Previously, when the CLI wanted to install a plugin, it used a special
method, `DownloadPlugin` on the `httpstate` backend to actually fetch
the tarball that had the plugin. The reason for this is largely tied
to history, at one point during a closed beta, we required presenting
an API key to download plugins (as a way to enforce folks outside the
beta could not download them) and because of that it was natural to
bake that functionality into the part of the code that interfaced with
the rest of the API from the Pulumi Service.
The downside here is that it means we need to host all the plugins on
`api.pulumi.com` which prevents community folks from being able to
easily write resource providers, since they have to manually manage
the process of downloading a provider to a machine and getting it on
the `$PATH` or putting it in the plugin cache.
To make this easier, we add a `--server` argument you can pass to
`pulumi plugin install` to control the URL that it attempts to fetch
the tarball from. We still have perscriptive guidence on how the
tarball must be
named (`pulumi-[<type>]-[<provider-name>]-vX.Y.Z.tar.gz`) but the base
URL can now be configured.
Folks publishing packages can use install scripts to run `pulumi
plugin install` passing a custom `--server` argument, if needed.
There are two improvements we can make to provide a nicer end to end
story here:
- We can augment the GetRequiredPlugins method on the language
provider to also return information about an optional server to use
when downloading the provider.
- We can pass information about a server to download plugins from as
part of a resource registration or creation of a first class
provider.
These help out in cases where for one reason or another where `pulumi
plugin install` doesn't get run before an update takes place and would
allow us to either do the right thing ahead of time or provide better
error messages with the correct `--server` argument. But, for now,
this unblocks a majority of the cases we care about and provides a
path forward for folks that want to develop and host their own
resource providers.
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