Preconditions and Postconditions

internal/configs/checks.go

@@ -0,0 +1,163 @@
+package configs
+
+import (
+	"fmt"
+	"unicode"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/gohcl"
+)
+
+// 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 is one or more full sentences, which would need to be in
+	// English for consistency with the rest of the error message output but
+	// can in practice be in any language as long as it ends with a period.
+	// 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.
+	ErrorMessage string
+
+	DeclRange hcl.Range
+}
+
+// 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 {
+		moreDiags := gohcl.DecodeExpression(attr.Expr, nil, &cr.ErrorMessage)
+		diags = append(diags, moreDiags...)
+		if !moreDiags.HasErrors() {
+			const errSummary = "Invalid validation error message"
+			switch {
+			case cr.ErrorMessage == "":
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  errSummary,
+					Detail:   "An empty string is not a valid nor useful error message.",
+					Subject:  attr.Expr.Range().Ptr(),
+				})
+			case !looksLikeSentences(cr.ErrorMessage):
+				// Because we're going to include this string verbatim as part
+				// of a bigger error message written in our usual style in
+				// English, we'll require the given error message to conform
+				// to that. We might relax this in future if e.g. we start
+				// presenting these error messages in a different way, or if
+				// Terraform starts supporting producing error messages in
+				// other human languages, etc.
+				// For pragmatism we also allow sentences ending with
+				// exclamation points, but we don't mention it explicitly here
+				// because that's not really consistent with the Terraform UI
+				// writing style.
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  errSummary,
+					Detail:   "The validation error message must be at least one full sentence starting with an uppercase letter and ending with a period or question mark.\n\nYour given message will be included as part of a larger Terraform error message, written as English prose. For broadly-shared modules we suggest using a similar writing style so that the overall result will be consistent.",
+					Subject:  attr.Expr.Range().Ptr(),
+				})
+			}
+		}
+	}
+
+	return cr, diags
+}
+
+// looksLikeSentence is a simple heuristic that encourages writing error
+// messages that will be presentable when included as part of a larger
+// Terraform error diagnostic whose other text is written in the Terraform
+// UI writing style.
+//
+// This is intentionally not a very strong validation since we're assuming
+// that module authors want to write good messages and might just need a nudge
+// about Terraform's specific style, rather than that they are going to try
+// to work around these rules to write a lower-quality message.
+func looksLikeSentences(s string) bool {
+	if len(s) < 1 {
+		return false
+	}
+	runes := []rune(s) // HCL guarantees that all strings are valid UTF-8
+	first := runes[0]
+	last := runes[len(runes)-1]
+
+	// If the first rune is a letter then it must be an uppercase letter.
+	// (This will only see the first rune in a multi-rune combining sequence,
+	// but the first rune is generally the letter if any are, and if not then
+	// we'll just ignore it because we're primarily expecting English messages
+	// right now anyway, for consistency with all of Terraform's other output.)
+	if unicode.IsLetter(first) && !unicode.IsUpper(first) {
+		return false
+	}
+
+	// The string must be at least one full sentence, which implies having
+	// sentence-ending punctuation.
+	// (This assumes that if a sentence ends with quotes then the period
+	// will be outside the quotes, which is consistent with Terraform's UI
+	// writing style.)
+	return last == '.' || last == '?' || last == '!'
+}
+
+var checkRuleBlockSchema = &hcl.BodySchema{
+	Attributes: []hcl.AttributeSchema{
+		{
+			Name:     "condition",
+			Required: true,
+		},
+		{
+			Name:     "error_message",
+			Required: true,
+		},
+	},
+}

internal/configs/experiments.go

@@ -209,6 +209,55 @@ func checkModuleExperiments(m *Module) hcl.Diagnostics {
 		}
 	}
 
+	if !m.ActiveExperiments.Has(experiments.PreconditionsPostconditions) {
+		for _, r := range m.ManagedResources {
+			for _, c := range r.Preconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Preconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+			for _, c := range r.Postconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Postconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+		}
+		for _, r := range m.DataResources {
+			for _, c := range r.Preconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Preconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+			for _, c := range r.Postconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Postconditions are experimental",
+					Detail:   "The resource preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+		}
+		for _, o := range m.Outputs {
+			for _, c := range o.Preconditions {
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Preconditions are experimental",
+					Detail:   "The output value preconditions feature is currently an opt-in experiment, subject to change in future releases based on feedback.\n\nActivate the feature for this module by adding preconditions_postconditions to the list of active experiments.",
+					Subject:  c.DeclRange.Ptr(),
+				})
+			}
+		}
+	}
+
 	return diags
 }