Consider triggering replacement for CopyFile when local file changes

[t0yv0]

Hello!

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Issue details

CopyFile does not sync the local file to the remote file when the local file changes. This can be implemented manually by using triggers with the hash of the local file forcing a replace. But perhaps should be the default behavior? If not, perhaps we can make this trigger example easy to find for new users.

Please see details worked out in the gist: https://gist.github.com/t0yv0/addb78402b3e91a910b2021bce7c8e1d

Also, tangentially, remote.Command that depends_on the CopyFile does not replace when CopyFile replaces. Is this expected? It can be made replace itself if given the same triggers as CopyFile in the current implementation.

Affected area/feature

[iwahbe]

CopyFile takes a local path string right now. I would be hesitant to add magic replacement semantics there. We could and should add the ability to specify an asset instead of a path, which would solve this for us.

[t0yv0]

That can be a good idea. I think the key here is the default documentation path that folks are looking at should enable the "keep the server file in sync with the local one sceanario" - that seems to me a better "default", even if it's not technically a default and needs some settings added.

[iwahbe]

I'm thinking something like

localPath: string
description: >
  The path to the file to be copied. This should be used only when you
  don't want CopyFile to trigger replacements because of underlying file
  changes.

  Mutually exclusive with `localFile`.

localFile: Asset
description: >
  The file to be copied.

  Mutually exclusive with `localPath`.

[t0yv0]

Can mention the trigger recipe into localPath.

If you want the path to sync upon changes, add a trigger to the `triggers` property with with the hash of the file contents.

[lukehoban]

Closing as duplicate of #33 and #42.