Add missing relationship constraint (#177)

* Add constraint "at least one"

the connector needs at least one field/envvar set/passed, otherwise it
will fail to run

* Check for zero-value of string slice

pkg/config/config.go

@@ -152,6 +152,8 @@ func DefineConfiguration(
 			mainCMD.MarkFlagsMutuallyExclusive(listFieldConstrainsAsStrings(constrain)...)
 		case field.RequiredTogether:
 			mainCMD.MarkFlagsRequiredTogether(listFieldConstrainsAsStrings(constrain)...)
+		case field.AtLeastOne:
+			mainCMD.MarkFlagsOneRequired(listFieldConstrainsAsStrings(constrain)...)
 		}
 	}
 

pkg/field/relationships.go

@@ -5,6 +5,7 @@ type Relationship int
 const (
 	RequiredTogether Relationship = iota + 1
 	MutuallyExclusive
+	AtLeastOne
 )
 
 type SchemaFieldRelationship struct {
@@ -25,3 +26,10 @@ func FieldsMutuallyExclusive(fields ...SchemaField) SchemaFieldRelationship {
 		Fields: fields,
 	}
 }
+
+func FieldsAtLeastOneUsed(fields ...SchemaField) SchemaFieldRelationship {
+	return SchemaFieldRelationship{
+		Kind:   AtLeastOne,
+		Fields: fields,
+	}
+}

pkg/field/validation.go

@@ -47,6 +47,8 @@ func Validate(c Configuration, v *viper.Viper) error {
 			isNonZero = v.GetInt(f.FieldName) != 0
 		case reflect.String:
 			isNonZero = v.GetString(f.FieldName) != ""
+		case reflect.Slice:
+			isNonZero = len(v.GetStringSlice(f.FieldName)) == 0
 		default:
 			return fmt.Errorf("field %s has unsupported type %s", f.FieldName, f.FieldType)
 		}
@@ -80,6 +82,9 @@ func validateConstraints(fieldsPresent map[string]int, relationships []SchemaFie
 		if present > 0 && present < len(relationship.Fields) && relationship.Kind == RequiredTogether {
 			return makeNeededTogetherError(fieldsPresent, relationship)
 		}
+		if present == 0 && relationship.Kind == AtLeastOne {
+			return makeAtLeastOneError(fieldsPresent, relationship)
+		}
 	}
 
 	return nil
@@ -106,3 +111,14 @@ func makeNeededTogetherError(fields map[string]int, relation SchemaFieldRelation
 
 	return fmt.Errorf("fields marked as needed together are missing: %s", strings.Join(found, ", "))
 }
+
+func makeAtLeastOneError(fields map[string]int, relation SchemaFieldRelationship) error {
+	var found []string
+	for _, f := range relation.Fields {
+		if fields[f.FieldName] == 0 {
+			found = append(found, f.FieldName)
+		}
+	}
+
+	return fmt.Errorf("at least one field was expected, any of: %s", strings.Join(found, ", "))
+}