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    registrar: replace "cmdtype" with an intent-based mechanism (API) · dfc51a482031
    Gregory Szorc authored
    Commands perform varied actions and repositories vary in their
    capabilities.
    
    Historically, the .hg/requires file has been used to lock out clients
    lacking a requirement. But this is a very heavy-handed approach and
    is typically reserved for cases where the on-disk storage format
    changes and we want to prevent incompatible clients from operating on
    a repo.
    
    Outside of the .hg/requires file, we tend to deal with things like
    optional, extension-provided features via checking at call sites.
    We'll either have checks in core or extensions will monkeypatch
    functions in core disabling incompatible features, enabling new
    features, etc.
    
    Things are somewhat tolerable today. But once we introduce alternate
    storage backends with varying support for repository features and
    vastly different modes of behavior, the current model will quickly
    grow unwieldy. For example, the implementation of the "simple store"
    required a lot of hacks to deal with stripping and verify because
    various parts of core assume things are implemented a certain way.
    Partial clone will require new ways of modeling file data retrieval,
    because we can no longer assume that all file data is already local.
    In this new world, some commands might not make any sense for certain
    types of repositories.
    
    What we need is a mechanism to affect the construction of repository
    (and eventually peer) instances so the requirements/capabilities
    needed for the current operation can be taken into account. "Current
    operation" can almost certainly be defined by a command. So it makes
    sense for commands to declare their intended actions.
    
    This commit introduces the "intents" concept on the command registrar.
    "intents" captures a set of strings that declare actions that are
    anticipated to be taken, requirements the repository must possess, etc.
    These intents will be passed into hg.repo(), which will pass them into
    localrepository, where they can be used to influence the object being
    created. Some use cases for this include:
    
    * For read-only intents, constructing a repository object that doesn't
      expose methods that can mutate the repository. Its VFS instances
      don't even allow opening a file with write access.
    * For read-only intents, constructing a repository object without
      cache invalidation logic. If the repo never changes during its lifetime,
      nothing ever needs to be invalidated and we don't need to do expensive
      things like verify the changelog's hidden revisions state is accurate
      every time we access repo.changelog.
    * We can automatically hide commands from `hg help` when the current
      repository doesn't provide that command. For example, an alternate
      storage backend may not support `hg commit`, so we can hide that
      command or anything else that would perform local commits.
    
    We already kind of had an "intents" mechanism on the registrar in the
    form of "cmdtype." However, it was never used. And it was limited to
    a single value. We really need something that supports multiple
    intents. And because intents may be defined by extensions and at this
    point are advisory, I think it is best to define them in a set rather
    than as separate arguments/attributes on the command.
    
    Differential Revision: https://phab.mercurial-scm.org/D3376
    dfc51a482031