Custom Query

Reported by pwb, 4 years ago.

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Monad.State
newtype WrappedState s a = WS { runWS :: State s a }
    deriving (Monad, MonadState state)

Note that the 'deriving' for MonadState has the wrong variable name. It should be rejected with something like "not in scope: type variable 'state'" but instead this happens:

[pwb@rhuidean tmp]$ ghc bug.hs 
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.4 for x86_64-unknown-linux):
	No match in record selector Var.tcTyVarDetails

Reported by chak, 12 years ago.

This is re my posting under the subject "Weak pointers,
garbage collection & deadlocks" on
glasgow-haskell-users.

Basically, the problem is that finaliser threads that
the next GC would generate aren't taken into account
when determining whether there are any more threads to
run.

I attach a program reproducing the error.

Reported by mikhail.vorozhtsov, 20 months ago.

The following program

import qualified OpenSSL.Session as SSL
import OpenSSL (withOpenSSL)
import System.Posix.Types (Fd(..))

main = withOpenSSL $ do
  ctx <- SSL.context
  ssl <- SSL.fdConnection ctx (Fd (-1))
  SSL.tryConnect ssl
  return ()

triggers a "PAP object entered!" crash in HsOpenSSL 0.10.1:

$ ghc-7.3.20110916 -fforce-recomp Bug.hs
[1 of 1] Compiling Main             ( Bug.hs, Bug.o )
Linking Bug ...
$ ./Bug 
Bug: internal error: PAP object entered!
    (GHC version 7.3.20110916 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
zsh: abort      ./Bug

The order of things is as follows (OpenSSL.Session module):

1. tryConnect calls sslTryHandshake

2. sslTryHandshake calls _ssl_connect which returns -1

3. sslTryHandshake calls _ssl_get_error which returns 5 (SSL_ERROR_SYSCALL)

4. sslTryHandshake calls throwSSLException which in this case is throw SSLIOError

5. Program crashes

If I either replace throw with throwIO or build HsOpenSSL with -O0, the crash goes away. I haven't checked 7.2.1, but 7.0.3 doesn't have this issue.