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Wait on child PID after failed exec#14

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dcoutts merged 1 commit intohaskell:masterfrom
DaveCTurner:patch-1
Jan 11, 2015
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Wait on child PID after failed exec#14
dcoutts merged 1 commit intohaskell:masterfrom
DaveCTurner:patch-1

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@DaveCTurner DaveCTurner commented Jan 9, 2015

On Linux, the following program generates 10 zombie processes, one for each time that callProcess tries to call a nonexistent process.

import Control.Concurrent
import Control.Exception
import Control.Monad
import System.IO
import System.Process

ignoreIOException :: IOException -> IO ()
ignoreIOException _ = return ()

main = do
  _ <- forkIO $ replicateM_ 10 $ callProcess "does-not-exist" [] `catch` ignoreIOException
  _ <- getLine
  return ()

The reason seems to be that the child PID leaks because the parent does not call waitpid() if the child has failed. This simple change seems to fix it.

@DaveCTurner DaveCTurner mentioned this pull request Jan 9, 2015
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dcoutts commented Jan 10, 2015

After looking at it for a while, I agree.

Minor point, as far as I can see (pid > 0) is necessarily true at that point in the code, and I'd add a comment, e.g.

     // We forked the child, but the child had a problem and stopped
     // so it's our responsibility to reap here as nobody else can.
     waitpid(pid, &unused, 0);

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DaveCTurner commented Jan 10, 2015

Seems reasonable. Do you want to make that change yourself or do you want another PR?

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DaveCTurner commented Jan 10, 2015

Ok, 77dc6dc takes your suggestions into account.

The docs for waitpid() say "If status is not NULL,..." which sort of implies that NULL is acceptable when you don't care about the status. So I've used that.

dcoutts added a commit that referenced this pull request Jan 11, 2015
@dcoutts
Merge pull request #14 from DaveCTurner/patch-1
29f0480 
Wait on child PID after failed exec
@dcoutts dcoutts merged commit 29f0480 into haskell:master Jan 11, 2015
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dcoutts commented Jan 11, 2015

Thanks!

hvr added a commit that referenced this pull request Jan 15, 2015
@hvr
Bump version to 1.2.2.0 and update changelog
93d8b62 
This adds changelog entries for the issues #14 and #15 addressed in
1.2.2 so far.
@hvr hvr mentioned this pull request Feb 12, 2015
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@apoikos apoikos mentioned this pull request Dec 11, 2019
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@DaveCTurner @dcoutts