Terraform v0.10 introduced .terraform/plugins as a cache directory for
automatically-installed plugins, Terraform v0.13 later reorganized the
directory structure inside but retained its purpose as a cache.
The local cache used to also serve as a record of specifically which
packages were selected in a particular working directory, with the intent
that a second run of "terraform init" would always select the same
packages again. That meant that in some sense it behaved a bit like a
local filesystem mirror directory, even though that wasn't its intended
purpose.
Due to some unfortunate miscommunications, somewhere a long the line we
published some documentation that _recommended_ using the cache directory
as if it were a filesystem mirror directory when working with Terraform
Cloud. That was really only working as an accident of implementation
details, and Terraform v0.14 is now going to break that because the source
of record for the currently-selected provider versions is now the
public-facing dependency lock file rather than the contents of an existing
local cache directory on disk.
After some consideration of how to move forward here, this commit
implements a compromise that tries to avoid silently doing anything
surprising while still giving useful guidance to folks who were previously
using the unsupported strategy. Specifically:
- The local cache directory will now be .terraform/providers rather than
.terraform/plugins, because .terraform/plugins is effectively "poisoned"
by the incorrect usage that we can't reliably distinguish from prior
version correct usage.
- The .terraform/plugins directory is now the "legacy cache directory". It
is intentionally _not_ now a filesystem mirror directory, because that
would risk incorrectly interpreting providers automatically installed
by Terraform v0.13 as if they were a local mirror, and thus upgrades
and checksum fetches from the origin registry would be blocked.
- Because of the previous two points, someone who _was_ trying to use the
legacy cache directory as a filesystem mirror would see installation
fail for any providers they manually added to the legacy directory.
To avoid leaving that user stumped as to what went wrong, there's a
heuristic for the case where a non-official provider fails installation
and yet we can see it in the legacy cache directory. If that heuristic
matches then we'll produce a warning message hinting to move the
provider under the terraform.d/plugins directory, which is a _correct_
location for "bundled" provider plugins that belong only to a single
configuration (as opposed to being installed globally on a system).
This does unfortunately mean that anyone who was following the
incorrectly-documented pattern will now encounter an error (and the
aforementioned warning hint) after upgrading to Terraform v0.14. This
seems like the safest compromise because Terraform can't automatically
infer the intent of files it finds in .terraform/plugins in order to
decide automatically how best to handle them.
The internals of the .terraform directory are always considered
implementation detail for a particular Terraform version and so switching
to a new directory for the _actual_ cache directory fits within our usual
set of guarantees, though it's definitely non-ideal in isolation but okay
when taken in the broader context of this problem, where the alternative
would be silent misbehavior when upgrading.
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