Url signing breaks if metadata field has null

[Agrumas]

I tried to create signed url with null in one of metafield and it breaks in an unexpected way:

TypeError: Cannot read property 'split' of undefined
    at Request.signedUrlSigner (app\node_modules\aws-sdk\lib\signers\presign.js:62:42)
    at Request.callListeners (app\node_modules\aws-sdk\lib\sequential_executor.js:105:20)
    at callNextListener (app\node_modules\aws-sdk\lib\sequential_executor.js:95:12)
    at app\node_modules\aws-sdk\lib\event_listeners.js:207:9
    at finish (app\node_modules\aws-sdk\lib\config.js:315:7)
    at getStaticCredentials (app\node_modules\aws-sdk\lib\config.js:342:7)
    at Config.getCredentials (app\node_modules\aws-sdk\lib\config.js:349:9)
    at Request.SIGN (app\node_modules\aws-sdk\lib\event_listeners.js:179:22)
    at Request.callListeners (app\node_modules\aws-sdk\lib\sequential_executor.js:101:18)
    at Request.emit (app\node_modules\aws-sdk\lib\sequential_executor.js:77:10)
  message: 'Cannot read property \'split\' of undefined',
  code: 'TypeError',
  time: 2017-06-26T13:17:28.344Z,
  retryDelay: 33.933526783304345

Example:

const S3 = require('aws-sdk/clients/s3');
const s3 = new S3({
  region: 'eu-central-1',
  signatureVersion: 'v4',
  credentials: {accessKeyId: 'accessKeyId', secretAccessKey: 'secretAccessKey'}
});

s3.getSignedUrl('putObject', {
  Metadata: {userId: null},
  Bucket: 'test',
  Key: 'test-key',
  ACL: 'authenticated-read',
  Expires: 3600,
}, (error, data) => {
  console.log(error, data);
});

Most likely this situation is the same as reported in #834

[chrisradek]

@Agrumas
The error is occurring because we're attempting to call toString on null when collecting the canonical headers to sign:
https://github.com/aws/aws-sdk-js/blob/v2.77.0/lib/signers/v4.js#L142

We should be throwing a more helpful error in this case.

That said, what are you expecting to happen by specifying null metadata in the presigned url? If you leave it out of your options, then your object should not include the metadata. If you want userId to appear, but me empty, you could also set it to an empty string.

[Agrumas]

@chrisradek error handling would be appreciated in that place because I spent a few hours trying to figure out what is wrong with Authentication part... The null isn't necessary in my case, just it was passed from other layer and I even wasn't aware of it. I would expect empty string, missing field, but not an error :)

[lock]

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