Prior to Terraform v0.12 it was possible for a provider to secretly set
some default arguments for the "connection" block, which most commonly
included a hard-coded type of "ssh" and a value from "host".
In the interests of "explicit is better than implicit", Terraform 0.12 no
longer has this feature and instead requires connection settings to be
written explicitly in terms of the resource's exported attributes. For
compatibility though, the upgrade tool will insert expressions that are
as close as possible to the logic the provider formerly implemented, or
in a few rare cases a TF-UPGRADE-TODO comment to fix it up manually.
Some of the existing resource type implementations have incredibly
complicated implementations of selecting a single host IP address to use
and don't expose the result of that as an attribute, so for now we handle
those via a complicated Terraform language expression achieving the same
result. Ideally these providers would introduce a new attribute that
exports the same address formerly exported as the hostname before their
initial v0.12-compatible release, in which case we can simplify these to
just reference the attribute in question. That would be preferable also
because it would allow use of that exported attribute in other contexts,
such as in a null_resource provisioner somewhere else or in an output
to allow a caller to deal with the SSH part itself.
This uses the fixed "superset" schema from the main terraform package to
apply our standard expression mapping, with the exception of "type" where
interpolation sequences are not supported due to the type being evaluated
early to retrieve the schema for decoding the rest.
Aside from the two special meta-arguments "connection" and "provisioner"
this is just our standard mapping from schema to conversion rules, using
the provisioner's configuration schema.
Martin Atkins