Terraform not accepting allow_major_version_upgrade - amazon-rds

Question

I have the following rds definition:
resource "aws_db_instance" "my-rds" {
allocated_storage = 20
engine = "mysql"
engine_version = "5.5.57"
instance_class = "db.t2.micro"
identifier = "my-db"
name = "somename"
username = "${var.RDS_USERNAME}" # username
password = "${var.RDS_PASSWORD}" # password
db_subnet_group_name = "${aws_db_subnet_group.some-subnet-group.name}"
parameter_group_name = "${aws_db_parameter_group.some-rds-parameter-group.name}"
multi_az = "false"
vpc_security_group_ids = ["${aws_security_group.some-sg.id}"]
storage_type = "gp2"
skip_final_snapshot = true
backup_retention_period = 30 # how long we re going to keep your backups
availability_zone = "${aws_subnet.some-private-1.availability_zone}"
tags {
Name = "some-tag-name"
}
}
So I am just adding:
allow_major_version_upgrade = true
... and getting
Error: Error applying plan:
1 error(s) occurred:
* aws_db_instance.my-rds: 1 error(s) occurred:
* aws_db_instance.my-rds: Error modifying DB Instance my-db: InvalidParameterCombination: No modifications were requested
status code: 400, request id: 2aed626f-6063-4b69-ac37-654bd783fd37
?

Answer 1

This may be related to this github issue or this one or other similar issues. Seems there is an issue with pending modifications versus applying them immediately. For example if I set up a DB like in your question and attempt to set allow_major_version_upgrade = true it fails the first run with the same error but the change happens and running apply again has no pending changes. However, if I also set apply_immediately = true it works on the first run without an error.
In addition when attempting to reproduce this I noticed that invalid parameters also produce this error, such as trying to specify an engine version that doesn't exist when changing the engine_version.