Make logging a proper plugin with state and cleanup method

[jpfr]

No description provided.

[Pro]

this should use logger

[jpfr]

Absolutely.
I would go so far not to include #include "ua_log_stdout.h" here at all.

[Pro]

this should use logger

[Pro]

Same here and in the rest of the file

[mlgiraud]

Couldn't we make the logger a singleton so that we can access it wherever we want to?
That way we wouldn't have to have all those pesky logger variables in all kinds of places.
IMHO that would clean up the code a bit.

Might make the logging calls easier as well. Example:

static UA_INLINE UA_FORMAT(3,4) void
UA_LOG_WARNING(UA_LogCategory category, const char *msg, ...) {
#if UA_LOGLEVEL <= 400
    UA_Logger logger* = UA_GetLogger();
    va_list args; va_start(args, msg);
    logger->log(UA_LOGLEVEL_WARNING, category, msg, args);
    va_end(args);
#endif
}

UA_LOG_WARNING(category, "my log message");

[jpfr]

Couldn't we make the logger a singleton so that we can access it wherever we want to?

Singletons in C are global variables.
http://wiki.c2.com/?GlobalVariablesAreBad

Especially, global variables that are not a constant are bad...
You might have several instances of client / server in one process.
At which point can you shut down the (stateful) logger?

[coveralls]

Coverage decreased (-0.009%) to 85.059% when pulling 4229a2d on stateful_logging into 885bad0 on master.

[jpfr]

@Pro After trying several times, I am unable to find the source of the problem on OSX.
It is related to the posix architecture definitions.

Do you have a hunch of how to fix this?

[Pro]

@jpfr this must be related to the removal of ua_log_stdout.h
I reverted that change in one commit (ad4f3e6) and now OS X is green, since the issue is not related to your changes. The macro redefinition is already happening on master, check e.g. https://travis-ci.org/open62541/open62541/jobs/462374570
Looks like it is not failing even though there are warnings and errors in the build.

So let's keep my commit in this branch and I will add a new PR which tries to solve the issue with the queue.h.

The PR is here:
#2268