Add basic API request throttling

A simple approach to throttle API requests and so keep within the rate
limits of the API. Can be enabled with "--throttle-limit" to specify
when throttling should start.
"--throttle-pause" defines the time to sleep between further API
requests.

github_backup/github_backup.py

@@ -331,6 +331,16 @@ def parse_args():
                         action='store_true',
                         dest='include_assets',
                         help='include assets alongside release information; only applies if including releases')
+    parser.add_argument('--throttle-limit',
+                        dest='throttle_limit',
+                        type=int,
+                        default=0,
+                        help='start throttling of GitHub API requests after this amount of API requests remain')
+    parser.add_argument('--throttle-pause',
+                        dest='throttle_pause',
+                        type=float,
+                        default=30.0,
+                        help='wait this amount of seconds when API request throttling is active (default: 30.0, requires --throttle-limit to be set)')
     return parser.parse_args()
 
 
@@ -439,6 +449,14 @@ def retrieve_data_gen(args, template, query_args=None, single_request=False):
         r, errors = _get_response(request, auth, template)
 
         status_code = int(r.getcode())
+        # be gentle with API request limit and throttle requests if remaining requests getting low
+        limit_remaining = int(r.headers.get('x-ratelimit-remaining', 0))
+        if limit_remaining <= args.throttle_limit:
+            log_info(
+                'API request limit hit: {} requests left, pausing further requests for {}s'.format(
+                    limit_remaining,
+                    args.throttle_pause))
+            time.sleep(args.throttle_pause)
 
         retries = 0
         while retries < 3 and status_code == 502: