Host becomes unresponsive during synchronous subprocess wait instead of timing out cleanly #305740
Description
Type: Performance Issue
Description
When an external subprocess stalls, the host can become unresponsive if the subprocess is waited on synchronously from a UI-thread or event-loop-owned execution path. I believe this is a separate resilience issue from the child-process bug itself.
I created a self-contained reproduction showing the host-side failure mode without depending on the original application code. The reproduction demonstrates that a blocking subprocess.wait() on the event-loop thread starves the host loop, while offloading the same wait preserves responsiveness.
This report is about the host freeze behavior, not the original child-process locking bug.
VS Code Version
Observed in a Windows environment. Exact VS Code build should be filled in from the affected machine when filing.
OS Version
Windows
Steps to Reproduce
- Run the self-contained reproduction in host_freeze_repro.py.
- Observe the
Blocking subprocess.wait() on event-loop threadsection. - Compare it with the
Offloaded subprocess.wait() preserves event loopsection. - Optionally run test_host_freeze_repro.py with pytest.
Commands:
python host_freeze_repro.py
python -m pytest test_host_freeze_repro.py -qExpected Behavior
- A slow or stalled subprocess should not freeze the host.
- UI/event-loop work should continue while the child process is pending.
- Timeout and cancellation should remain serviceable.
- The host should surface an error state rather than becoming unresponsive.
Actual Behavior
- If the host performs a synchronous wait on the same thread that services events, event-loop progress stalls.
- In the repro, heartbeat activity pauses for approximately the same duration as the child-process wait.
- The offloaded version does not show the same starvation pattern.
Does This Issue Occur When All Extensions Are Disabled?
The original symptom involved extension-driven subprocess execution, so the real-world scenario is extension-related. The attached repro is extension-independent and demonstrates the host-side design risk directly.
Reproduction Files
Why I Think This Belongs In VS Code
Even if an extension or child process misbehaves, the host should avoid blocking the execution path responsible for responsiveness. If VS Code or an extension-host layer exposes or encourages a synchronous subprocess wait pattern in a latency-sensitive path, that is a host resilience problem.
Likely Fix Area
Please inspect the process-launch and process-wait path used by the extension host or related integration layers for any of the following on a responsiveness-critical thread:
- synchronous
wait()orrun()calls - blocking process output draining
- sync-over-async bridges around subprocess completion
The likely fix is to move waiting and stream handling off the host thread or use a fully asynchronous subprocess flow.
VS Code version: Code 1.113.0 (cfbea10, 2026-03-24T15:07:18+01:00)
OS version: Windows_NT x64 10.0.26100
Modes:
System Info
| Item | Value |
|---|---|
| CPUs | Intel(R) Core(TM) Ultra 7 165U (14 x 2688) |
| GPU Status | 2d_canvas: enabled GPU0: VENDOR= 0x8086, DEVICE=0x7d45 [Intel(R) Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.8332 ACTIVE GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.7309 Machine model name: Machine model version: direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off |
| Load (avg) | undefined |
| Memory (System) | 15.45GB (2.71GB free) |
| Process Argv | --crash-reporter-id 5635b230-cd5f-4ce9-8a4c-135551ad6c28 |
| Screen Reader | no |
| VM | 0% |
Process Info
CPU % Mem MB PID Process
0 220 21256 code
0 328 3276 gpu-process
0 136 19096 file-watcher [1]
0 159 21136 pty-host
0 107 4420 C:\WINDOWS\System32\WindowsPowerShell\v1.0\powershell.exe -noexit -command "try { . \"c:\Program Files\Microsoft VS Code\cfbea10c5f\resources\app\out\vs\workbench\contrib\terminal\common\scripts\shellIntegration.ps1\" } catch {}"
0 87 9316 C:\WINDOWS\System32\WindowsPowerShell\v1.0\powershell.exe -noexit -command "try { . \"c:\Program Files\Microsoft VS Code\cfbea10c5f\resources\app\out\vs\workbench\contrib\terminal\common\scripts\shellIntegration.ps1\" } catch {}"
0 13 14404 conpty-agent
0 13 17188 conpty-agent
0 142 25496 C:\WINDOWS\System32\WindowsPowerShell\v1.0\powershell.exe -NoProfile -ExecutionPolicy Bypass -Command "Import-Module 'c:\Users\adbudl\.vscode\extensions\ms-vscode.powershell-2025.4.0\modules\PowerShellEditorServices\PowerShellEditorServices.psd1'; Start-EditorServices -HostName 'Visual Studio Code Host' -HostProfileId 'Microsoft.VSCode' -HostVersion '2025.4.0' -BundledModulesPath 'c:\Users\adbudl\.vscode\extensions\ms-vscode.powershell-2025.4.0\modules' -EnableConsoleRepl -StartupBanner \"PowerShell Extension v2025.4.0
Copyright (c) Microsoft Corporation.
https://aka.ms/vscode-powershell
Type 'help' to get help.
