🐌 Handle MyPurdue request throttling (#78)

MyPurdue has recently started throttling incoming requests, preventing
Purdue.io CatalogSync from successfully syncing:

> We are sorry, but the site has received too many requests.

The exact behavior of the throttle logic is unclear, but brief
experimentation has revealed that after being throttled, you must wait a
minimum of 60 seconds from your last request to avoid being throttled
again.

This change implements an exponential delay with a 60-second base time
when the throttling message is received to allow the sync to complete.

Author: haydenmc

src/Scraper/Connections/MyPurdueConnection.cs

@@ -29,7 +29,11 @@ private enum HttpMethod { GET, POST };
         private readonly HttpClient httpClient;
 
         // How many request attempts should be made before failure
-        private const int MAX_RETRIES = 5;
+        private const int MAX_RETRIES = 6;
+
+        // MyPurdue now throttles our requests, and only seems to let us through
+        // after waiting a minimum of one minute.
+        private const int THROTTLE_DELAY_BASE_MS = 60000;
 
         public MyPurdueConnection(ILogger<MyPurdueConnection> logger)
         {
@@ -146,11 +150,19 @@ public async Task<string> GetSectionListPageAsync(string termCode, string subjec
                 {
                     this.logger.LogError("Received non-success status code '{}' on " +
                         "GetSectionListPageAsync.", request.StatusCode);
+                    continue;
                 }
-                else
+
+                var content = await request.Content.ReadAsStringAsync();
+                if (content.Contains("We are sorry, but the site has received too many requests."))
                 {
-                    return await request.Content.ReadAsStringAsync();
+                    var retryDelayMs = THROTTLE_DELAY_BASE_MS + ExponentialRetryDelayMs(attempts);
+                    this.logger.LogError("MyPurdue is throttling requests. Retrying in {}ms...",
+                        retryDelayMs);
+                    await Task.Delay(retryDelayMs);
+                    continue;
                 }
+                return content;
             }
             throw new ApplicationException(
                 "Exceeded retries attempting to query MyPurdue section list");
@@ -315,5 +327,10 @@ private async Task<HttpResponseMessage> Request(HttpMethod method, string url,
             }
             return result;
         }
+
+        private static int ExponentialRetryDelayMs(int retryNumber)
+        {
+            return (int)(Math.Pow(2, retryNumber) * 1000);
+        }
     }
 }