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Date: Tue, 26 Nov 2013 06:12:36 +0000 (UTC)
From: "Michael Allen (JIRA)" <jira@apache.org>
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Message-ID: <JIRA.12681104.1385405606887.29242.1385446356686@arcas>
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Subject: [jira] [Commented] (ACCUMULO-1928) Class loader issues with
 building auth/auth/perm extensions
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    [ https://issues.apache.org/jira/browse/ACCUMULO-1928?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13832349#comment-13832349 ] 

Michael Allen commented on ACCUMULO-1928:
-----------------------------------------

I don't think there's any real point to on the fly reloading of those particular classes, but if that's the case then some pieces of documentation should make it clear that you want to avoid putting those kinds of classes in lib/ext.

We might need to give some care to thinking about where AccumuloVFSClassLoader is used, because it should not be used indiscriminately wherein classes loaded by multiple different class loaders may interact with each other.  I don't know of a good way to audit the code for this, as these bugs are incredibly subtle ideas not subject to a good static analysis, other than just literally looking at every spot AccumuloVFSClassLoader (and its helpers like Property.createInstanceFromPropertyName()) gets used and make sure that the objects are more or less "self contained" within their own little class loader universes.  

(Now this bug is starting to sound like a Doctor Who episode.  Time for me to go to bed.  :)  )

> Class loader issues with building auth/auth/perm extensions
> -----------------------------------------------------------
>
>                 Key: ACCUMULO-1928
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-1928
>             Project: Accumulo
>          Issue Type: Bug
>    Affects Versions: 1.5.0, 1.6.0
>            Reporter: Michael Allen
>
> In the course of building extensions to take advantage of the {{Authenticator}}, {{Authorizor}} and {{PermissionHandler}} interfaces, I ran into several issues with how the extension classes were loaded (and reloaded) using {{AccumuloVFSClassloader}}.  These issues ultimately meant that we had to make the decision to put these extensions into {{$ACCUMULO_HOME/lib}}, rather than {{$ACCUMULO_HOME/lib/ext}}, so that they would not be reloaded and we would not encounter those issues.
> We should figure out how to fix up the classloader such that it makes it possible to put extensions like these into {{$$ACCUMULO_HOME/lib/ext}}, or we should document that it is not recommended to do so.
> So what were the problems?
> 1. In 1.5.0, Authentication tokens from within the extension could not be loaded via the normal creation pathway.  This pathway in {{CredentialHelper}} includes the following line of code:
> {code}
> Object obj = Class.forName(tokenClass).newInstance();
> {code}
> In 1.6 this code has been replaced with a call in {{AuthenticationToken.AuthenticationTokenSerializer.deserialize()}} that uses {{AccumuloVFSClassloader}}, but I wonder if we shouldn't also make a 1.5.X change to at least make it possible to do it there?  Otherwise anyone trying to create an extension for 1.5.X can only use {{PasswordToken}}s
> 2.  Even when you do or don't fix #1, it leads you on to a different, less tractable problem.
> The {{Authenticatior}} etc. classes are loaded by {{SecurityOperation}} singleton, using {{AccumuloVFSClassloader}} and the objects are held within the singleton, which is held statically by the {{SecurityOperation}} class.  That's all well and good, and let's call that {{AccumuloVSFClassloader}} instances that loaded those classes {{CL1}}.
> But now let's say you stick a new version of your JAR file into {{$ACCUMULO_HOME/lib/ext}}.  {{AccumuloVFSClassloader}} happily creates a new classloader (call it {{CL2}}) for you and reloads your classes.  *However*, the objects instantiated by {{SecurityOperation}} still owe their lineage to {{CL1}}.  When a new authentication token comes in and gets loaded by {{CL2}}, its class name might look like a class from {{CL1}}, but when the {{Authenticator}} that came from {{CL1}} tries to cast that {{CL2}}-based object, you get {{ClassCastException}}s and are basically toast at that point.
> One possible fix here would be for {{AccumuloVFSClassloader}} to somehow notify interested parties that classes had reloaded so that they could flush away any old classes if desired.  There'd be some synchronization issues around doing this, to make sure that the security stuff still worked reliably through this process.  You'd have no guarantee, though, that the rest of the code wouldn't be hanging on to {{CL1}}-based object references and not letting go.  Beating those bugs out may require too much coder discipline to be worth the feature.
> Another possible fix is to just say "don't do that for security-related features and put them in {{$ACCUMULO_HOME/lib}}.  That's kind of giving up on being able to reload these at runtime, but maybe that's OK.  Generally speaking, reloading your security assumptions might be something you want to take the tablet server down for.


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