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fix(mercury): gracefully handle mercury rebalancing #5106

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8e0f835
fix(mercury): gracefully handle mercury rebalancing
Shreyas281299 Jul 20, 2026
a9a8559
Merge branch 'next' into mercury-4000-fix
Shreyas281299 Jul 23, 2026
9c0d114
fix(mercury): add JS_SDK_MERCURY_CLOSE_4000 metric
Shreyas281299 Jul 27, 2026
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49 changes: 46 additions & 3 deletions packages/@webex/internal-plugin-mercury/src/mercury.js
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@ import {
} from './errors';

const normalReconnectReasons = ['idle', 'done (forced)', 'pong not received', 'pong mismatch'];
const MERCURY_CLOSE_4000_METRIC = 'JS_SDK_MERCURY_CLOSE_4000';

const Mercury = WebexPlugin.extend({
namespace: 'Mercury',
Expand Down Expand Up @@ -850,6 +851,29 @@ const Mercury = WebexPlugin.extend({
return handlers;
},

_submitMercuryClose4000Metric(sessionId, event, options) {
const {action, isActiveSocket, messageType} = options;

try {
this.webex.internal.metrics.submitClientMetrics(MERCURY_CLOSE_4000_METRIC, {

@chatgpt-codex-connector chatgpt-codex-connector Bot Jul 27, 2026

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P2 Badge Handle the metric submission promise

When the client-metrics request fails asynchronously—such as during the same network disruption that closed Mercury—submitClientMetrics() returns a rejected promise, but this call discards it. The surrounding try/catch only handles synchronous throws, so the failure becomes an unhandled rejection and can terminate Node clients instead of merely logging the metric failure as intended; attach a rejection handler to the returned promise.

Useful? React with 👍 / 👎.

fields: {
action,
close_code: event.code,
close_reason: event.reason || '',
is_active_socket: isActiveSocket,
message_type: messageType,
session_id: sessionId,
},
tags: {
action,
message_type: messageType,
},
});
} catch (error) {
this.logger.warn(`${this.namespace}: failed to submit Mercury 4000 close metric`, error);
}
},

_onclose(sessionId, event, sourceSocket) {
// I don't see any way to avoid the complexity or statement count in here.
/* eslint complexity: [0] */
Expand Down Expand Up @@ -901,9 +925,28 @@ const Mercury = WebexPlugin.extend({
break;
case 4000:
// metric: disconnect
this.logger.info(`${this.namespace}: socket ${sessionId} replaced; will not reconnect`);
if (isActiveSocket) this._emit(sessionId, 'offline.replaced', event);
// If not active, nothing to do
if (reason === 'replaced') {

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P2 Badge Keep bare 4000 closes in the replaced path

When the active socket closes with code 4000 but no reason is provided, this condition now falls into the reconnect branch, emitting offline.transient and calling _reconnect; previously every 4000 close was treated as offline.replaced, and the existing mercury-events unit coverage still exercises {code: 4000} as a replacement. In clients or browsers that do not surface the close reason, a replaced socket will now reconnect and can churn against the intended replacement connection instead of staying closed.

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this._submitMercuryClose4000Metric(sessionId, event, {
action: 'no_action',
isActiveSocket,
messageType: 'replaced',
});
this.logger.info(`${this.namespace}: socket ${sessionId} replaced; will not reconnect`);
if (isActiveSocket) this._emit(sessionId, 'offline.replaced', event);
} else {
this._submitMercuryClose4000Metric(sessionId, event, {
action: isActiveSocket ? 'reconnect' : 'ignore_non_active',
isActiveSocket,
messageType: 'other',
});
this.logger.info(
`${this.namespace}: socket ${sessionId} disconnected with 4000: ${event.reason}; reconnecting`
);
if (isActiveSocket) {
this._emit(sessionId, 'offline.transient', event);
this._reconnect(socketUrl, sessionId);
}
}
break;
case 4001:
// replaced during shutdown
Expand Down
122 changes: 122 additions & 0 deletions packages/@webex/internal-plugin-mercury/test/unit/spec/mercury.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1280,6 +1280,128 @@ describe('plugin-mercury', () => {
});
});

describe('#_onclose() with code 4000', () => {
let mockSocket, anotherSocket;

beforeEach(() => {
mockSocket = {
url: 'ws://active-socket.com',
removeAllListeners: sinon.stub(),
};
anotherSocket = {
url: 'ws://old-socket.com',
removeAllListeners: sinon.stub(),
};
mercury.socket = mockSocket;
mercury.sockets.set(mercury.defaultSessionId, mockSocket);
mercury.connected = true;
sinon.stub(mercury, '_emit');
sinon.stub(mercury, '_reconnect');
sinon.stub(mercury, 'unset');
});

afterEach(() => {
mercury._emit.restore();
mercury._reconnect.restore();
mercury.unset.restore();
});

it('should not reconnect when active socket is replaced', () => {
const closeEvent = {
code: 4000,
reason: 'Replaced',
};

mercury._onclose(mercury.defaultSessionId, closeEvent, mockSocket);

assert.calledOnceWithExactly(
webex.internal.metrics.submitClientMetrics,
'JS_SDK_MERCURY_CLOSE_4000',
{
fields: {
action: 'no_action',
close_code: 4000,
close_reason: 'Replaced',
is_active_socket: true,
message_type: 'replaced',
session_id: mercury.defaultSessionId,
},
tags: {
action: 'no_action',
message_type: 'replaced',
},
}
);
assert.calledWith(mercury._emit, mercury.defaultSessionId, 'offline.replaced', closeEvent);
assert.notCalled(mercury._reconnect);
});

it('should reconnect active socket when 4000 has an unknown reason', () => {
const closeEvent = {
code: 4000,
reason: 'Unexpected close',
};

mercury._onclose(mercury.defaultSessionId, closeEvent, mockSocket);

assert.calledOnceWithExactly(
webex.internal.metrics.submitClientMetrics,
'JS_SDK_MERCURY_CLOSE_4000',
{
fields: {
action: 'reconnect',
close_code: 4000,
close_reason: 'Unexpected close',
is_active_socket: true,
message_type: 'other',
session_id: mercury.defaultSessionId,
},
tags: {
action: 'reconnect',
message_type: 'other',
},
}
);
assert.calledWith(mercury._emit, mercury.defaultSessionId, 'offline.transient', closeEvent);
assert.calledWith(
mercury._reconnect,
mockSocket.url,
mercury.defaultSessionId
);
});

it('should not reconnect a non-active socket with an unknown 4000 reason', () => {
const closeEvent = {
code: 4000,
reason: 'Unexpected close',
};

mercury._onclose(mercury.defaultSessionId, closeEvent, anotherSocket);

assert.calledOnceWithExactly(
webex.internal.metrics.submitClientMetrics,
'JS_SDK_MERCURY_CLOSE_4000',
{
fields: {
action: 'ignore_non_active',
close_code: 4000,
close_reason: 'Unexpected close',
is_active_socket: false,
message_type: 'other',
session_id: mercury.defaultSessionId,
},
tags: {
action: 'ignore_non_active',
message_type: 'other',
},
}
);
assert.notCalled(mercury._emit);
assert.notCalled(mercury._reconnect);
assert.isTrue(mercury.connected);
});
});

describe('#_onclose() with code 4001 (shutdown replacement)', () => {
let mockSocket, anotherSocket;

Expand Down
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