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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-9972) new APIs for listStatus and globStatus to deal with symlinks
Date Tue, 24 Sep 2013 17:24:04 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-9972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13776512#comment-13776512
] 

Colin Patrick McCabe commented on HADOOP-9972:
----------------------------------------------

bq. I mean listStatus(Path, PathOption) should call into listLinkStatus(it is HDFS::listStatus
which is a primitive RPC call), not the other way around. I wonder how can we implement listStatus(Path,
PathOption) without the primitive of listLinkStatus(Path)?

FileSystem#listStatus(Path, PathOption) should just be an abstract function which is implemented
by DistributedFilesystem and other implementation classes.  DistributedFileSystem, and the
other FileSystem implementations, need to get access to the other things in PathOption, such
as the error handler.  Also, if we want to add more options in the future, we don't want to
create listLinkStatusWithFoo and listLinkStatusWithFooAndBar.  Just listStatus(Path, PathOption).

I understand that bash globs ignore errors.  But that's not really a good reason why we shouldn't.
 Hadoop and HDFS exist in an environment where there are unreliable networks.  So if globStatus
swallows unresolved symlink errors, you could find yourself in a situation where your cross-filesystem
symlink fails, and you silently operate on data that isn't what you think you're operating
on.  There are also compatibility reasons not to ignore errors-- errors were not ignored in
branch-1.  We discussed this on HADOOP-9929.
                
> new APIs for listStatus and globStatus to deal with symlinks
> ------------------------------------------------------------
>
>                 Key: HADOOP-9972
>                 URL: https://issues.apache.org/jira/browse/HADOOP-9972
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: fs
>    Affects Versions: 2.1.1-beta
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>
> Based on the discussion in HADOOP-9912, we need new APIs for FileSystem to deal with
symlinks.  The issue is that code has been written which is incompatible with the existence
of things which are not files or directories.  For example,
> there is a lot of code out there that looks at FileStatus#isFile, and
> if it returns false, assumes that what it is looking at is a
> directory.  In the case of a symlink, this assumption is incorrect.
> It seems reasonable to make the default behavior of {{FileSystem#listStatus}} and {{FileSystem#globStatus}}
be fully resolving symlinks, and ignoring dangling ones.  This will prevent incompatibility
with existing MR jobs and other HDFS users.  We should also add new versions of listStatus
and globStatus that allow new, symlink-aware code to deal with symlinks as symlinks.

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