config: validation error when output is missing value field

Also lists out invalid keys in errmsg when they are present

Closes #4398

config/config.go

@@ -500,16 +500,23 @@ func (c *Config) Validate() error {
 
 	// Check that all outputs are valid
 	for _, o := range c.Outputs {
-		invalid := false
-		for k, _ := range o.RawConfig.Raw {
-			if k != "value" {
-				invalid = true
-				break
+		var invalidKeys []string
+		valueKeyFound := false
+		for k := range o.RawConfig.Raw {
+			if k == "value" {
+				valueKeyFound = true
+			} else {
+				invalidKeys = append(invalidKeys, k)
 			}
 		}
-		if invalid {
+		if len(invalidKeys) > 0 {
 			errs = append(errs, fmt.Errorf(
-				"%s: output should only have 'value' field", o.Name))
+				"%s: output has invalid keys: %s",
+				o.Name, strings.Join(invalidKeys, ", ")))
+		}
+		if !valueKeyFound {
+			errs = append(errs, fmt.Errorf(
+				"%s: output is missing required 'value' key", o.Name))
 		}
 
 		for _, v := range o.RawConfig.Variables {

config/config_test.go

@@ -196,6 +196,13 @@ func TestConfigValidate_outputBadField(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_outputMissingEquals(t *testing.T) {
+	c := testConfig(t, "validate-output-missing-equals")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_pathVar(t *testing.T) {
 	c := testConfig(t, "validate-path-var")
 	if err := c.Validate(); err != nil {

config/module/test-fixtures/validate-child-good/child/main.tf

@@ -1,3 +1,3 @@
 variable "memory" {}
 
-output "result" {}
+output "result" { value = "foo" }

config/test-fixtures/validate-output-missing-equals/main.tf

@@ -0,0 +1,3 @@
+output "whoops" {
+  value "wheresmyequals"
+}