Hadoop fs s3a lib not working with temporary credentials in AWS Lambda

Trying to use the hadoop fs s3a library in AWS lambda with temporary credentials but it's not possible because of the way the AWSCredentialsProviderChain is defined under https://github.com/apache/hadoop/blob/29ae25801380b94442253c4202dee782dc4713f5/hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java

Specifically the following code is used to initialise the credentials chain

AWSCredentialsProviderChain credentials = new AWSCredentialsProviderChain(
        new BasicAWSCredentialsProvider(accessKey, secretKey),
        new InstanceProfileCredentialsProvider(),
        new AnonymousAWSCredentialsProvider()
    );

The above works fine when the EC2 metadata endpoint is available (i.e. running on an EC2 instance) however it doesn't work properly when the environment variables are used to define credentials as it happens in AWS Lambda. Amazon suggests to use the EnvironmentVariableCredentialsProvider in AWS Lambda.

To summarise and suggest an alternative I think that the DefaultAWSCredentialsProviderChain could be used instead of the InstanceProfileCredentialsProvider and that would cover the following cases:

If you think that the above change would be useful I could investigate more about what the required changes would be and submit a patch.