fix: memory leak in terminal process #279172

SimonSiefke · 2025-11-24T14:36:29Z

Sets the this._ptyProcess to undefined on dispose so that ptyProcess can be garbage collected on dispose.
Sets the this._processStartupComplete to undefined on dispose so that the promise can be garbage collected

Before

When running a task 17 times, the number of various objects and functions grows each time:

{
  "namedFunctionCount3": [
    {
      "count": 40,
      "delta": 17,
      "name": "EventEmitter",
      "sourceLocation": "node:events:219:21"
    },
    {
      "count": 39,
      "delta": 34,
      "name": "EventEmitter2",
      "sourceLocation": "vscode/node_modules/node-pty/lib/eventEmitter2.js:7:26",
      "originalName": null
    },
    {
      "count": 38,
      "delta": 34,
      "name": "anonymous",
      "sourceLocation": "vscode/node_modules/node-pty/lib/eventEmitter2.js:14:39",
      "originalName": null
    },
    {
      "count": 32,
      "delta": 17,
      "name": "Buffer",
      "sourceLocation": "node:buffer:270:15"
    },
    {
      "count": 24,
      "delta": 17,
      "name": "ReadableState",
      "sourceLocation": "node:internal/streams/readable:261:22"
    },
    {
      "count": 22,
      "delta": 17,
      "name": "WritableState",
      "sourceLocation": "node:internal/streams/writable:303:22"
    },
    {
      "count": 20,
      "delta": 17,
      "name": "ChildProcessMonitor",
      "sourceLocation": "vscode/out/vs/platform/terminal/node/childProcessMonitor.js:24:13",
      "originalLocation": "src/vs/platform/terminal/node/childProcessMonitor.ts:52:19",
      "originalName": "ChildProcessMonitor"
    },
    {
      "count": 20,
      "delta": 17,
      "name": "StringDecoder",
      "sourceLocation": "node:string_decoder:80:22"
    },
    {
      "count": 20,
      "delta": 17,
      "name": "UnixTerminal",
      "sourceLocation": "vscode/node_modules/node-pty/lib/unixTerminal.js:45:25",
      "originalName": null
    },
    {
      "count": 20,
      "delta": 17,
      "name": "ReadStream",
      "sourceLocation": "node:tty:50:19"
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "vscode/node_modules/node-pty/lib/terminal.js:170:30",
      "originalName": null
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "vscode/node_modules/node-pty/lib/terminal.js:171:28",
      "originalName": null
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "vscode/node_modules/node-pty/lib/unixTerminal.js:140:43",
      "originalName": null
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "vscode/node_modules/node-pty/lib/unixTerminal.js:105:43",
      "originalName": null
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "vscode/node_modules/node-pty/lib/terminal.js:91:33",
      "originalName": null
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "vscode/node_modules/node-pty/lib/terminal.js:92:33",
      "originalName": null
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "vscode/out/vs/base/common/decorators.js:59:34",
      "originalLocation": "src/vs/base/common/decorators.ts:83:31",
      "originalName": "debounce"
    }
  ],
  "isLeak": true
}

After

When running a task 17 times, there are much less leaked functions and objects:

{
  "namedFunctionCount3": [
    {
      "count": 20,
      "delta": 17,
      "name": "ChildProcessMonitor",
      "sourceLocation": "file:///home/simon/.cache/repos/vscode/out/vs/platform/terminal/node/childProcessMonitor.js:24:13",
      "originalLocation": "src/vs/platform/terminal/node/childProcessMonitor.ts:52:19",
      "originalName": "ChildProcessMonitor"
    },
    {
      "count": 19,
      "delta": 17,
      "name": "anonymous",
      "sourceLocation": "file:///home/simon/.cache/repos/vscode/out/vs/base/common/decorators.js:59:34",
      "originalLocation": "src/vs/base/common/decorators.ts:83:31",
      "originalName": "debounce"
    }
  ],
  "isLeak": true
}

vs-code-engineering · 2025-11-24T14:36:55Z

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@Tyriar

Matched files:

src/vs/platform/terminal/node/terminalProcess.ts

Tyriar

@SimonSiefke do you know why this is necessary? Disposing of a TerminalProcess is mean to release all handles to it and therefore free it and it's own references up for GC?

