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From "Andy Isaacson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4178) shell scripts should not close stderr
Date Mon, 19 Nov 2012 18:48:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4178?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13500496#comment-13500496
] 

Andy Isaacson commented on HDFS-4178:
-------------------------------------

bq. (I was under the misimpression that fd 0-2 were reserved unless explicit opened, or that
they were redirected to /dev/null after dropping the controlling terminal)

Yeah, it's really surprising that this pitfall is left lurking for people to stumble into!
 There's no credible use case for leaving fd 0,1,2 closed during process startup and it would
be a huge win for {{_start}} to open {{/dev/null}} as appropriate before running {{main()}},
but unfortunately I've confirmed that this is not done and I actually did experimentally trigger
the "glibc detected bad free"-in-my-datafile failure mode under glibc 2.13 running with {{at}}.

Thanks for committing the fix!
                
> shell scripts should not close stderr
> -------------------------------------
>
>                 Key: HDFS-4178
>                 URL: https://issues.apache.org/jira/browse/HDFS-4178
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: scripts
>    Affects Versions: 2.0.2-alpha
>            Reporter: Andy Isaacson
>            Assignee: Andy Isaacson
>             Fix For: 3.0.0, 2.0.3-alpha
>
>         Attachments: hdfs4178.txt
>
>
> The {{start-dfs.sh}} and {{stop-dfs.sh}} scripts close stderr for some subprocesses using
the construct
> bq. {{2>&-}}
> This is dangerous because child processes started up under this scenario will re-use
filedescriptor 2 for opened files.  Since libc and many other codepaths assume that filedescriptor
2 can be written to in error conditions, this can potentially result in data corruption.
> Much better to redirect stderr using the construct {{2>/dev/null}}.

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