AWS SAM模板解析失败：块映射语法错误排查求助

问题场景 #

编写的AWS SAM模板在解析时出现Failed to parse template: while parsing a block mapping错误，错误指向Policies中BucketName下的!Sub标签位置。模板代码如下：

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  example

Parameters:
  EnvironmentPrefix:
    Type: String
    Default: "test-"

Resources:
  S3JsonLoggerFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: src/handlers/s3-json-logger.s3JsonLoggerHandler
      Runtime: nodejs18.x
      Architectures:
        - x86_64
      MemorySize: 128
      Timeout: 60
      Policies:
        - S3ReadPolicy:
            BucketName:
              !Fn::ImportValue:
                !Sub "${EnvironmentPrefix}example-bucket"
      Events:
        S3NewObjectEvent:
          Type: S3
          Properties:
            Bucket:
              !Fn::ImportValue:
                !Sub "${EnvironmentPrefix}example-bucket"
            Events: s3:ObjectCreated:*
            Filter:
              S3Key:
                Rules:
                  - Name: prefix
                    Value: "changelog/"

错误信息：

Error: Failed to parse template: while parsing a block mapping
  in "<unicode string>", line 23, column 13:
                BucketName:
                ^
expected <block end>, but found '<tag>'
  in "<unicode string>", line 25, column 17:
                    !Sub "${EnvironmentPrefix}example-b ...
                    ^

错误原因 #

这是YAML语法解析规则导致的错误：!Fn::ImportValue（或短格式!ImportValue）仅接受字符串类型的参数，但原模板中把!Sub标签作为嵌套的块映射项放在!Fn::ImportValue下方，YAML解析器无法识别这种结构，误以为是新的块映射，因此抛出解析失败的错误。

修复方案 #

将!Sub作为!ImportValue的直接参数，调整为以下两种合法写法之一，同时注意同步修正Events部分的Bucket字段（存在同样问题）：

写法1：短格式标签嵌套（推荐） #

Policies:
  - S3ReadPolicy:
      BucketName: !ImportValue !Sub "${EnvironmentPrefix}example-bucket"
Events:
  S3NewObjectEvent:
    Type: S3
    Properties:
      Bucket: !ImportValue !Sub "${EnvironmentPrefix}example-bucket"

写法2：完整函数语法 #

Policies:
  - S3ReadPolicy:
      BucketName:
        !Fn::ImportValue: !Sub "${EnvironmentPrefix}example-bucket"
Events:
  S3NewObjectEvent:
    Type: S3
    Properties:
      Bucket:
        !Fn::ImportValue: !Sub "${EnvironmentPrefix}example-bucket"

修正后的完整模板 #

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  example

Parameters:
  EnvironmentPrefix:
    Type: String
    Default: "test-"

Resources:
  S3JsonLoggerFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: src/handlers/s3-json-logger.s3JsonLoggerHandler
      Runtime: nodejs18.x
      Architectures:
        - x86_64
      MemorySize: 128
      Timeout: 60
      Policies:
        - S3ReadPolicy:
            BucketName: !ImportValue !Sub "${EnvironmentPrefix}example-bucket"
      Events:
        S3NewObjectEvent:
          Type: S3
          Properties:
            Bucket: !ImportValue !Sub "${EnvironmentPrefix}example-bucket"
            Events: s3:ObjectCreated:*
            Filter:
              S3Key:
                Rules:
                  - Name: prefix
                    Value: "changelog/"

内容的提问来源于stack exchange，提问作者Alex R