presignedPostPolicy method

Future<PostPolicyResult> presignedPostPolicy(PostPolicy postPolicy)

presignedPostPolicy can be used in situations where we want more control on the upload than what presignedPutObject() provides. i.e Using presignedPostPolicy we will be able to put policy restrictions on the object's name bucket expiry Content-Type

Implementation

Future<PostPolicyResult> presignedPostPolicy(PostPolicy postPolicy) async {
  if (_client.anonymous) {
    throw MinioAnonymousRequestError(
      'Presigned POST policy cannot be generated for anonymous requests',
    );
  }

  final region = await getBucketRegion(postPolicy.formData['bucket']!);
  final date = DateTime.now().toUtc();
  final dateStr = makeDateLong(date);

  if (postPolicy.policy['expiration'] == null) {
    // 'expiration' is mandatory field for S3.
    // Set default expiration date of 7 days.
    final expires = DateTime.now().toUtc();
    expires.add(const Duration(days: 7));
    postPolicy.setExpires(expires);
  }

  postPolicy.policy['conditions'].add(['eq', r'$x-amz-date', dateStr]);
  postPolicy.formData['x-amz-date'] = dateStr;

  postPolicy.policy['conditions']
      .add(['eq', r'$x-amz-algorithm', 'AWS4-HMAC-SHA256']);
  postPolicy.formData['x-amz-algorithm'] = 'AWS4-HMAC-SHA256';

  postPolicy.policy['conditions'].add(
    ['eq', r'$x-amz-credential', '$accessKey/${getScope(region, date)}'],
  );
  postPolicy.formData['x-amz-credential'] =
      '$accessKey/${getScope(region, date)}';

  if (sessionToken != null) {
    postPolicy.policy['conditions']
        .add(['eq', r'$x-amz-security-token', sessionToken]);
  }

  final policyBase64 = jsonBase64(postPolicy.policy);
  postPolicy.formData['policy'] = policyBase64;

  final signature =
      postPresignSignatureV4(region, date, secretKey, policyBase64);

  postPolicy.formData['x-amz-signature'] = signature;
  final url = _client
      .getBaseRequest(
        'POST',
        postPolicy.formData['bucket'],
        null,
        region,
        null,
        null,
        null,
        null,
      )
      .url;
  final portStr = (port == 80 || port == 443) ? '' : ':$port';
  final urlStr = '${url.scheme}://${url.host}$portStr${url.path}';
  return PostPolicyResult(postURL: urlStr, formData: postPolicy.formData);
}