Service update ignores existing log-driver when setting log-opt

as reported in https://github.com/docker/docker.github.io/issues/8745 by @Tomcat-Engineering

When updating a service (docker service update), any log-opt specified is ignored unless a log-driver is also set when updating.

this looks to be a combination of something that's by design, and (what I think is) a bug in the docker service update logic; docker service create and update should follow slightly different logic (and I think they currently use the same).

• expected: setting log-opt for a service that does not have a logging-driver configured isn't supported
  • reason for this is https://github.com/moby/moby/issues/31515#issuecomment-321011661 that in a swarm setup, all instances of a service should use the same configuration (including logging-driver), so when specifying options, they must be accompanied with the driver
• t.b.d.: creating a service with a log-opt set, but no logging driver silently ignores the option
  • should it produce an error? a warning?
• bug: setting log-opt for a service that has a logging driver configured (when created), but without specifying that driver during update is silently ignored.
  • when updating a service, the CLI should take the existing logging-driver, and update the options

Taking a full example;

docker service create --log-driver=json-file --log-opt max-file=5 --name loggie nginx:alpine
docker service inspect --format='{{json .Spec.TaskTemplate.LogDriver}}' loggie
{"Name":"json-file","Options":{"max-file":"5"}}

Updating without logging-driver set doesn't take effect. IMO this should work, given that the service already had a logging-driver configured. It should still fail (or warn) if no logging-driver was configured yet on the service.

docker service update --log-opt max-file=15 loggie
docker service inspect --format='{{json .Spec.TaskTemplate.LogDriver}}' loggie
{"Name":"json-file","Options":{"max-file":"5"}}

Updating with a logging driver specified does;

docker service update --log-driver=json-file --log-opt max-file=15 loggie
docker service inspect --format='{{json .Spec.TaskTemplate.LogDriver}}' loggie
{"Name":"json-file","Options":{"max-file":"15"}}

We should fix the update case:

• if a logging-driver is set on the service, keep that driver and set the log-opts
• if no logging-driver is set, and none is specified when updating; produce a warning (or error)
  • this validation should be handled daemon-side
• when creating a service; produce a warning (or error) if a log-opt is set, but no log-driver
  • this validation should be handled daemon-side

Start with the docker service update and docker service create entry points and reproduce the commands in the issue, then inspect how log-driver and log-opt values are assembled. Compare the update behavior with the existing service specification. Done means existing drivers are preserved during option-only updates and the unspecified create/update validation behavior is implemented consistently.