From 166ca356950dfefef782fe0ca4ad7fc3a325e06e Mon Sep 17 00:00:00 2001
From: Stefan Sundin <git@stefansundin.com>
Date: Wed, 29 Aug 2018 19:25:44 -0700
Subject: [PATCH 1/2] Support resetting parameters in aws_db_parameter_group
 and aws_rds_cluster_parameter_group.

---
 aws/resource_aws_db_parameter_group.go        | 20 +++++++++++++++++++
 ...esource_aws_rds_cluster_parameter_group.go | 20 +++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/aws/resource_aws_db_parameter_group.go b/aws/resource_aws_db_parameter_group.go
index faafcb3583b..1dc9ef2e2a1 100644
--- a/aws/resource_aws_db_parameter_group.go
+++ b/aws/resource_aws_db_parameter_group.go
@@ -295,6 +295,26 @@ func resourceAwsDbParameterGroupUpdate(d *schema.ResourceData, meta interface{})
 			}
 			d.SetPartial("parameter")
 		}
+
+		// Reset parameters that have been removed
+		parameters, err = expandParameters(os.Difference(ns).List())
+		if err != nil {
+			return err
+		}
+		if len(parameters) > 0 {
+			parameterGroupName := d.Get("name").(string)
+			resetOpts := rds.ResetDBParameterGroupInput{
+				DBParameterGroupName: aws.String(parameterGroupName),
+				Parameters:           parameters,
+			}
+
+			log.Printf("[DEBUG] Reset DB Parameter Group: %s", resetOpts)
+			_, err = rdsconn.ResetDBParameterGroup(&resetOpts)
+			if err != nil {
+				return fmt.Errorf("Error resetting DB Parameter Group: %s", err)
+			}
+			d.SetPartial("parameter")
+		}
 	}
 
 	if err := setTagsRDS(rdsconn, d, d.Get("arn").(string)); err != nil {
diff --git a/aws/resource_aws_rds_cluster_parameter_group.go b/aws/resource_aws_rds_cluster_parameter_group.go
index 577a5b6f4b3..5e4d4b19ebd 100644
--- a/aws/resource_aws_rds_cluster_parameter_group.go
+++ b/aws/resource_aws_rds_cluster_parameter_group.go
@@ -234,6 +234,26 @@ func resourceAwsRDSClusterParameterGroupUpdate(d *schema.ResourceData, meta inte
 			}
 			d.SetPartial("parameter")
 		}
+
+		// Reset parameters that have been removed
+		parameters, err = expandParameters(os.Difference(ns).List())
+		if err != nil {
+			return err
+		}
+		if len(parameters) > 0 {
+			parameterGroupName := d.Get("name").(string)
+			resetOpts := rds.ResetDBClusterParameterGroupInput{
+				DBClusterParameterGroupName: aws.String(parameterGroupName),
+				Parameters:                  parameters,
+			}
+
+			log.Printf("[DEBUG] Reset DB Cluster Parameter Group: %s", resetOpts)
+			_, err = rdsconn.ResetDBClusterParameterGroup(&resetOpts)
+			if err != nil {
+				return fmt.Errorf("Error resetting DB Cluster Parameter Group: %s", err)
+			}
+			d.SetPartial("parameter")
+		}
 	}
 
 	if err := setTagsRDS(rdsconn, d, d.Get("arn").(string)); err != nil {

From a4963dd5f9bae50972bedfb44692a2d1ceb362e1 Mon Sep 17 00:00:00 2001
From: Stefan Sundin <git@stefansundin.com>
Date: Fri, 31 Aug 2018 11:36:26 -0700
Subject: [PATCH 2/2] Try to add a test.

---
 aws/resource_aws_db_parameter_group_test.go | 48 +++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/aws/resource_aws_db_parameter_group_test.go b/aws/resource_aws_db_parameter_group_test.go
index 7f7ba29372f..a3aca8e80be 100644
--- a/aws/resource_aws_db_parameter_group_test.go
+++ b/aws/resource_aws_db_parameter_group_test.go
@@ -291,6 +291,15 @@ func TestAccAWSDBParameterGroup_basic(t *testing.T) {
 						"aws_db_parameter_group.bar", "arn", regexp.MustCompile(fmt.Sprintf("^arn:[^:]+:rds:[^:]+:\\d{12}:pg:%s", groupName))),
 				),
 			},
+			{
+				Config: testAccAWSDBParameterGroupConfig(groupName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSDBParameterGroupExists("aws_db_parameter_group.bar", &v),
+					testAccCheckAWSDBParameterGroupAttributes(&v, groupName),
+					testAccCheckAWSDBParameterNotUserDefined("aws_db_parameter_group.bar", "collation_connection"),
+					testAccCheckAWSDBParameterNotUserDefined("aws_db_parameter_group.bar", "collation_server"),
+				),
+			},
 		},
 	})
 }
@@ -539,6 +548,45 @@ func testAccCheckAWSDBParameterGroupExists(n string, v *rds.DBParameterGroup) re
 	}
 }
 
+func testAccCheckAWSDBParameterNotUserDefined(n, paramName string) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		rs, ok := s.RootModule().Resources[n]
+		if !ok {
+			return fmt.Errorf("Not found: %s", n)
+		}
+
+		if rs.Primary.ID == "" {
+			return fmt.Errorf("No DB Parameter Group ID is set")
+		}
+
+		conn := testAccProvider.Meta().(*AWSClient).rdsconn
+
+		opts := rds.DescribeDBParametersInput{
+			DBParameterGroupName: aws.String(rs.Primary.ID),
+			Source:               aws.String("user"),
+		}
+		fmt.Printf("test: %s\n", rs.Primary.ID)
+
+		userDefined := false
+		conn.DescribeDBParametersPages(&opts, func(page *rds.DescribeDBParametersOutput, lastPage bool) bool {
+			for _, param := range page.Parameters {
+				fmt.Printf("param: %s %s\n", *param.ParameterName, *param.Source)
+				if *param.ParameterName == paramName {
+					userDefined = true
+					return false
+				}
+			}
+			return true
+		})
+
+		if userDefined {
+			return fmt.Errorf("DB Parameter is user defined")
+		}
+
+		return nil
+	}
+}
+
 func randomString(strlen int) string {
 	rand.Seed(time.Now().UTC().UnixNano())
 	const chars = "abcdefghijklmnopqrstuvwxyz"