command: Fix init flags silent exit bug

[alisdair]

When using -flag=value with Powershell, unquoted values are broken into separate arguments. This means that the following command:

terraform init -backend-config=./backend.conf

is interpreted by Terraform as:

terraform init -backend-config= ./backend.conf

This results in an empty backend-config setting (which is semantically valid!) followed by a custom configuration path (pointing at a file).

Due to a bug where we could exit without printing diagnostics, this would result in a silent failure that was very difficult to diagnose. See #25266.

[codecov]

Codecov Report

Merging #25300 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
command/init.go	71.04% <100.00%> (+0.59%)	⬆️
dag/walk.go	93.16% <0.00%> (+0.62%)	⬆️
backend/remote/backend_common.go	55.25% <0.00%> (+0.67%)	⬆️
dag/marshal.go	54.44% <0.00%> (+1.11%)	⬆️
terraform/node_resource_plan.go	93.44% <0.00%> (+1.63%)	⬆️
communicator/communicator.go	83.33% <0.00%> (+3.70%)	⬆️

[apparentlymart]

Good catch! Looks like not showing the error messages here was a result of an incorrect adaptation of an old implementation prior to diagnostics.

With that said, you got me thinking about the -backend-config= (zero-length value after =) case a little: isn't it the case that this must either be a filename or a key=value string? If so, I think that an empty string could never be either of those, and so we could potentially have a more tailored error message here noting that the option value is invalid.

This will still, of course, remain confusing under PowerShell: PowerShell's insistence on inserting unrequested spaces in the command line means that the result would still be pretty confusing, and appear as a Terraform bug rather than a PowerShell parsing quirk:

PS C:\> terraform init -backend-config=./foo

Error: Module directory ./foo does not exist

Error: -backend-config= requires an argument

Without being able to reliably detect that the calling program is PowerShell here I don't really see any robust way to do better: what the user entered and what PowerShell passes to us are different in an infuriatingly-subtle yet significant way. 😖

[alisdair]

With that said, you got me thinking about the -backend-config= (zero-length value after =) case a little: isn't it the case that this must either be a filename or a key=value string? If so, I think that an empty string could never be either of those, and so we could potentially have a more tailored error message here noting that the option value is invalid.

Sadly I don't think this is possible. While I can't find where this is documented (if it is), the command interprets an empty string value as meaning "remove all previous backend-config overrides":

terraform/command/init.go

Lines 777 to 781 in 69b94ec

	if len(items) == 1 && items[0].Value == "" {
	// Explicitly remove all -backend-config options.
	// We do this by setting an empty but non-nil ConfigOverrides.
	return configs.SynthBody("-backend-config=''", synthVals), diags
	}

This was introduced in #21439.

I'm also not satisfied with this current solution to the bug, but at least having some error message displayed would give the user a chance of figuring out that there's a quoting issue happening. I'm not targeting this at 0.13.0 anyway (since we're close to an RC), so maybe a better solution will occur to us before merge 🤞

[apparentlymart]

Oh yes, I'd forgotten about that tricky overloading of -backend-config=. That is a good point and I agree.

[alisdair]

I'm not targeting this at 0.13.0 anyway (since we're close to an RC)

After thinking about this some more I've changed my mind. Given the tiny scope of this change, I think it's low enough risk that it's better to merge now than leave this silent failure edge case to trip someone else up.

[ghost]

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