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

[Stephan Herrmann <stephan.herrmann@xxxxxxxxx>]

Hi Tom,

Thanks for your comprehensive answer.
Yes, we can easily agree that SynchronousLogListener is dangerous :)

More questions inline ...

On 05/16/2016 04:14 PM, Thomas Watson wrote:
> 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.

Sounds promising, but ...

>  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.

... is that s.t. I have to register? How?
Can I hook this up so that all entries do (eventually) appear in the
Errors view and perhaps the persistent log file?

> 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'm using it for all kinds of things. The particular dangerous case was logging
profiling data at level INFO.

> 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! :-)


On 05/16/2016 04:31 PM, Thomas Watson wrote:
> I forgot to mention how the framework logs.  Typically we do not actually log.  Instead we publish framework events which get picked
> up by the LogService implementation.  Since you are a framework extension you could consider doing the same thing using Equinox
> container APIs.  The container allows for events of type error, warning or info.  But not that again only the error types will get
> written to the persistent eclipse log.   One way to do this would be with the following code.

Works nicely for ERROR, but again: can I achieve that also WARNING and INFO are
"visible" to users? Right now they seem to go to /dev/null


thanks,
Stephan