[GHC] #13167: GC and weak reference finalizers and exceptions
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When GC runs a number of finalizers in a row, and the first of them throws an exception, then other finalizers are ignored. The relevant piece of code is [https://github.com/ghc/ghc/blob/fb4092642f057f258d07cd6979925f4e2579eda6/lib... here]. The following program reproduces the issue: {{{ import Data.IORef import Control.Monad import Control.Exception import System.Mem main :: IO () main = do run run run run performMajorGC performMajorGC run :: IO () run = do ref <- newIORef () void $ mkWeakIORef ref $ do putStr "." throwIO $ ErrorCall "failed" }}} I expect it to output "....", but I get only "." The issue makes it unsafe to rely on finalizer for resource cleanup because unrelated finalizer (e.g. from some other library) may prevent your finalizer from running. Actually I was sure the issue is known, but today I tried to find a reference to it, and failed. If it is by design, then it should be documented somewhere. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * cc: simonmar (added) Comment: Adding `simonmar` who is king of finalisers. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonmar): Yes, this is technically a bug. We could have `runFinalizerBatch` catch and discard exceptions (or print them to stdout; it's really a bug if a finalizer throws an exception). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by andrewthad): I've open a PR that adds docs explaining this behavior. https://github.com/ghc/ghc/pull/51 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by Phyx-): * cc: Phyx- (added) Comment: Replying to [comment:2 simonmar]:
Yes, this is technically a bug. We could have `runFinalizerBatch` catch and discard exceptions (or print them to stdout; it's really a bug if a finalizer throws an exception).
Any suggestions on how? the I/O manager seems to rely on using finalizers to flush buffers, however for some reason my new I/O manager's finalizers never get called. I assume because something else threw an exception. I've modified `runFinalizerBatch` to handle exceptions (unless I misunderstood the code) {{{ runFinalizerBatch :: Int -> Array# (State# RealWorld -> State# RealWorld) -> IO () runFinalizerBatch (I# n) arr = let go m = IO $ \s -> case m of 0# -> (# s, () #) _ -> let !m' = m -# 1# in case indexArray# arr m' of { (# io #) -> case catch# (\p -> (# io p, () #)) (\_ s'' -> (# s'', () #)) s of { (# s', _ #) -> unIO (go m') s' }} in go n }}} With `mio` the output varies between `..` and `....`, so not very deterministic. with `winio` none of it ever runs... Any ideas what else might be making these handlers not run at all? or how I can debug this? for `winio` I can imagine one of the finalizers deadlocking, an ffi call maybe. But the mio output is strange. If we're going to rely on them for managing handles and buffers they would ideally be a bit more robust... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonmar): The example program in the description doesn't wait for the finalizer threads to complete, so it might exit while there are still finalizers alive. This is why you don't see deterministic output. Probably adding a `threadDelay 1000000` at the end would be enough. A deadlock sounds like a plausible explanation for the behaviour you're seeing. The fix to `runFinalizerBatch` looks good - we should probably do that anyway (I'm sure you could construct a test case to demonstrate the problem that it fixes). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Phyx-): ah indeed, `threadDelay` worked! Ok I'll whip up a testcase and patch. I'll see if I can't add some debug code to figure out which finalizer is deadlocking. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4693 Wiki Page: | -------------------------------------+------------------------------------- Changes (by Phyx-): * status: new => patch * differential: => Phab:D4693 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13167: GC and weak reference finalizers and exceptions
-------------------------------------+-------------------------------------
Reporter: Yuras | Owner: (none)
Type: bug | Status: patch
Priority: normal | Milestone:
Component: Compiler | Version: 8.0.1
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4693
Wiki Page: |
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Comment (by Ben Gamari
#13167: GC and weak reference finalizers and exceptions -------------------------------------+------------------------------------- Reporter: Yuras | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4693 Wiki Page: | -------------------------------------+------------------------------------- Changes (by Phyx-): * status: patch => closed * resolution: => fixed * milestone: => 8.6.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13167#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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