fix: check for potential null futures in CancellationSupport.execute()

Author: sebthom

org.eclipse.lsp4e/src/org/eclipse/lsp4e/internal/CancellationSupport.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2303 Red Hat Inc. and others.
+ * Copyright (c) 2023 Red Hat Inc. and others.
  * This program and the accompanying materials are made
  * available under the terms of the Eclipse Public License 2.0
  * which is available at https://www.eclipse.org/legal/epl-2.0/
@@ -11,11 +11,14 @@
  *******************************************************************************/
 package org.eclipse.lsp4e.internal;
 
+import static org.eclipse.lsp4e.internal.NullSafetyHelper.castNullable;
+
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.CompletableFuture;
 
+import org.eclipse.lsp4e.LanguageServerPlugin;
 import org.eclipse.lsp4j.jsonrpc.CancelChecker;
 
 /**
@@ -38,7 +41,14 @@ public CancellationSupport() {
 	}
 
 	public <T> CompletableFuture<T> execute(CompletableFuture<T> future) {
-		this.futuresToCancel.add(future);
+		if (castNullable(future) == null) {
+			// Although 'future' is expected to be non-null per contract, we perform a null
+			// check to catch potential issues in asynchronous code and eagerly log an error
+			// if null is encountered.
+			LanguageServerPlugin.logError(new NullPointerException("[future] must not be null")); //$NON-NLS-1$
+		} else {
+			this.futuresToCancel.add(future);
+		}
 		return future;
 	}
 
@@ -58,12 +68,10 @@ public void cancel() {
 	@Override
 	public void checkCanceled() {
 		// When LSP requests are called (ex : 'textDocument/completion') the LSP
-		// response
-		// items are used to compose some UI item (ex : LSP CompletionItem are translate
-		// to Eclipse ICompletionProposal).
+		// response items are used to compose some UI item (ex : LSP CompletionItem are
+		// translate to Eclipse ICompletionProposal).
 		// If the cancel occurs after the call of those LSP requests, the component
-		// which uses the LSP responses
-		// can call checkCanceled to stop the UI creation.
+		// which uses the LSP responses can call checkCanceled to stop the UI creation.
 		if (cancelled) {
 			throw new CancellationException();
 		}