perf: dispose mocha runner on run completion to prevent memory leak on rerun

[mschile]

• Closes https://github.com/cypress-io/cypress-services/issues/13184

Additional details

Problem

In cypress open mode, repeatedly rerunning a spec caused memory to grow monotonically and eventually crash the renderer process with V8 process OOM. The leak got worse with each rerun.

Root causes

1. process uncaughtException listener accumulation (every rerun)

At the end of a Mocha run, packages/driver/src/cypress/runner.ts was calling _runner.removeAllListeners(). That only clears listeners on the Runner emitter itself — it does not remove the uncaughtException listener Mocha installs on process during _runner.run() (see mocha/lib/runner.js:1035):

self._addEventListener(process, 'uncaughtException', self.uncaught);

self.uncaught is a bound function whose [[BoundThis]] slot holds the Runner. Because process persists across reruns, the handler list accumulated one entry per rerun — each anchoring an old Runner and everything it retains (suite tree, tests, command queue, snapshots, logs).

2. Cypress instance retention via top error listeners (first rerun only)

setTopOnError in packages/driver/src/cypress/cy.ts runs its full body only once (because top.onerror is configurable: false). The onTopError closure it creates captured the Cypress parameter from that first call. Because the top.addEventListener('error') and top.addEventListener('unhandledrejection') listeners added in that first call can never be removed, Cypress₀ — and all the test data it retained — was permanently anchored in memory.

Additionally, the inline noop get()/set() functions passed to Object.defineProperty(top, 'onerror') shared a V8 closure context with onTopError, creating a second retention path via top.onerror's [[Get]] slot.

Fixes

runner.ts: Call _runner.dispose() instead of _runner.removeAllListeners(). dispose() walks Mocha's internal _eventListeners bookkeeping to remove each externally-installed listener from its target — including the uncaughtException listener on process. Also removed a redundant _runner.removeAllListeners() call from the abort() path.

cy.ts: Mirror the existing curCy pattern by adding a module-level curCypress variable that is updated on every run. Replace the closed-over Cypress parameter in onTopError with curCypress, so the closure no longer permanently captures Cypress₀. Also move the noop onerror getter/setter to module scope so they do not share a V8 closure context with onTopError.

Steps to test

1. Open any memory-intensive project with cypress open.
2. Open a spec and let it run to completion.
3. Click Rerun several times.
4. Observe DevTools Memory — before the fix, heap grew monotonically with each rerun. After the fix it should plateau.

A unit regression test is included at packages/driver/test/unit/cypress/runner.spec.ts that:

• Exercises the real $Runner.create(...).run(...) path.
• Asserts process.listenerCount('uncaughtException') returns to baseline after a run ends.
• Simulates 5 rerun cycles and asserts listener count stays at baseline (no accumulation).

Run with: yarn workspace @packages/driver test test/unit/cypress/runner.spec.ts.

How has the user experience changed?

• cypress open reruns no longer leak memory across rerun cycles.
• Long-running open-mode sessions no longer accumulate heap and crash the renderer.

Before (5 runs):

After (6 runs):

PR Tasks

• Have tests been added/updated?
• [na] Has a PR for user-facing changes been opened in cypress-documentation?
• [na] Have API changes been updated in the type definitions?

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Note

Medium Risk
Touches core runner lifecycle and global error handling; mistakes could cause missed/duplicated error reporting or unexpected runner teardown behavior during open-mode reruns.

Overview
Prevents cypress open from leaking memory across spec reruns by disposing the underlying Mocha Runner on run completion (switching from _runner.removeAllListeners() to _runner.dispose()), ensuring Mocha’s process-level uncaughtException listener is removed and prior runner state can be garbage collected.

Updates setTopOnError in cy.ts to stop closures from permanently capturing the first Cypress instance by using module-level curCypress (and module-scoped noop top.onerror accessors), so top-frame error/unhandledrejection handlers don’t retain old run data. Adds a new unit spec (runner.spec.ts) asserting dispose() is called and process listeners don’t accumulate, and documents the fix in the cli/CHANGELOG.md.

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[cypress]

cypress    Run #70025

Run Properties:   Passed #70025  •  1d83fa3ad7: Update CHANGELOG for version 15.14.1

Project	cypress
Branch Review	claude/jovial-mahavira
Run status	Passed #70025
Run duration	16m 45s
Commit	1d83fa3ad7: Update CHANGELOG for version 15.14.1
Committer	Matt Schile
View all properties for this run ↗︎

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Test results
Failures	0
Flaky	1
Pending	10
Skipped	0
Passing	658
View all changes introduced in this branch ↗︎

UI Coverage  0%
Untested elements	4
Tested elements	0

Accessibility  100%
Failed rules	0 critical   0 serious   0 moderate   0 minor
Failed elements	0

[AtofStryker]

Great work on this! Really encouraging after looking at the heap snapshots.

[cypress-bot]

Released in 15.14.1.

This comment thread has been locked. If you are still experiencing this issue after upgrading to
Cypress v15.14.1, please open a new issue.