Wait until item exists

IA doesn't immediately create the item on CreateMultipartUpload, so if it didn't already exist, UploadPart would fail for a while and we'd waste bandwidth.

ia-upload-stream

@@ -212,6 +212,19 @@ def upload(item, filename, metadata, *, iaConfigFile = None, partSize = 100*1024
 		uploadId = m[1]
 		logger.info(f'Got upload ID {uploadId}')
 
+		# Wait for the item to exist; if the above created the item, it takes a little while for IA to actually create the bucket, and uploads would fail with a 404 until then.
+		for attempt in range(1, tries + 1):
+			logger.info(f'Checking for existence of {item}')
+			r = requests.get(f'https://s3.us.archive.org/{item}/', headers = headers)
+			if r.status_code == 200:
+				break
+			sleepTime = min(3 ** attempt, 30)
+			retrying = f', retrying after {sleepTime} seconds' if attempt < tries else ''
+			logger.error(f'Got status code {r.status_code} from IA S3 on checking for item existence{retrying}')
+			if attempt == tries:
+				raise UploadError('Item still does not exist', r = r, uploadId = uploadId, parts = parts)
+			time.sleep(sleepTime)
+
 	# Upload the data in parts
 	if parts is None:
 		parts = []