logging: lazy representation, interpolation, formatting, etc.

[zooko]

In Mojo Nation's and Mnet's logging system, you could pass a format string and arguments, like this:

log("A %s had a %s, leading to a %s.", myplate, eggsplosion, mess)

And the logging system would invoke formatstr % args only if the log message was interesting enough to write to the log file. If it was not going to be written, then the logging system lazily avoided formatting it.

I would like for Tahoe's logging system to have such a feature.

See also ticket #57.

[zooko]

This is super thoroughly fixed by foolscap logging, which not only doesn't interpolate the arguments into a string until the last possible moment, but which also has a bunch of other features to minimize the network and CPU load of running logging when you're not actually looking at the logs.