From fa6162bb77bbabc63c1d93009841b0c49e8c8e92 Mon Sep 17 00:00:00 2001
From: sebthom <sebthom@users.noreply.github.com>
Date: Fri, 31 Oct 2025 19:50:15 +0100
Subject: [PATCH] fix(jsonrpc): cancel locally first and still emit
 $/cancelRequest once

- Prevent a race where a server's RequestCancelled response completes
the future with ResponseErrorException before local cancellation,
breaking expected CancellationException behavior.
- Change cancel(boolean) to cancel the future locally first, then
(root-only) send $/cancelRequest.
- Adjust cancelRequest() to emit once even if the future is already
completed locally by removing the isDone() short-circuit; dedup remains
via cancelSent.
---
 .../eclipse/lsp4j/jsonrpc/JsonRpcRequestFuture.java   | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/JsonRpcRequestFuture.java b/org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/JsonRpcRequestFuture.java
index 45a1c9bd..3d7e9ca4 100644
--- a/org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/JsonRpcRequestFuture.java
+++ b/org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/JsonRpcRequestFuture.java
@@ -93,16 +93,18 @@ public <U> JsonRpcRequestFuture<U> applyToEitherAsync(final CompletionStage<? ex
 
 	@Override
 	public boolean cancel(final boolean mayInterruptIfRunning) {
-		// Root cancel ensures cancelRequest() behavior once before local cancellation
+		// First cancel locally to ensure future is in a cancelled state
+		// before any concurrent response can complete it exceptionally.
+		final boolean cancelled = super.cancel(mayInterruptIfRunning);
+		// Only the root future should emit the protocol cancel notification.
 		if (root == this) {
 			try {
 				cancelRequest();
 			} catch (final RuntimeException ex) {
-				// catching potential exception to ensure local cancel still proceeds
 				LOG.log(Level.WARNING, ex.getMessage(), ex);
 			}
 		}
-		return super.cancel(mayInterruptIfRunning);
+		return cancelled;
 	}
 
 	/**
@@ -119,9 +121,6 @@ public JsonRpcRequestFuture<?> getRoot() {
 	 */
 	public boolean cancelRequest() {
 		final JsonRpcRequestFuture<?> root = getRoot();
-		if (root.isDone()) {
-			return false;
-		}
 		if (root.cancelSent.compareAndSet(false, true)) {
 			root.cancelAction.run();
 			return true;