Backport of backend: Validate remote backend Terraform version into v0.12

[alisdair]

Backport

This PR is a manual backport of #26947 targeted at the 0.12 branch for future release, likely in 0.12.30.

The below text is copied from the body of the original PR.

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When using the enhanced remote backend, a subset of all Terraform operations are supported. Of these, only plan and apply can be executed on the remote infrastructure (e.g. Terraform Cloud). Other operations run locally and use the remote backend for state storage.

This causes problems when the local version of Terraform does not match the configured version from the remote workspace. If the two versions are incompatible, an import or state mv operation can cause the remote workspace to be unusable until a manual fix is applied.

To prevent this from happening accidentally, this commit introduces a check that the local Terraform version and the configured remote workspace Terraform version are compatible. This check is skipped for commands which do not write state, and can also be disabled by the use of a new command-line flag, -ignore-remote-version.

Terraform version compatibility is defined as:

• For all releases before 0.14.0, local must exactly equal remote, as two different versions cannot share state;
• 0.14.0 to 1.0.x are compatible, as we will not change the state version number until at least Terraform 1.1.0;
• Versions after 1.1.0 must have the same major and minor versions, as we will not change the state version number in a patch release.

If the two versions are incompatible, a diagnostic is displayed, advising that the error can be suppressed with -ignore-remote-version. When this flag is used, the diagnostic is still displayed, but as a warning instead of an error.

Commands which will not write state can assert this fact by calling the helper meta.ignoreRemoteBackendVersionConflict, which will disable the checks. Those which can write state should instead call the helper meta.remoteBackendVersionCheck, which will return diagnostics for display.

In addition to these explicit paths for managing the version check, we have an implicit check in the remote backend's state manager initialization method. Both of the above helpers will disable this check. This fallback is in place to ensure that future code paths which access state cannot accidentally skip the remote version check.

Release plans

This change is intended to ship with 0.15.0, and we also intend to backport to earlier release series. The current plan is 0.14, 0.13, and 0.12.

Screenshot

Here's a session which demonstrates both the error and warning forms of the diagnostics:

[codecov]

Codecov Report

Merging #27002 (a9063ef) into v0.12 (c5c0268) will decrease coverage by 1.19%.
The diff coverage is 55.83%.

Impacted Files	Coverage Δ
command/state_mv.go	42.12% <4.34%> (-1.84%)	⬇️
command/state_rm.go	43.15% <8.33%> (-2.21%)	⬇️
command/untaint.go	37.58% <17.39%> (-0.35%)	⬇️
command/meta_backend.go	53.96% <22.22%> (-1.27%)	⬇️
command/taint.go	38.35% <27.27%> (-0.38%)	⬇️
command/import.go	50.42% <44.44%> (-0.45%)	⬇️
command/state_meta.go	46.15% <50.00%> (-2.52%)	⬇️
command/state_push.go	35.36% <71.42%> (+0.79%)	⬆️
backend/remote/backend_mock.go	62.57% <72.72%> (+0.02%)	⬆️
backend/remote/backend.go	61.70% <100.00%> (+3.68%)	⬆️
... and 669 more

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