Limit ReservedThread creation (#13837)

Updated the `ReservedThreadExecutor` default value for `maxThreads` to be `0`, meaning the capacity.
In this way, by default, the reserved threads that get created are at most 2 times the capacity, rather than an unlimited number.

Signed-off-by: Oleksandr Krutko <alexander.krutko@gmail.com>

Author: arsenalzp

jetty-core/jetty-util/src/main/java/org/eclipse/jetty/util/thread/ReservedThreadExecutor.java

@@ -74,7 +74,7 @@ public ReservedThreadExecutor(Executor executor, int capacity)
      */
     public ReservedThreadExecutor(Executor executor, int capacity, int minSize)
     {
-        this(executor, capacity, minSize, -1);
+        this(executor, capacity, minSize, 0);
     }
 
     /**
@@ -83,7 +83,7 @@ public ReservedThreadExecutor(Executor executor, int capacity, int minSize)
      *                 is calculated based on a heuristic from the number of available processors and
      *                 thread pool type.
      * @param minSize The minimum number of reserve Threads that the algorithm tries to maintain, or -1 for a heuristic value.
-     * @param maxPending The maximum number of reserved Threads to start, or -1 for no limit.
+     * @param maxPending The maximum number of reserved Threads to start, 0 (default) for thread pool capacity or -1 for no limit.
      */
     public ReservedThreadExecutor(Executor executor, int capacity, int minSize, int maxPending)
     {
@@ -92,9 +92,14 @@ public ReservedThreadExecutor(Executor executor, int capacity, int minSize, int
         _minSize = minSize < 0 ? Math.min(1, _threads.capacity()) : minSize;
         if (_minSize > _threads.capacity())
             throw new IllegalArgumentException("minSize larger than capacity");
-        _maxPending = maxPending;
-        if (_maxPending == 0)
-            throw new IllegalArgumentException("maxPending cannot be 0");
+
+        if (maxPending < 0)
+            _maxPending = -1;
+        else if (maxPending == 0)
+            _maxPending = _threads.capacity();
+        else
+            _maxPending = maxPending;
+
         if (LOG.isDebugEnabled())
             LOG.debug("{}", this);
         installBean(_executor);
@@ -227,7 +232,7 @@ public boolean tryExecute(Runnable task)
 
     private void startReservedThread()
     {
-        if (_maxPending > 0 && _pending.incrementAndGet() >= _maxPending)
+        if (_maxPending > 0 && _pending.incrementAndGet() > _maxPending)
         {
             _pending.decrementAndGet();
             return;