tfsdk: Ensure Config, Plan, and State PathMatches will return null an…

…d unknown parent paths (hashicorp#410)

This will also now return an error diagnostic if a given path expression is invalid for the schema data.

The `(path.Expression).MergeExpressions(...path.Expression) Expressions` is a helper method for the common use case of combining an attribute path expression with an incoming collection of expressions to perform generic validation across attributes.

path/expression.go

@@ -131,6 +131,15 @@ func (e Expression) Matches(path Path) bool {
 	return e.steps.Matches(path.Steps())
 }
 
+// MatchesParent returns true if the given Path is a valid parent for the
+// Expression. This is helpful for determining if a child Path would
+// potentially match the full Expression during depth-first traversal. Any
+// relative expression steps, such as ExpressionStepParent, are automatically
+// resolved before matching.
+func (e Expression) MatchesParent(path Path) bool {
+	return e.steps.MatchesParent(path.Steps())
+}
+
 // Merge returns a copied expression either with the steps of the given
 // expression added to the end of the existing steps, or overwriting the
 // steps if the given expression was a root expression.
@@ -152,6 +161,24 @@ func (e Expression) Merge(other Expression) Expression {
 	return copiedExpression
 }
 
+// MergeExpressions returns collection of expressions that calls Merge() on
+// the current expression with each of the others.
+func (e Expression) MergeExpressions(others ...Expression) Expressions {
+	var result Expressions
+
+	if len(others) == 0 {
+		result.Append(e)
+
+		return result
+	}
+
+	for _, other := range others {
+		result.Append(e.Merge(other))
+	}
+
+	return result
+}
+
 // Resolve returns a copied expression with any relative steps, such as
 // ExpressionStepParent, resolved. This is not necessary before calling methods
 // such as Matches(), however it can be useful before returning the String()

path/expression_steps.go

@@ -84,6 +84,37 @@ func (s ExpressionSteps) Matches(pathSteps PathSteps) bool {
 	return true
 }
 
+// MatchesParent returns true if the given PathSteps match each ExpressionStep
+// until there are no more PathSteps. This is helpful for determining if the
+// PathSteps would potentially match the full ExpressionSteps during
+// depth-first traversal.
+//
+// Any ExpressionStepParent will automatically be resolved.
+func (s ExpressionSteps) MatchesParent(pathSteps PathSteps) bool {
+	resolvedExpressionSteps := s.Resolve()
+
+	// Empty expression should not match anything to prevent false positives.
+	// Ensure to not return false on an empty path since walking a path always
+	// starts with no steps.
+	if len(resolvedExpressionSteps) == 0 {
+		return false
+	}
+
+	// Path steps deeper than or equal to the expression steps should not match
+	// as a potential parent.
+	if len(pathSteps) >= len(resolvedExpressionSteps) {
+		return false
+	}
+
+	for stepIndex, pathStep := range pathSteps {
+		if !resolvedExpressionSteps[stepIndex].Matches(pathStep) {
+			return false
+		}
+	}
+
+	return true
+}
+
 // NextStep returns the first ExpressionStep and the remaining ExpressionSteps.
 func (s ExpressionSteps) NextStep() (ExpressionStep, ExpressionSteps) {
 	if len(s) == 0 {