Session execute Ensure on only single Task.

CinemaLib/Webshare/Session.cs

@@ -27,6 +27,8 @@ public sealed class Session
   private TimeSpan _nextLoginBackoff;
   private DateTime _nextValidTrial;
 
+  private Task<bool>? _ensureValidTask;
+
   /// <summary>
   /// Constructs a session from credentials and optionally a possibly live token.
   /// </summary>
@@ -80,11 +82,21 @@ public sealed class Session
       // Fast track positive check
       return true;
 
-    if (await RefreshUserDataAsync(cancel) && (_isVip ?? false))
+    // Serialize threads so we do not bounce the server too much
+    TaskCompletionSource<bool> task = new TaskCompletionSource<bool>();
+    Task<bool>? resultT = Interlocked.CompareExchange(ref _ensureValidTask, task.Task, null);
+    if (resultT != null)
+      // We are not the thread that will determine it
+      return await resultT;
+
+    bool result = false;
+    try
+    {
+      if (_token != null && await RefreshUserDataAsync(_token, cancel) && (_isVip ?? false))
       {
         // Valid VIP (slower with API check)
         _nextValidTrial = now + TokenValidPeriod;
-      return true;
+        return result = true;
       }
 
       if (now > _nextLoginTrial)
@@ -106,13 +118,13 @@ public sealed class Session
           token = null;
         }
 
-      if (token != null && await RefreshUserDataAsync(cancel) && (_isVip ?? false))
+        if (token != null && await RefreshUserDataAsync(token, cancel) && (_isVip ?? false))
         {
           // Valid token with VIP
           _nextLoginBackoff = InitialLoginBackoff;
           _token = token;
           _nextValidTrial = now + TokenValidPeriod;
-        return true;
+          return result = true;
 
         }
         else
@@ -129,7 +141,13 @@ public sealed class Session
       _token = null;
       _isVip = null;
       _vipDaysLeft = null;
-    return false;
+      return result = false;
+    }
+    finally
+    {
+      task.SetResult(result);
+      GC.KeepAlive(Interlocked.Exchange(ref _ensureValidTask, null));
+    }
   }
 
   /// <summary>
@@ -226,7 +244,7 @@ public sealed class Session
   /// Gets user data and indirectly validates the session token.
   /// </summary>
   /// <returns>True if refresh has been successful and session token is valid. -or- False otherwise.</returns>
-  private async Task<bool> RefreshUserDataAsync(CancellationToken cancel)
+  private async Task<bool> RefreshUserDataAsync(string? token, CancellationToken cancel)
   {
     _isVip = null;
     _vipDaysLeft = null;
@@ -234,7 +252,7 @@ public sealed class Session
     // Example response: <?xml version="1.0" encoding="UTF-8"?><response><status>OK</status>...??...<app_version>30</app_version></response>
     Uri dataUri = new Uri(WebshareApiUri, "user_data/");
     HttpResponseMessage dataRes = await Program._http.PostAsync(dataUri, new FormUrlEncodedContent(new[] {
-      new KeyValuePair<string, string>(TokenKeyName, _token ?? "")
+      new KeyValuePair<string, string>(TokenKeyName, token ?? "")
       }), cancel
     );
     if (!dataRes.IsSuccessStatusCode)