SimonSiefke · 2025-12-08T21:37:39Z

Hi @Tyriar ,
I don't think it's necessary in general, but rather it can helpful sometimes to set properties to undefined on dispose to reduce the amount of leaked data.

I believe the whole TerminalProcess instance is leaking to due some kind of issue with debounce.

By setting this_ptyProcess in TerminalProcess to undefined, it ensures this._ptyProcess can be garbage collected even though TerminalProcess instances might still be leaking.

But you are definitly right: When there would be no other leaks, this.ptyProcess would be garbage collected automatically also.

About the promise, I'm not entirely sure. I think it's best to always set all promise properties to undefined on dispose.

anthonykim1 · 2025-12-09T00:09:41Z

@SimonSiefke Do you know how I can repro this memory leak?
If its disposable leak I thought they showed up in console of dev tool.

I've tried creating and deleting terminal but that alone did not seem to show the leak on dev tool console. Are you watching the processes in some specific tool that shows the leak

SimonSiefke · 2025-12-09T08:48:25Z

Hey @anthonykim1,

I have this repo that I'm using: https://github.com/SimonSiefke/vscode-memory-leak-finder/

Prerequisites: Ensure you have node 24 or later installed.

Test script:

git clone [email protected]:SimonSiefke/vscode-memory-leak-finder.git &&
cd vscode-memory-leak-finder &&
npm ci &&
node packages/cli/bin/test.js --cwd packages/e2e   --only terminal.create --runs 97 --check-leaks --measure named-function-count3 --run-skipped-tests-anyway --measure-after --inspect-ptyhost && 
cat .vscode-memory-leak-finder-results/pty-host/named-function-count3/terminal.create.json

With local vscode version (adjust `--vscode-path` to your local vscode path):

git clone [email protected]:SimonSiefke/vscode-memory-leak-finder.git &&
cd vscode-memory-leak-finder &&
npm ci &&
node packages/cli/bin/test.js --cwd packages/e2e   --only terminal.create --runs 97 --check-leaks --measure named-function-count3 --run-skipped-tests-anyway --measure-after --inspect-ptyhost --vscode-path "/home/simon/.cache/repos/vscode/scripts/code.sh"  &&
cat .vscode-memory-leak-finder-results/pty-host/named-function-count3/terminal.create.json

The script launches vscode, takes a heapsnapshot of pty-host, runs an e2e tests which creates and closes a terminal 97 times and takes a heapsnapshot of pty-host again. It then compares the heapsnapshot location arrays to see how many of each function have been added between the heapsnapshots. That's how it generates this data:

