Run long-spanning tasks cancellably on background in the (Java) LSP server.

java/java.lsp.server/src/org/netbeans/modules/java/lsp/server/protocol/PriorityQueueRun.java

@@ -0,0 +1,199 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.netbeans.modules.java.lsp.server.protocol;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map.Entry;
+import java.util.SortedMap;
+import java.util.TreeMap;
+import java.util.concurrent.CancellationException;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import org.openide.util.RequestProcessor;
+
+public class PriorityQueueRun {
+
+    private static final PriorityQueueRun INSTANCE = new PriorityQueueRun();
+    private static final RequestProcessor WORKER = new RequestProcessor(PriorityQueueRun.class.getName() + "-worker", 1, false, false);
+    private static final RequestProcessor DELAY = new RequestProcessor(PriorityQueueRun.class.getName() + "-delay", 1, false, false);
+
+    public static PriorityQueueRun getInstance() {
+        return INSTANCE;
+    }
+
+    private final SortedMap<Priority, List<TaskDescription<?, ?>>> priority2Tasks = new TreeMap<>((p1, p2) -> -p1.compareTo(p2));
+    private Priority currentPriority;
+    private CancelCheck currentTaskCheck;
+
+    public <P, R> CompletableFuture<R> runTask(Priority priority, CancellableTask<P, R> task, P data) {
+        CompletableFuture<R> result = new CompletableFuture<>();
+
+        synchronized (this) {
+            priority2Tasks.computeIfAbsent(priority, __ -> new ArrayList<>())
+                          .add(new TaskDescription(task, data, result));
+
+            scheduleNext();
+        }
+
+        return result;
+    }
+
+    public <P, R> CompletableFuture<R> runTask(Priority priority, CancellableTask<P, R> task, P data, int delay) {
+        CompletableFuture<R> result = new CompletableFuture<>();
+        DELAY.post(() -> {
+            if (result.isCancelled()) {
+                return ; //already cancelled
+            }
+            synchronized (this) {
+                priority2Tasks.computeIfAbsent(priority, __ -> new ArrayList<>())
+                              .add(new TaskDescription(task, data, result));
+
+                scheduleNext();
+            }
+        }, delay);
+
+        return result;
+    }
+
+    private synchronized void scheduleNext() {
+        Priority foundPriority = null;
+        List<TaskDescription<?, ?>> foundTasks = null;
+
+        for (Entry<Priority, List<TaskDescription<?, ?>>> e : priority2Tasks.entrySet()) {
+            if (!e.getValue().isEmpty()) {
+                foundPriority = e.getKey();
+                foundTasks = e.getValue();
+                break;
+            }
+        }
+
+        if (foundPriority == null) {
+            return ;
+        }
+
+        if (currentPriority == null) {
+            Priority thisPriority = currentPriority = foundPriority;
+            TaskDescription thisTask = foundTasks.remove(0);
+            CancelCheck thisTaskCheck = currentTaskCheck = new CancelCheck();
+
+            WORKER.post(() -> {
+                thisTask.result.whenComplete((__, exc) -> {
+                    if (exc instanceof CancellationException) {
+                        thisTaskCheck.cancel();
+                    }
+                });
+
+                if (thisTask.result.isCancelled()) {
+                    //nothing to do anymore:
+                    return ;
+                }
+
+                Object result = null;
+                Throwable exception = null;
+
+                try {
+                    result = thisTask.task.compute(thisTask.data, thisTaskCheck);
+                } catch (Throwable t) {
+                    exception = t;
+                }
+
+                synchronized (PriorityQueueRun.this) {
+                    currentPriority = null;
+                    currentTaskCheck = null;
+
+                    if (!thisTask.result.isCancelled()) {
+                        if (exception != null) {
+                            thisTask.result.completeExceptionally(exception);
+                        } else if (thisTaskCheck.isCancelled()) {
+                            priority2Tasks.computeIfAbsent(thisPriority, __ -> new ArrayList<>())
+                                          .add(0, thisTask);
+                        } else {
+                            thisTask.result.complete(result);
+                        }
+                    }
+                }
+
+                scheduleNext();
+            });
+        }
+
+        if (currentPriority != null && currentPriority.compareTo(foundPriority) < 0) {
+            //cancel the currently running task:
+            currentTaskCheck.cancel();
+        }
+    }
+
+    private static final class TaskDescription<P, R> {
+        private final CancellableTask<P, R> task;
+        private final P data;
+        private final CompletableFuture<R> result;
+
+        public TaskDescription(CancellableTask<P, R> task, P data, CompletableFuture<R> result) {
+            this.task = task;
+            this.data = data;
+            this.result = result;
+        }
+    }
+
+    public interface CancellableTask<P, R> {
+        public R compute(P param, CancelCheck cancel) throws Exception;
+    }
+
+    public interface CancelCallback {
+        public void cancel();
+    }
+
+    public final class CancelCheck {
+        private final AtomicBoolean cancelled = new AtomicBoolean();
+        private final AtomicReference<CancelCallback> cancelCallback = new AtomicReference<>();
+
+        private CancelCheck() {}
+
+        public boolean isCancelled() {
+            return cancelled.get();
+        }
+
+        public void registerCancel(CancelCallback callback) {
+            cancelCallback.set(callback);
+            if (cancelled.get()) {
+                callback.cancel();
+            }
+        }
+
+        void cancel() {
+            cancelled.set(true);
+
+            CancelCallback callback = cancelCallback.get();
+
+            if (callback != null) {
+                callback.cancel();
+            }
+        }
+    }
+
+    public enum Priority {
+        BELOW_LOW,
+        LOW,
+        NORMAL,
+        HIGH,
+        HIGHER;
+    }
+}