[libvirt-users] is libvirt java binding thread safe?

[Daniel P. Berrange]

On Fri, Sep 03, 2010 at 01:02:09PM +0200, Matthias Bolte wrote:
> Ah, okay, now I understand what you want to say.
> 
> You have two threads A and B. When A runs on its own then it's just
> fine, but when you add B (that triggers an exception on purpose and
> ignores it) then A picks it up and reports it. This is the point where
> thread safety comes to mind.
> 
> libvirt stores errors in thread-local-storage. It uses
> pthread_key_create/pthread_{get,set}specific for this (or
> TlsAlloc/Tls{Get,Set}Value on Windows).
> 
> I think what's happening here is that all your threads in Java share
> the same thread-local-storage. Therefore, thread A can pickup the
> error triggered by thread B, that should not happen.
> 
> I'm not sure how to fix that.

Looking at the java code, I believe the problem is that the java
bindings are *not* using the threadsafe error APIs:

In Connect.java

    /**
     * call the error handling logic. Should be called after every libvirt call
     * 
     * @throws LibvirtException
     */
    protected void processError() throws LibvirtException {
        ErrorHandler.processError(libvirt, VCP);
    }

Which calls into

    public static void processError(Libvirt libvirt, ConnectionPointer conn) throws LibvirtException {
        virError vError = new virError();
        int errorCode = libvirt.virConnCopyLastError(conn, vError);

And virConnCopyLastError is *not* threadsafe:


/**
 * virConnCopyLastError:
 * @conn: pointer to the hypervisor connection
 * @to: target to receive the copy
 *
 * Copy the content of the last error caught on that connection
 *
 * This method is not protected against access from multiple
 * threads. In a multi-threaded application, always use the
 * global virGetLastError() API which is backed by thread
 * local storage.

Regards,
Daniel
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