REPL: thread blocked indefinitely in an MVar operation

[shirouto]

Description

While running forM_ [0..4096 print] in etlas repl, I got

eta: panic! (the 'impossible' happened)                                                                                                                                                                                                       
  (Eta version 0.8.6b2):                                                                                                                                                                                                                      
        java.lang.RuntimeException: Failed during evalIO of Eta REPL expression                                                                                                                                                               
        at eta.serv.REPLClassLoader.evalIO(REPLClassLoader.java:263)                                                                                                                                                                          
        at eta.serv.Utils.evalIO(Utils.java:42)                                                                                                                                                                                               
        at main.eta.serv.Run$sat$33.applyV(Run.hs:84)                                                                                                                                                                                         
        at eta.runtime.exception.Exception.catch_(Exception.java:132)                                                                                                                                                                         
        at main.eta.serv.Run.run1(Run.hs:181)                                                                                                                                                                                                 
        at main.Lib.$wa(Lib.hs:23)
        at main.Main$sat$1.applyV(Main.hs:36)
        at eta.runtime.exception.Exception.maskUninterruptible(Exception.java:69)
        at main.Main.main6(Main.hs:37)
        at main.Main.main4(Main.hs:34)
        at main.Main.main1(Main.hs:32)
        at main.Main$main1.applyV(Main.hs)
        at eta.runtime.exception.Exception.catch_(Exception.java:132)
        at main.Main.main7(Main.hs)
        at main.Main.DZCmain(Main.hs:25)
        at main.Main$DZCmain.applyV(Main.hs:25)
        at eta.runtime.stg.Closures$EvalLazyIO.enter(Closures.java:125)
        at eta.runtime.stg.Capability.schedule(Capability.java:254)
        at eta.runtime.stg.Capability.scheduleClosure(Capability.java:210)
        at eta.runtime.Runtime.evalLazyIO(Runtime.java:372)
        at eta.runtime.Runtime.main(Runtime.java:365)
        at eta.main.main(Unknown Source)
Caused by: java.lang.RuntimeException: Failed during evalIOInternal of Eta REPL expression
        at eta.serv.REPLClassLoader.evalIOInternal(REPLClassLoader.java:381)
        at eta.serv.REPLClassLoader.evalIO(REPLClassLoader.java:261)
        ... 21 more
Caused by: java.lang.reflect.InvocationTargetException
        at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at eta.serv.REPLClassLoader.evalIOInternal(REPLClassLoader.java:379)
        ... 22 more
Caused by: eta.runtime.exception.EtaException: thread blocked indefinitely in an MVar operation
        at eta.runtime.stg.Capability.detectMVarDeadlock(Capability.java:711)
        at eta.runtime.stg.Capability.idleLoop(Capability.java:698)
        at eta.runtime.stg.Capability.blockedLoop(Capability.java:736)
        at eta.runtime.stg.Capability.blockedLoop(Capability.java:732)
        at eta.runtime.concurrent.Concurrent.takeMVar(Concurrent.java:79)
        at base.ghc.io.handle.Internals.$wa2(Internals.hs:164)
        at base.ghc.io.handle.Internals$a3.applyV(Internals.hs:133)
        at eta.runtime.exception.Exception.maskAsyncExceptions(Exception.java:45)
        at base.ghc.io.handle.Internals.$wa4(Internals.hs:131)
        at base.ghc.io.handle.Internals.$wa3(Internals.hs:237)
        at base.ghc.io.handle.Internals.wantWritableHandle1(Internals.hs:227)
        at base.ghc.io.Handle.hFlush1(Handle.hs:305)
        at base.ghc.io.Handle.hFlush(Handle.hs:305)
        at base.ghc.io.Handle$hFlush.apply1V(Handle.hs:305)

This is probably related to a stack overflow issue similar to #866 and #543. Compiling the code with -fno-ignore-interface-pragmas and -fenable-rewrite-rules or -O1 is a way around, but these optimizations are not available in the present etlas repl.

Expected Behavior

The code should run just fine.

Actual Behavior

The repl dumps a stack trace.

Possible Fix

As referred in this comment, one can re-write the code using trampolineIO or a stack managed IO approach.

Steps to Reproduce

1. etlas repl
2. Evaluate forM_ [0..4096] print