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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3574) Fix small race and do some cleanup in GetImageServlet
Date Wed, 27 Jun 2012 21:03:43 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3574?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13402549#comment-13402549
] 

Colin Patrick McCabe commented on HDFS-3574:
--------------------------------------------

It seems like there is still some TOCTOUs here, since we're getting the length of the file
and then reading the file afterwards.  What if the length of the file changes?  Maybe we don't
care about this case, but perhaps there should be a comment that this race exists.

I also don't completely understand the logic behind this:
{code}
if (!imageFile.exists()) {
  // Potential race where the file was deleted while we were in the
  // process of setting headers!
  throw new FileNotFoundException();
}
// send fsImage
TransferFsImage.getFileServer(response, imageFile, fis, getThrottler(conf));
{code}

Can't the imageFile be deleted immediately after the imageFile.exists() check and before the
call to TransferFsImage#getFileServer?  So the original TOCTOU isn't really fixed either.

It seems like the only real fix would be creating a hardlink to a temporary file, so that
if a user deleted the original file we were concerned with, the data would not be lost.  Alternately
we could buffer the file in memory ahead of time, so that we know the length of what we're
going to send and it can't disappear.  But that probably isn't feasible for large files, at
least.
                
> Fix small race and do some cleanup in GetImageServlet
> -----------------------------------------------------
>
>                 Key: HDFS-3574
>                 URL: https://issues.apache.org/jira/browse/HDFS-3574
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: name-node
>    Affects Versions: 3.0.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Minor
>         Attachments: hdfs-3574.txt
>
>
> There's a very small race window in GetImageServlet, if the following interleaving occurs:
> - The Storage object returns some local file in the storage directory (eg an edits file
or image file)
> - *Race*: some other process removes the file
> - GetImageServlet calls file.length() which returns 0, since it doesn't exist. It thus
faithfully sets the Content-Length header to 0
> - getFileClient() throws FileNotFoundException when trying to open the file. But, since
we call response.getOutputStream() before this, the headers have already been sent, so we
fail to send the "404" or "500" response that we should.
> Thus, the client sees a 0-length Content-Length followed by 0 lengths of content, and
thinks it successfully has downloaded the target file, where in fact it downloads an empty
one.
> I saw this in practice during the "edits synchronization" phase of recovery while working
on HDFS-3077, though it could apply on existing code paths, as well, I believe.

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