Fix deadlock on busy servers with small thread pools, close #100

common/src/main/java/net/william278/husksync/database/Database.java

@@ -114,10 +114,9 @@ public abstract class Database {
      * <b>(Internal)</b> Prune user data for a given user to the maximum value as configured.
      *
      * @param user The user to prune data for
-     * @return A future returning void when complete
      * @implNote Data snapshots marked as {@code pinned} are exempt from rotation
      */
-    protected abstract CompletableFuture<Void> rotateUserData(@NotNull User user);
+    protected abstract void rotateUserData(@NotNull User user);
 
     /**
      * Deletes a specific {@link UserDataSnapshot} entry for a user from the database, by its UUID.

common/src/main/java/net/william278/husksync/database/MySqlDatabase.java

@@ -297,8 +297,7 @@ public class MySqlDatabase extends Database {
     }
 
     @Override
-    protected CompletableFuture<Void> rotateUserData(@NotNull User user) {
-        return CompletableFuture.runAsync(() -> {
+    protected void rotateUserData(@NotNull User user) {
         final List<UserDataSnapshot> unpinnedUserData = getUserData(user).join().stream()
                 .filter(dataSnapshot -> !dataSnapshot.pinned()).toList();
         if (unpinnedUserData.size() > plugin.getSettings().maxUserDataSnapshots) {
@@ -317,7 +316,6 @@ public class MySqlDatabase extends Database {
                 plugin.log(Level.SEVERE, "Failed to prune user data from the database", e);
             }
         }
-        });
     }
 
     @Override
@@ -362,7 +360,8 @@ public class MySqlDatabase extends Database {
                     plugin.log(Level.SEVERE, "Failed to set user data in the database", e);
                 }
             }
-        }).thenRun(() -> rotateUserData(user).join());
+            this.rotateUserData(user);
+        });
     }
 
     @Override