IAM role service principal constantly wants to update if using json + heredoc

[drewmullen]

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Terraform CLI and Terraform AWS Cloud Control Provider Version

Affected Resource(s)

• awscc_iam_role

Terraform Configuration Files

Please include all Terraform configurations required to reproduce the bug. Bug reports without a functional reproduction may be closed without investigation.

resource "awscc_iam_role" "flow_log_cloudwatch" {
  role_name   = "vpc-flow-log"
  description = "VPC Flow logs for"
  # tags = var.tags
  assume_role_policy_document = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "VPCFlowLogs",
      "Effect": "Allow",
      "Principal": {
        "Service": ["vpc-flow-logs.amazonaws.com"]
      },
      "Action": "sts:AssumeRole"
    }
  ]
}
EOF
}

Expected Behavior

no update

Actual Behavior

  # awscc_iam_role.flow_log_cloudwatch will be updated in-place
  ~ resource "awscc_iam_role" "flow_log_cloudwatch" {
      ~ assume_role_policy_document = jsonencode(
          ~ {
              ~ Statement = [
                  ~ {
                      ~ Principal = {
                          ~ Service = "vpc-flow-logs.amazonaws.com" -> [
                              + "vpc-flow-logs.amazonaws.com",
                            ]
                        }
                        # (3 unchanged elements hidden)
                    },
                ]
                # (1 unchanged element hidden)
            }
        )
        id                          = "test-vpc-flow-log-yxik2ig8"
      - max_session_duration        = 3600 -> null
        # (5 unchanged attributes hidden)
    }

Steps to Reproduce

1. terraform apply
2. terraform apply

[drewmullen]

NOTE: this behavior does not happen if you use the data source:

data "aws_iam_policy_document" "flow_log_cw_assume_role" {
  statement {
    principals {
      type        = "Service"
      identifiers = ["vpc-flow-logs.amazonaws.com"]
    }

    effect = "Allow"

    actions = ["sts:AssumeRole"]
  }
}

resource "awscc_iam_role" "flow_log_cloudwatch" {
  role_name   = "${var.name}-vpc-flow-log-${random_string.suffix.result}"
  description = "VPC Flow logs for ${var.name}"
  # tags = var.tags
  assume_role_policy_document = data.aws_iam_policy_document.flow_log_cw_assume_role.json
}

[drewmullen]

FYI: when using embedded json, removing the [] resolves this issue

      "Principal": {
        "Service": "vpc-flow-logs.amazonaws.com"
      },

[ewbankkit]

Relates https://github.com/hashicorp/terraform-provider-awscc/issues/509.