Fix HttpShardHandlerFactory to use an unbounded queue if the pool siz…

…e is limited, to avoid rejected tasks in wide distributed searches

solr/core/src/java/org/apache/solr/handler/component/HttpShardHandlerFactory.java

@@ -27,6 +27,7 @@
 import java.util.concurrent.CompletionService;
 import java.util.concurrent.ExecutorCompletionService;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.SynchronousQueue;
 import java.util.concurrent.TimeUnit;
 
@@ -194,9 +195,7 @@ public void init(PluginInfo info) {
       r.setSeed(Long.parseLong(v));
     }
 
-    BlockingQueue<Runnable> blockingQueue = (this.queueSize == -1) ?
-        new SynchronousQueue<Runnable>(this.accessPolicy) :
-        new ArrayBlockingQueue<Runnable>(this.queueSize, this.accessPolicy);
+    BlockingQueue<Runnable> blockingQueue = getBlockingQueue();
 
     this.commExecutor = new ExecutorUtil.MDCAwareThreadPoolExecutor(
         this.corePoolSize,
@@ -226,6 +225,21 @@ public void init(PluginInfo info) {
     this.loadbalancer = createLoadbalancer(defaultClient);
   }
 
+  private BlockingQueue<Runnable> getBlockingQueue() {
+    if (this.queueSize == -1) {
+      if (this.maximumPoolSize == Integer.MAX_VALUE) {
+        return new SynchronousQueue<Runnable>(this.accessPolicy);
+      } else {
+        // A limited maximumPoolSize would result in new tasks being rejected
+        // once the pool is full, with SynchronousQueue, so it's better to accept
+        // as many as we're offered and let the pool work through them
+        return new LinkedBlockingQueue<>();
+      }
+    } else {
+      return new ArrayBlockingQueue<Runnable>(this.queueSize, this.accessPolicy);
+    }
+  }
+
   protected ModifiableSolrParams getClientParams() {
     ModifiableSolrParams clientParams = new ModifiableSolrParams();
     clientParams.set(HttpClientUtil.PROP_SO_TIMEOUT, soTimeout);