cli: Only rewrite provider locks file if changed

[alisdair]

If the provider locks have not changed, there is no need to rewrite the locks file. Preventing this needless rewrite should allow Terraform to operate in a read-only directory, so long as the provider requirements don't change.

Slightly related to #27630, but serving a different use case. The goal here is to make a smaller change which can be back-ported to 0.14, and still allow being able to run Terraform in a read-only directory to allow upgrade from 0.13 to 0.14.

[codecov]

Codecov Report

Merging #28230 (786d992) into main (340543d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
command/init.go	56.90% <100.00%> (+0.50%)	⬆️
terraform/evaluate.go	52.47% <0.00%> (-0.42%)	⬇️
terraform/node_resource_plan.go	98.05% <0.00%> (+1.94%)	⬆️
communicator/communicator.go	80.70% <0.00%> (+3.50%)	⬆️

[alisdair]

📝

[alisdair]

I think this TODO was out of date. Immediately above we already check if newLocks is different from previousLocks and output two different messages. The same applies to v0.14:

terraform/command/init.go

Lines 845 to 872 in 1c68b7b

	if !newLocks.Equal(previousLocks) {
	if previousLocks.Empty() {
	// A change from empty to non-empty is special because it suggests
	// we're running "terraform init" for the first time against a
	// new configuration. In that case we'll take the opportunity to
	// say a little about what the dependency lock file is, for new
	// users or those who are upgrading from a previous Terraform
	// version that didn't have dependency lock files.
	c.Ui.Output(c.Colorize().Color(`
	Terraform has created a lock file [bold].terraform.lock.hcl[reset] to record the provider
	selections it made above. Include this file in your version control repository
	so that Terraform can guarantee to make the same selections by default when
	you run "terraform init" in the future.`))
	} else {
	c.Ui.Output(c.Colorize().Color(`
	Terraform has made some changes to the provider dependency selections recorded
	in the .terraform.lock.hcl file. Review those changes and commit them to your
	version control system if they represent changes you intended to make.`))
	}
	}
	// TODO: Check whether newLocks is different from previousLocks and mention
	// in the UI if so. We should emit a different message if previousLocks was
	// empty, because that indicates we were creating a lock file for the first
	// time and so we need to introduce the user to the idea of it.
	moreDiags = c.replaceLockedDependencies(newLocks)
	diags = diags.Append(moreDiags)

[apparentlymart]

Agreed. I'm pretty sure I just saw the TODO, TODID it and then forgot to remove the comment afterwards.

[ghost]

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