[Libguestfs] nbdkit design: varying the rate limit parameter

[Richard W.M. Jones]

I'm embellishing this quite a lot, but this was an alternative
proposal we discussed on IRC, originated by Pino Toscano:

We'd add a "reload" callback to plugins and filters.  Calling this is
triggered when the server receives SIGHUP [currently ignored by the
server].  The plugins and filters could treat this any way they
wanted, but a suggestion would be that if they are taking
configuration from a file then they would reload the information from
that file.

Note that nbdkit itself does not have a configuration file, and we
don't really plan one, so this doesn't affect the server.  Only the
error filter uses an external "configuration file" (of sorts) but
that's a special case so I don't think it would use this file.

We'd then implement the rate filter with new parameters rate-file and
connection-rate-file which would read the rate out of a file, and wire
up the reload callback to re-read those files.

I think we'd likely want the reload callback to serialise with all
other callbacks, even for fully parallel plugins.

Rich.

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