terraform/internal/configs/checks.go

package configs

import (
	"fmt"

	"github.com/hashicorp/hcl/v2"
	"github.com/hashicorp/terraform/internal/addrs"
	"github.com/hashicorp/terraform/internal/lang"
)

// CheckRule represents a configuration-defined validation rule, precondition,
// or postcondition. Blocks of this sort can appear in a few different places
// in configuration, including "validation" blocks for variables,
// and "precondition" and "postcondition" blocks for resources.
type CheckRule struct {
	// Condition is an expression that must evaluate to true if the condition
	// holds or false if it does not. If the expression produces an error then
	// that's considered to be a bug in the module defining the check.
	//
	// The available variables in a condition expression vary depending on what
	// a check is attached to. For example, validation rules attached to
	// input variables can only refer to the variable that is being validated.
	Condition hcl.Expression

	// ErrorMessage should be one or more full sentences, which should be in
	// English for consistency with the rest of the error message output but
	// can in practice be in any language. The message should describe what is
	// required for the condition to return true in a way that would make sense
	// to a caller of the module.
	//
	// The error message expression has the same variables available for
	// interpolation as the corresponding condition.
	ErrorMessage hcl.Expression

	DeclRange hcl.Range
}

// validateSelfReferences looks for references in the check rule matching the
// specified resource address, returning error diagnostics if such a reference
// is found.
func (cr *CheckRule) validateSelfReferences(checkType string, addr addrs.Resource) hcl.Diagnostics {
	var diags hcl.Diagnostics
	refs, _ := lang.References(cr.Condition.Variables())
	for _, ref := range refs {
		var refAddr addrs.Resource

		switch rs := ref.Subject.(type) {
		case addrs.Resource:
			refAddr = rs
		case addrs.ResourceInstance:
			refAddr = rs.Resource
		default:
			continue
		}

		if refAddr.Equal(addr) {
			diags = diags.Append(&hcl.Diagnostic{
				Severity: hcl.DiagError,
				Summary:  fmt.Sprintf("Invalid reference in %s", checkType),
				Detail:   fmt.Sprintf("Configuration for %s may not refer to itself.", addr.String()),
				Subject:  cr.Condition.Range().Ptr(),
			})
			break
		}
	}
	return diags
}

// decodeCheckRuleBlock decodes the contents of the given block as a check rule.
//
// Unlike most of our "decode..." functions, this one can be applied to blocks
// of various types as long as their body structures are "check-shaped". The
// function takes the containing block only because some error messages will
// refer to its location, and the returned object's DeclRange will be the
// block's header.
func decodeCheckRuleBlock(block *hcl.Block, override bool) (*CheckRule, hcl.Diagnostics) {
	var diags hcl.Diagnostics
	cr := &CheckRule{
		DeclRange: block.DefRange,
	}

	if override {
		// For now we'll just forbid overriding check blocks, to simplify
		// the initial design. If we can find a clear use-case for overriding
		// checks in override files and there's a way to define it that
		// isn't confusing then we could relax this.
		diags = diags.Append(&hcl.Diagnostic{
			Severity: hcl.DiagError,
			Summary:  fmt.Sprintf("Can't override %s blocks", block.Type),
			Detail:   fmt.Sprintf("Override files cannot override %q blocks.", block.Type),
			Subject:  cr.DeclRange.Ptr(),
		})
		return cr, diags
	}

	content, moreDiags := block.Body.Content(checkRuleBlockSchema)
	diags = append(diags, moreDiags...)

	if attr, exists := content.Attributes["condition"]; exists {
		cr.Condition = attr.Expr

		if len(cr.Condition.Variables()) == 0 {
			// A condition expression that doesn't refer to any variable is
			// pointless, because its result would always be a constant.
			diags = diags.Append(&hcl.Diagnostic{
				Severity: hcl.DiagError,
				Summary:  fmt.Sprintf("Invalid %s expression", block.Type),
				Detail:   "The condition expression must refer to at least one object from elsewhere in the configuration, or else its result would not be checking anything.",
				Subject:  cr.Condition.Range().Ptr(),
			})
		}
	}

	if attr, exists := content.Attributes["error_message"]; exists {
		cr.ErrorMessage = attr.Expr
	}

	return cr, diags
}

var checkRuleBlockSchema = &hcl.BodySchema{
	Attributes: []hcl.AttributeSchema{
		{
			Name:     "condition",
			Required: true,
		},
		{
			Name:     "error_message",
			Required: true,
		},
	},
}