala:24)
at mill.bsp.worker.MillBuildServer.evaluate(MillBuildServer.scala:1012)
at mill.bsp.worker.MillBuildServer.$anonfun$23(MillBuildServer.scala:537)
at scala.collection.StrictOptimizedIterableOps.map(StrictOptimizedIterableOps.scala:100)
at scala.collection.StrictOptimizedIterableOps.map$(StrictOptimizedIterableOps.scala:87)
at scala.collection.immutable.Map$Map1.map(Map.scala:259)
at mill.bsp.worker.MillBuildServer.buildTargetCompile$$anonfun$1(MillBuildServer.scala:529)
at mill.bsp.worker.MillBuildServer.$anonfun$41(MillBuildServer.scala:922)
... 14 more

[lihaoyi]

Thanks for the report @nk2IsHere, could you email me a copy of the log files in custom-java/out/mill-bsp-out/mill-bsp so I can see if there's any clues in there?

[lihaoyi]

@nk2IsHere I didn't see anything obvious in the logs you send me, could you help minimize a repro and attach it to this issue?

[nk2IsHere]

Hey! A small update from me - it seems after rebooting macOS i cannot reproduce it (same goes for #5965) - so my current assumption is it is something related to long sessions with mill.
I will keep looking for ways to reproduce it and if ill encounter it again, i will ping you here :)