Re: [equinox-dev] Safe logging from a WeavingHook?

["Thomas Watson" <tjwatson@xxxxxxxxxx>]

Hi

The Eclipse Log API (org.eclipse.core.runtime.ILog)
has the unfortunate contract that log listeners registered with it will
be called synchronously.  This can cause the issues you are seeing
with class loading if we get into a circularity.  In Equinox we actually
have one and only one log implementation and this is the OSGi LogService
implementation.  In Equinox we have an extension of the specification
in the org.eclipse.equinox.log package.  Here is where we have the
interface org.eclipse.equinox.log.SynchronousLogListener which allows us
to implement the contract of org.eclipse.core.runtime.ILog on top of.  All
other log APIs in Eclipse and Equinox are all implemented on top of this
single OSGi Log Service implementation.

In the extended API Equinox provides
to the OSGi Service API we also have the concept of named loggers.  Now
the org.eclipse.core.runtime.ILogListener that are registered will only
listen to a very specific logger named "org.eclipse.equinox.logger".
 This logger happens to also be the logger which also writes to the
persistent log for eclipse.  You could consider using the equinox
extended log service instead by calling org.eclipse.equinox.log.ExtendedLogService.getLogger(String)
and supplying your own logger name.  Anything logged here will be
sent to LogListeners asynchronously, unless you happen to have someone
implementing the SynchronousLogListener and paying attention to your logger
name.  But be aware that nobody is going to see your logs until there
is a org.osgi.service.log.LogService registered to listen to your logs.
 The one exception to this rule is when the LogEntry is of type ERROR,
in that case the log will be sent to the persistent eclipse log file BUT
it WILL NOT be sent to the listeners registered with the org.eclipse.core.runtime.ILog.

I'm unsure what you are trying to log,
but if it is simply ERRORs then I would use the standard org.osgi.service.log.LogService

I should also mention that most all
the concepts of the Equinox extended logger API in org.eclipse.equinox.log
will be included in the future OSGi R7 specification.  The one exception
is the SynchronousLogListener, which I believe to be a dangerous thing
to spec, and I think you may agree! :-)

Tom





From:      
 Stephan Herrmann <stephan.herrmann@xxxxxxxxx>
To:      
 equinox-dev@xxxxxxxxxxx
Date:      
 05/15/2016 07:07 AM
Subject:    
   [equinox-dev]
Safe logging from a WeavingHook?
Sent by:    
   equinox-dev-bounces@xxxxxxxxxxx

---

Hi,

When implementing a WeavingHook, is there a safe way to perform logging?

I'm asking because my current strategy recently broke when adding
the AERI error reporter to the mix:

I'm acquiring a log basically from
    Platform.getLog(bundleContext.getBundle())
(with extra caution to see if the platform is ready).

This works well until others register log listeners of their own.
Such breakage is tracked in https://bugs.eclipse.org/493566
showing a stack trace of what looks like deadly re-entrance,
causing NoClassDefFoundError.

I know that the Equinox framework uses a logger of its own,
which - I assume - does not support any log listeners, right?
Is it possible for a weaving hook implementation to log into
the framework log?

If no safe log is available, are there any points in the hook
protocol, where it is safe to log using a platform log?
I'm thinking of queueing log events until modified(WovenClass),
but even during that method I don't know if any class loading
is active further down the stack that may cause the same problem.
Do I have to maintain my own thread-local stack of classes
being defined to wait for a point when this stack is empty?
Or is it possible to get this information from the framework?

Interesting, how such a basic functionality like logging can blow
up a system, if both class loading and logging are extensible ...

thanks,
Stephan
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