Release the parallelism slot when a dispatched message rejects

[Ngo Quoc Viet (NgoQuocViet2001)]

Problem

With maxParallelism configured, the message queue's slot accounting lives only in the fulfilment callback:

// jsonrpc/src/common/connection.ts
inFlight++;
...
if (result instanceof Promise) {
    result.then(() => {
        inFlight--;
        triggerMessageQueue();
    }).catch((error) => {
        logger.error(`Processing message queue failed: ${error.toString()}`);
    });
}

Written as .then(onFulfilled).catch(onRejected), a rejection skips onFulfilled entirely. The .catch logs, but never runs inFlight-- and never re-pumps. inFlight is the gate triggerMessageQueue checks:

if (maxParallelism !== -1 && inFlight >= maxParallelism) {
    return;
}

so one rejected dispatch leaves the counter permanently at the limit and the queue stops draining. The connection stays open and Listening; there is no error event, no close, nothing but a single log line, while every later request and notification sits in messageQueue forever.

Trigger

The promise the queue awaits is handleRequest's, which ends in messageWriter.write(...). A response that cannot be serialized rejects it. A handler throwing a ResponseError whose data is not JSON-serializable is enough — toJson() copies data onto the wire message:

server.onRequest(first, () => {
    const circular = {}; circular.self = circular;
    throw new ResponseError(ErrorCodes.InternalError, 'boom', circular);
});
server.onRequest(second, () => 'handled');

With { maxParallelism: 1 }, the request after it is never answered.

A handler that merely throws is not affected — that is caught inside handleRequest and turned into a well-formed replyError. It is the failure to write the response that escapes.

Fix

Use the two-argument form of then, so both settlements do the same bookkeeping. The log line is unchanged.

result.then(() => {
    inFlight--;
    triggerMessageQueue();
}, (error) => {
    inFlight--;
    logger.error(`Processing message queue failed: ${error.toString()}`);
    triggerMessageQueue();
});

The maxParallelism === -1 default never reaches the gate, so unlimited-parallelism behaviour is untouched.

Test plan

• Added Parallelism - a rejected dispatch releases its slot to jsonrpc/src/node/test/connection.test.ts, beside the existing Parallelism - limited case and reusing its TestDuplex pair.
• Ran: npm run compile:jsonrpc then the node suite from jsonrpc/ → 68 passing.
• Checked: reverting only connection.ts fails the new test with 'timed out' — the second request is never answered.
• Ran: npx eslint src in jsonrpc/ → clean.

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