{
  "namedFunctionCount3": [
    {
      "count": 233,
      "delta": 218,
      "name": "Timeout",
      "sourceLocation": "node:internal/timers:174:13"
    },
    {
      "count": 201,
      "delta": 194,
      "name": "EventEmitter2",
      "sourceLocation": "resources/app/node_modules/node-pty/lib/eventEmitter2.js:7:26",
      "originalLocation": "src/eventEmitter2.ts:15:0",
      "originalName": ""
    },
    {
      "count": 200,
      "delta": 194,
      "name": "anonymous",
      "sourceLocation": "resources/app/node_modules/node-pty/lib/eventEmitter2.js:14:39",
      "originalLocation": "src/eventEmitter2.ts:21:21",
      "originalName": ""
    },
    {
      "count": 121,
      "delta": 97,
      "name": "EventEmitter",
      "sourceLocation": "node:events:219:21"
    },
    {
      "count": 113,
      "delta": 97,
      "name": "Buffer",
      "sourceLocation": "node:buffer:270:15"
    },
    {
      "count": 105,
      "delta": 97,
      "name": "ReadableState",
      "sourceLocation": "node:internal/streams/readable:261:22"
    },
    {
      "count": 103,
      "delta": 97,
      "name": "WritableState",
      "sourceLocation": "node:internal/streams/writable:303:22"
    },
    {
      "count": 101,
      "delta": 97,
      "name": "UnixTerminal",
      "sourceLocation": "resources/app/node_modules/node-pty/lib/unixTerminal.js:45:25",
      "originalLocation": "src/unixTerminal.ts:56:14",
      "originalName": ""
    },
    {
      "count": 101,
      "delta": 97,
      "name": "ReadStream",
      "sourceLocation": "node:tty:50:19"
    },
    {
      "count": 101,
      "delta": 97,
      "name": "Ge",
      "sourceLocation": "resources/app/out/vs/platform/terminal/node/ptyHostMain.js:32:497",
      "originalLocation": "src/vs/platform/terminal/node/childProcessMonitor.ts:52:19",
      "originalName": "ChildProcessMonitor"
    },
    {
      "count": 101,
      "delta": 97,
      "name": "StringDecoder",
      "sourceLocation": "node:string_decoder:80:22"
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/out/vs/platform/terminal/node/ptyHostMain.js:25:79452",
      "originalLocation": "src/vs/base/common/decorators.ts:121:32",
      "originalName": "throttle"
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/out/vs/platform/terminal/node/ptyHostMain.js:32:8059",
      "originalLocation": "src/vs/platform/terminal/node/terminalProcess.ts:337:20",
      "originalName": "TerminalProcess.setupPtyProcess"
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/out/vs/platform/terminal/node/ptyHostMain.js:32:7812",
      "originalLocation": "src/vs/platform/terminal/node/terminalProcess.ts:319:20",
      "originalName": "TerminalProcess.setupPtyProcess"
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/node_modules/node-pty/lib/terminal.js:92:33",
      "originalLocation": "src/terminal.ts:97:21",
      "originalName": ""
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/node_modules/node-pty/lib/terminal.js:91:33",
      "originalLocation": "src/terminal.ts:96:20",
      "originalName": ""
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/node_modules/node-pty/lib/unixTerminal.js:140:43",
      "originalLocation": "src/unixTerminal.ts:165:29",
      "originalName": ""
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/node_modules/node-pty/lib/unixTerminal.js:105:43",
      "originalLocation": "src/unixTerminal.ts:123:30",
      "originalName": ""
    },
    {
      "count": 100,
      "delta": 97,
      "name": "anonymous",
      "sourceLocation": "resources/app/out/vs/platform/terminal/node/ptyHostMain.js:25:78961",
      "originalLocation": "src/vs/base/common/decorators.ts:83:31",
      "originalName": "debounce"
    }
  ],
  "isLeak": true
}

SVG

With npm run build-charts it one can see the json visualized as svg.

The locations that have stayed the same are black, the locations that have been added / leaked are red.

SimonSiefke added 3 commits November 24, 2025 15:08

fix: memory leak in terminal process

4f3d624

clean up promise

8138a81

Merge branch 'main' into fix/memory-leak-terminal-process-3

9520b2e

vs-code-engineering bot assigned anthonykim1 Nov 24, 2025

SimonSiefke mentioned this pull request Nov 24, 2025

Possible memory leak in debounce #279177

Open

Merge branch 'main' into fix/memory-leak-terminal-process-3

7deb5e1

Tyriar mentioned this pull request Dec 8, 2025

Promise memory leak when creating terminal (pty-host) #276958

Closed

Tyriar self-assigned this Dec 8, 2025

Merge branch 'main' into fix/memory-leak-terminal-process-3

ff0ce2c

Tyriar reviewed Dec 8, 2025

View reviewed changes

Tyriar added this to the December / January 2026 milestone Dec 8, 2025

Don't override dispose

4cf8c56

Tyriar approved these changes Dec 9, 2025

View reviewed changes

Tyriar enabled auto-merge December 9, 2025 14:08

sandy081 approved these changes Dec 9, 2025

View reviewed changes

Tyriar merged commit dcb7026 into microsoft:main Dec 9, 2025
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fix: memory leak in terminal process #279172

fix: memory leak in terminal process #279172

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fix: memory leak in terminal process #279172

fix: memory leak in terminal process #279172

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SimonSiefke commented Nov 24, 2025

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SimonSiefke commented Dec 9, 2025

Test script:

With local vscode version (adjust --vscode-path to your local vscode path):

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With local vscode version (adjust `--vscode-path` to your local vscode path):