tfprotov5: Document Terraform 0.12-14 Requirement for UpgradeResourceStateResponse.UpgradedState.Msgpack

[bflad]

Does this documenation exist?

• This is new documentation
• This is an enhancement to existing documentation

Where would you expect to find this documentation?

• On terraform.io
• In the GoDoc for this module
• In this repo as a markdown file
• Somewhere else

Description

When running a protocol version 5 server via tf5server (or via tf6to5server), Terraform CLI 0.12 through 0.14 return an error such as:

    provider_test.go:11: Step 1/1 error: Error running post-apply plan: exit status 1
        
        Error: EOF

(Yes, that's it.) If UpgradeResourceState.Response.UpgradedState has the JSON field, but not Msgpack. We should document this requirement in the protocol version 5 definition and/or UpgradeResourceStateResponse type.

Discovered because terraform-plugin-framework only passes through the JSON field and ignores the Msgpack in its placeholder UpgradeResourceState handling.

References

• https://github.com/hashicorp/terraform-plugin-framework/blob/bf0240fa43e10ac4c3ac454338f75fe928e9f048/tfsdk/serve.go#L493-L497
• Add terraform-plugin-mux@v0.5.0 testing providers terraform-provider-corner#44

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