\" -LogLevel 'Warning' -LogPath 'c:\Users\adbudl\AppData\Roaming\Code\logs\20260327T122112\window1\exthost\ms-vscode.powershell' -SessionDetailsPath 'c:\Users\adbudl\AppData\Roaming\Code\User\globalStorage\ms-vscode.powershell\sessions\PSES-VSCode-21256-143946.json' -FeatureFlags @() "
0 13 28708 conpty-agent
0 107 22452 utility-network-service
0 396 24824 window [1] (Preview HOST_FREEZE_ISSUE_BODY_COPILOT.md - freeze-issue - Visual Studio Code)
0 133 25168 window
0 164 25456 shared-process
0 610 31712 extension-host [1]
0 115 7984 "C:\Program Files\Microsoft VS Code\Code.exe" "c:\Program Files\Microsoft VS Code\cfbea10c5f\resources\app\extensions\markdown-language-features\dist\serverWorkerMain" --node-ipc --clientProcessId=31712
0 117 17952 "C:\Program Files\Microsoft VS Code\Code.exe" "c:\Program Files\Microsoft VS Code\cfbea10c5f\resources\app\extensions\json-language-features\server\dist\node\jsonServerMain" --node-ipc --clientProcessId=31712
0 15 27956 c:\Users\adbudl\.vscode\extensions\ms-python.vscode-python-envs-1.24.0-win32-x64\python-env-tools\bin\pet.exe server
0 12 24144 C:\WINDOWS\system32\conhost.exe 0x4
0 564 28948 electron-nodejs (bundle.js)
0 99 32164 crashpad-handler
Workspace Info
| Window (Preview HOST_FREEZE_ISSUE_BODY_COPILOT.md - freeze-issue - Visual Studio Code)
| Folder (freeze-issue): 13 files
| File types: md(6) py(3) ps1(2) json(1)
| Conf files:;
Extensions (14)
| Extension | Author (truncated) | Version |
|---|---|---|
| copilot-chat | Git | 0.41.2 |
| vscode-github-actions | git | 0.31.2 |
| rainbow-csv | mec | 3.24.1 |
| debugpy | ms- | 2025.18.0 |
| python | ms- | 2026.4.0 |
| vscode-pylance | ms- | 2026.1.1 |
| vscode-python-envs | ms- | 1.24.0 |
| cmake-tools | ms- | 1.22.28 |
| cpp-devtools | ms- | 0.4.6 |
| cpptools | ms- | 1.31.3 |
| cpptools-extension-pack | ms- | 1.5.1 |
| makefile-tools | ms- | 0.12.17 |
| powershell | ms- | 2025.4.0 |
| vscode-yaml | red | 1.21.0 |
(1 theme extensions excluded)
A/B Experiments
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binariesv615:30325510
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dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
6abeh943:31336334
envsactivate1:31464700
cloudbuttont:31379625
aihoversummaries_f:31469309
3efgi100_wstrepl:31403338
use-responses-api:31390855
defaultusage:31403421
je187915:31401257
ec5jj548:31422691
cp_cls_t_966_ss:31454198
a9239246:31483273
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ge8j1254_inline_auto_hint_haiku:31431912
38bie571_auto:31478677
cp_cls_t_1081:31454832
ia-use-proxy-models-svc:31452481
e9c30283:31461165
test_control1:31471004
g_63ac8346:31467999
idci7584:31464702
showingstats:31481873
534a6447:31478749
hg17d649:31458077
nes-extended-on:31455476
chat:31457767
8hig5102:31480529
67jbj424:31484169
bg_compact_t:31481650
t-some:31484756
cpptoolson-v2:31475363
db0gd219:31481871
ec1eg703:31484515
client_tst_c:31484206
po_split_t:31483953
ddid_c:31478207
nes-perm-reb-0:31481661