Implement serial rate limiter with backoff

src/Jellyfin.Plugin.ListenBrainz.Api/BaseClient.cs

@@ -1,9 +1,13 @@
+using System.Net;
 using System.Net.Http.Headers;
 using System.Text;
+using Jellyfin.Plugin.ListenBrainz.Api.Exceptions;
 using Jellyfin.Plugin.ListenBrainz.Api.Interfaces;
+using Jellyfin.Plugin.ListenBrainz.Api.Resources;
 using Jellyfin.Plugin.ListenBrainz.Http;
 using Jellyfin.Plugin.ListenBrainz.Http.Exceptions;
 using Jellyfin.Plugin.ListenBrainz.Http.Interfaces;
+using Jellyfin.Plugin.ListenBrainz.Http.Services;
 using Microsoft.Extensions.Logging;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Serialization;
@@ -26,6 +30,10 @@ public class BaseClient : HttpClient
         ContractResolver = new DefaultContractResolver { NamingStrategy = new SnakeCaseNamingStrategy() }
     };
 
+    private readonly object _lock = new();
+    private readonly ILogger _logger;
+    private readonly ISleepService _sleepService;
+
     /// <summary>
     /// Initializes a new instance of the <see cref="BaseClient"/> class.
     /// </summary>
@@ -35,6 +43,8 @@ public class BaseClient : HttpClient
     protected BaseClient(IHttpClientFactory httpClientFactory, ILogger logger, ISleepService? sleepService)
         : base(httpClientFactory, logger, sleepService)
     {
+        _logger = logger;
+        _sleepService = sleepService ?? new DefaultSleepService();
     }
 
     /// <summary>
@@ -90,12 +100,53 @@ protected BaseClient(IHttpClientFactory httpClientFactory, ILogger logger, ISlee
     private async Task<TResponse?> DoRequest<TResponse>(HttpRequestMessage requestMessage, CancellationToken cancellationToken)
         where TResponse : IListenBrainzResponse
     {
-        var response = await SendRequest(requestMessage, cancellationToken);
+        HttpResponseMessage response;
+        try
+        {
+            Monitor.Enter(_lock);
+            while (true)
+            {
+                response = await SendRequest(requestMessage, cancellationToken);
+                if (response.StatusCode == HttpStatusCode.TooManyRequests)
+                {
+                    _logger.LogDebug("Rate limit reached, will retry after new window opens");
+                    HandleRateLimit(response);
+                    continue;
+                }
+
+                _logger.LogDebug("Did not hit any rate limits, all OK");
+                break;
+            }
+        }
+        finally
+        {
+            Monitor.Exit(_lock);
+        }
+
         var stringContent = await response.Content.ReadAsStringAsync(cancellationToken);
         var result = JsonConvert.DeserializeObject<TResponse>(stringContent, SerializerSettings);
         if (result is null) throw new InvalidResponseException("Response deserialized to NULL");
 
         result.IsOk = response.IsSuccessStatusCode;
         return result;
     }
+
+    private void HandleRateLimit(HttpResponseMessage response)
+    {
+        // X-RateLimit-Reset-In: Number of seconds when current time window expires
+        var header = response.Headers.FirstOrDefault(h => h.Key == Headers.RateLimitResetIn);
+        var resetIn = header.Value.FirstOrDefault();
+        if (resetIn is null)
+        {
+            throw new ListenBrainzException("No 'rate limit reset in' value available, cannot continue");
+        }
+
+        if (!int.TryParse(resetIn, out var resetInSec))
+        {
+            throw new ListenBrainzException("Invalid value for 'rate limit reset in', cannot continue");
+        }
+
+        _logger.LogDebug("Waiting for {Seconds} seconds before trying again", resetInSec);
+        _sleepService.Sleep(resetInSec);
+    }
 }

src/Jellyfin.Plugin.ListenBrainz.Api/Resources/Headers.cs

@@ -0,0 +1,27 @@
+namespace Jellyfin.Plugin.ListenBrainz.Api.Resources;
+
+/// <summary>
+/// ListenBrainz API custom headers.
+/// </summary>
+public static class Headers
+{
+    /// <summary>
+    /// Number of requests allowed in given time window.
+    /// </summary>
+    public const string RateLimitLimit = "X-RateLimit-Limit";
+
+    /// <summary>
+    /// Number of requests remaining in current time window.
+    /// </summary>
+    public const string RateLimitRemaining = "X-RateLimit-Remaining";
+
+    /// <summary>
+    /// Number of seconds when current time window expires (recommended: this header is resilient against clients with incorrect clocks).
+    /// </summary>
+    public const string RateLimitResetIn = "X-RateLimit-Reset-In";
+
+    /// <summary>
+    /// UNIX epoch number of seconds (without timezone) when current time window expires.
+    /// </summary>
+    public const string RateLimitReset = "X-RateLimit-Reset";
+}