SharedIniFileCredentials/AWS_STS_REGIONAL_ENDPOINTS=regional fails if there's no '[default]' profile

[rix0rrr]

Describe the bug

When using SharedIniFileCredentials with:

• A profile name supplied as argument (not via AWS_PROFILE).
• A profile that uses assumerole credentials
• AWS_STS_REGIONAL_ENDPOINTS=regional

Then the credential retrieval fails if there's not a [default] profile with a region (even if both profiles involved in the AssumeRole operation do have a region in them). I'm assuming if there is a [default] profile then the provider will use the wrong region--I did not try to confirm that.

The problem does not appear when I configure AWS_PROFILE which makes me think there's some code that's forgetting to pass on the profile constructor argument to the right object.

Is the issue in the browser/Node.js?
Node.js

If on Node.js, are you running this on AWS Lambda?
No

Details of the browser/Node.js version
v12.12.0

SDK version number
2.713.0

To Reproduce (observed behavior)

The following code:

process.env.AWS_SDK_LOAD_CONFIG = '1';
process.env.AWS_STS_REGIONAL_ENDPOINTS = 'regional';

const AWS = require('aws-sdk');

const creds = new AWS.SharedIniFileCredentials({ profile: 'huijbers-admin-role' });

const chain = new AWS.CredentialProviderChain([creds]);
chain.resolvePromise().then(console.log);

Fails when given the following config:

============ ~/.aws/credentials =======================
[huijbers-admin-user]
aws_access_key_id = AKIA****************
aws_secret_access_key = YASbc2*****************

[huijbers-admin-role]
role_arn = arn:aws:iam::993655754359:role/Assumable
source_profile = huijbers-admin-user

============ ~/.aws/config =======================
[profile huijbers-admin-role]
region = eu-west-1

[profile huijbers-admin-user]
region = eu-west-1

The error is the following:

(node:66546) UnhandledPromiseRejectionWarning: ConfigError: Missing region in config
    at Request.optInRegionalEndpoint (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/services/sts.js:75:30)
    at Request.callListeners (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/sequential_executor.js:106:20)
    at Request.emit (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/sequential_executor.js:78:10)
    at Request.emit (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/request.js:688:14)
    at Request.transition (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/request.js:22:10)
    at AcceptorStateMachine.runTo (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/state_machine.js:14:12)
    at Request.runTo (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/request.js:408:15)
    at Request.send (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/request.js:372:10)
    at features.constructor.makeRequest (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/service.js:205:27)
    at features.constructor.svc.<computed> [as assumeRole] (/Users/huijbers/Temp/repro/node_modules/aws-sdk/lib/service.js:677:23)

And succeeds when the following is added to ~/.aws/config:

[default]
region = eu-west-1

Expected behavior
I would have expected the region from the huijbers-admin-role profile to be used.

Additional context

Originally reported in aws/aws-cdk#9937

[rix0rrr]

Isn't the solution to change this:

aws-sdk-js/lib/credentials/shared_ini_file_credentials.js

Lines 219 to 222 in f1c689f

	var sts = new STS({
	credentials: sourceCredentials,
	httpOptions: this.httpOptions
	});

To read:

    var sts = new STS({
      region: roleProfile['region'],
      credentials: sourceCredentials,
      httpOptions: this.httpOptions
    });

[rix0rrr]

Oh, what I proposed won't actually work because of this piece of code:

aws-sdk-js/lib/util.js

Lines 969 to 974 in 5ae5a7d

	for (var i = 0, profileNames = Object.keys(profilesFromConfig); i < profileNames.length; i++) {
	profiles[profileNames[i]] = profilesFromConfig[profileNames[i]];
	}
	for (var i = 0, profileNames = Object.keys(profilesFromCreds); i < profileNames.length; i++) {
	profiles[profileNames[i]] = profilesFromCreds[profileNames[i]];
	}

Given:

========== config ===============
[profile foo]
region = us-east-1

========= credentials ==============
[foo]
aws_access_key_id = 123

The "config" profile will be completely overwritten (not merged) by the credentials profile and the end result will be:

{
  "foo": {
    "aws_access_key_id": "123"
  }
}

So "region" will have been dropped from it.

Is this on purpose?