I found the flag --force to be a strange name for skipping a preview,
since that name is usually reserved for operations that might be harmful
and yet you're coercing a tool to do it anyway, knowing there's a chance
you're going to shoot yourself in the foot.
I also found that what I almost always want in the situation where
--force was being used is to actually just run a preview and have the
confirmation auto-accepted. Going straight to --force isn't the right
thing in a CI scenario, where you actually want to run a preview first,
just to ensure there aren't any issues, before doing the update.
In a sense, there are four options here:
1. Run a preview, ask for confirmation, then do an update (the default).
2. Run a preview, auto-accept, and then do an update (the CI scenario).
3. Just run a preview with neither a confirmation nor an update (dry run).
4. Just do an update, without performing a preview beforehand (rare).
This change enables all four workflows in our CLI.
Rather than have an explosion of flags, we have a single flag,
--preview, which can specify the mode that we're operating in. The
following are the values which correlate to the above four modes:
1. "": default (no --preview specified)
2. "auto": auto-accept preview confirmation
3. "only": only run a preview, don't confirm or update
4. "skip": skip the preview altogether
As part of this change, I redid a bit of how the preview modes
were specified. Rather than booleans, which had some illegal
combinations, this change introduces a new enum type. Furthermore,
because the engine is wholly ignorant of these flags -- and only the
backend understands them -- it was confusing to me that
engine.UpdateOptions stored this flag, especially given that all
interesting engine options _also_ accepted a dryRun boolean. As of
this change, the backend.PreviewBehavior controls the preview options.
6ad785d5c4