Terraform配置CloudFront动态块报错：参数定义需以换行结尾

我在AWS CloudFront Distribution资源中编写了如下ordered_cache_behavior动态块：

dynamic "ordered_cache_behavior" {
  for_each = var.ordered_cache_behaviors
  content {
    path_pattern              = ordered_cache_behavior.value.path_pattern
    allowed_methods           = ordered_cache_behavior.value.allowed_methods
    cached_methods            = ordered_cache_behavior.value.cached_methods
    target_origin_id          = var.origin_id
    cache_policy_id           = var.cache_policy_ids["${var.policy_prefix}${ordered_cache_behavior.value.cache_policy_name}"]
    origin_request_policy_id  = ordered_cache_behavior.value.path_pattern = "/" ? var.origin_request_policy_ids["whitelist_policy"] : null
    dynamic "lambda_function_association" {
      for_each = var.enable_auth ? var.default_cache_behavior.lambda_function_association : ordered_cache_behavior.value.lambda_function_association
      content {
        event_type   = lambda_function_association.value.event_type
        include_body = lambda_function_association.value.include_body
        lambda_arn   = lambda_function_association.value.lambda_arn != "" ? lambda_function_association.value.lambda_arn : local.lambda_mapping[lambda_function_association.value.event_type]
      }
    }
    compress               = ordered_cache_behavior.value.compress
    viewer_protocol_policy = ordered_cache_behavior.value.viewer_protocol_policy
  }
}

运行后触发如下错误：

│ Error: Missing newline after argument
│ 
│   on main.tf line 111, in resource "aws_cloudfront_distribution" "web_distribution":
│  111:       origin_request_policy_id  = ordered_cache_behavior.value.path_pattern = "/" ? var.origin_request_policy_ids["origin_1H_1D_plp_pdp_whitelist_whitelist_none"] : null
│ 
│ An argument definition must end with a newline.

我的需求是根据path_pattern是否为"/"来设置origin_request_policy_id，但不清楚错误原因，请问问题出在哪？

问题根源 #

错误提示看似是换行问题，但实际是三元表达式的判断符号用错了：你用了单个等号=，这是Terraform里的赋值操作符，而判断值相等需要用双等号==。

当你写ordered_cache_behavior.value.path_pattern = "/"时，Terraform会把它理解为赋值操作，后面跟着的? ... : ...就成了无效语法，导致解析混乱，才抛出了“缺少换行”的错误提示。

修正方案 #

把判断符号改成==即可，修正后的origin_request_policy_id行代码如下：

origin_request_policy_id  = ordered_cache_behavior.value.path_pattern == "/" ? var.origin_request_policy_ids["whitelist_policy"] : null

替换后重新执行Terraform命令，语法错误就能解决，同时也能实现根据path_pattern是否为"/"来设置对应策略ID的需求。

内容的提问来源于stack exchange，提问作者MBA