Backport of filesystem functions, don't cache unknown function results into v1.14

internal/lang/function_results.go

@@ -51,6 +51,16 @@ func (f *FunctionResults) CheckPrior(name string, args []cty.Value, result cty.V
 // CheckPriorProvider compares the provider function call against any cached
 // results, and returns an error if the result does not match a prior call.
 func (f *FunctionResults) CheckPriorProvider(provider addrs.Provider, name string, args []cty.Value, result cty.Value) error {
+	// Don't cache unknown values. We could technically store types and
+	// refinements for validation, but we don't currently have a way to
+	// serialize those in the plan. Unknowns are also handled much more
+	// gracefully throughout the evaluation system, whereas invalid data is
+	// harder to trace back to the source since it's usually only visible due to
+	// unexpected side-effects.
+	if !result.IsKnown() {
+		return nil
+	}
+
 	argSum := sha256.New()
 
 	if !provider.IsZero() {

internal/lang/function_results_test.go

@@ -72,7 +72,7 @@ func TestFunctionCache(t *testing.T) {
 				result:   cty.False,
 			},
 			// result changed from unknown => false
-			expectErr: true,
+			expectErr: false,
 		},
 		{
 			first: testCall{
@@ -172,6 +172,9 @@ func TestFunctionCache(t *testing.T) {
 			if err != nil && !test.expectErr {
 				t.Fatal(err)
 			}
+			if err == nil && test.expectErr {
+				t.Fatal("expected error")
+			}
 
 			// reload the data to ensure we validate identically
 			newResults := NewFunctionResultsTable(results.GetHashes())