GHC uses O_NONBLOCK on regular files, which has no effect, and blocks the runtime

This is the outcome of https://mail.haskell.org/pipermail/ghc-devs/2018-May/015749.html

Reading through the code of readRawBufferPtr the first line jumped to my eye:

  | isNonBlocking fd = unsafe_read -- unsafe is ok, it can't block

This looks suspicious.

On Linux, if fd is a a descriptor to a regular file (on disk, a networked filesystem, or a block device), then O_NONBLOCK will have no effect, yet unsafe_read is used which will block the running OS thread.

You can read more about O_NONBLOCK not working on regular files on Linux here:

• https://www.nginx.com/blog/thread-pools-boost-performance-9x/
• https://stackoverflow.com/questions/8057892/epoll-on-regular-files
• https://jvns.ca/blog/2017/06/03/async-io-on-linux--select--poll--and-epoll/
• https://groups.google.com/forum/#!topic/comp.os.linux.development.system/K-fC-G6P4EA

And indeed, the following program does NOT keep printing things in the printing thread, and instead blocks for 30 seconds:

module Main where

import           Control.Concurrent
import           Control.Monad
import qualified Data.ByteString as BS
import           System.Environment

main :: IO ()
main = do
  args <- getArgs
  case args of
    [file] -> do

      forkIO $ forever $ do
        putStrLn "still running"
        threadDelay 100000 -- 0.1 s
      bs <- BS.readFile file
      putStrLn $ "Read " ++ show (BS.length bs) ++ " bytes"

    _ -> error "Pass 1 argument (a file)"

when compiled with

~/.stack/programs/x86_64-linux/ghc-8.2.2/bin/ghc --make -O -threaded blocking-regular-file-read-test.hs

on my Ubuntu 16.04 and on a 2GB file like

./blocking-regular-file-read-test /mnt/images/ubuntu-18.04-desktop-amd64.iso

And strace -f -e open,read on it shows:

open("/mnt/images/ubuntu-18.04-desktop-amd64.iso", O_RDONLY|O_NOCTTY|O_NONBLOCK) = 11
read(11,  <unfinished ...>

So GHC is trying to use O_NONBLOCK on regular files, which cannot work and will block when used through unsafe foreign calls like that.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	lehins, nh2
Operating system
Architecture