Consider removing exported error types

[kmoe]

The tfexec exported error types - for example, tfexec.ErrCLIUsage and tfexec.ErrTFVersionMismatch - are implemented by parsing Terraform's human-readable error output using regular expressions.

Since this log output is subject to frequent changes and not covered by the v1.0 compatibility promises, users of these error types are at risk of having their code broken by new Terraform versions, requiring them (in the best case) to keep terraform-exec updated.

This error parsing code also seems to be a leading cause of terraform-exec nightly test failures.

Are these error types useful enough to consumers to continue maintaining them? Should we remove them prior to v1.0.0? Are there any really good use cases?

[bflad]

To gain some insight on potential usage in the wild, I spent some time with GitHub's code search (fuzzy matching on tfexec.{TYPE} because it'd be difficult to ignore non-tfexec things and most Go code doesn't use aliasing), e.g.

Error Type	External GitHub Repo Usage	Code Search URL
ErrConfigInvalid	1	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrConfigInvalid%22
ErrLockIdInvalid	0	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrLockIdInvalid%22
ErrMissingVar	2	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrMissingVar%22
ErrNoConfig	1	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrNoConfig%22
ErrNoInit	1	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrNoInit%22
ErrNoWorkspace	3	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrNoWorkspace%22
ErrStateLocked	1	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrStateLocked%22
ErrStatePlanRead	0	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrStatePlanRead%22
ErrTFVersionMismatch	0	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrTFVersionMismatch%22
ErrWorkspaceExists	2	https://cs.github.com/?scopeName=All+repos&scope=&q=language%3Ago+%22tfexec.ErrWorkspaceExists%22

What I found was:

• https://github.com/hashicorp/consul-terraform-sync (ErrNoWorkspace / ErrWorkspaceExists)
• https://github.com/ministryofjustice/cloud-platform-cli (ErrNoWorkspace / ErrWorkspaceExists)
• https://github.com/weaveworks/tf-controller (ErrStateLocked)
• https://github.com/nunchistudio/temporal-land/tree/main/integrations/terraform
• https://github.com/CoverGo/terraform-wrapper (really just a terraform-exec fork)

I personally think that these removals are okay given the maintenance burden.