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[PR #15894] Add missing lock guards on Terminal access #30773

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opened 2026-01-31 09:42:51 +00:00 by claunia · 0 comments
claunia commented 2026-01-31 09:42:51 +00:00
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Original Pull Request: https://github.com/microsoft/terminal/pull/15894

State: closed
Merged: Yes

Terminal is used concurrently by at least 4 threads. The table
below lists the class members and the threads that access them
to the best of my knowledge. Where:

  • UI: UI Thread
  • BG: Background worker threads (winrt::resume_background)
  • RD: Render thread
  • VT: VT connection thread
UI BG RD VT
_pfnWriteInput x x x
_pfnWarningBell x
_pfnTitleChanged x
_pfnCopyToClipboard x
_pfnScrollPositionChanged x x x
_pfnCursorPositionChanged x
_pfnTaskbarProgressChanged x
_pfnShowWindowChanged x
_pfnPlayMidiNote x
_pfnCompletionsChanged x
_renderSettings x x x
_stateMachine x x
_terminalInput x x
_title x x x
_startingTitle x x
_startingTabColor x
_defaultCursorShape x x
_systemMode x x x
_snapOnInput x x
_altGrAliasing x
_suppressApplicationTitle x x
_trimBlockSelection x
_autoMarkPrompts x
_taskbarState x x
_taskbarProgress x x
_workingDirectory x x
_fontInfo x x
_selection x x x x
_blockSelection x x x
_wordDelimiters x x
_multiClickSelectionMode x x x
_selectionMode x x x
_selectionIsTargetingUrl x x x
_selectionEndpoint x x x
_anchorInactiveSelectionEndpoint x x x
_mainBuffer x x x x
_altBuffer x x x x
_mutableViewport x x x
_scrollbackLines x
_detectURLs x
_altBufferSize x x x x
_deferredResize x x
_scrollOffset x x x x
_patternIntervalTree x x x x
_lastKeyEventCodes x
_currentPromptState x x

Only 7 members are specific to one thread and don't require locking.
All other members require some for of locking to be safe for use.

To address the issue this changeset adds LockForReading/LockForWriting
calls everywhere _terminal is accessed in ControlCore/HwndTerminal.
Additionally, to ensure these issues don't pop up anymore, it adds to
all Terminal functions a debug assertion that the lock is being held.

Finally, because this changeset started off rather modest, it contains
changes that I initially made without being aware about the extent of
the issue. It simplifies the access around _patternIntervalTree by
making _InvalidatePatternTree() directly use that member.
Furthermore, it simplifies _terminal->SetCursorOn(!IsCursorOn()) to
BlinkCursor(), allowing the code to be shared with HwndTerminal.

Ideally Terminal should not be that much of a class so that we don't
need such coarse locking. Splitting out selection and rendering state
should allow deduplicating code with conhost and use finer locking.

Closes #9617

Validation Steps Performed

I tried to use as many Windows Terminal features as I could and fixed
every occurrence of _assertLocked() failures.

**Original Pull Request:** https://github.com/microsoft/terminal/pull/15894 **State:** closed **Merged:** Yes --- `Terminal` is used concurrently by at least 4 threads. The table below lists the class members and the threads that access them to the best of my knowledge. Where: * UI: UI Thread * BG: Background worker threads (`winrt::resume_background`) * RD: Render thread * VT: VT connection thread | | UI | BG | RD | VT | |------------------------------------|----|----|----|----| | `_pfnWriteInput` | x | x | | x | | `_pfnWarningBell` | | | | x | | `_pfnTitleChanged` | | | | x | | `_pfnCopyToClipboard` | | | | x | | `_pfnScrollPositionChanged` | x | x | | x | | `_pfnCursorPositionChanged` | | | | x | | `_pfnTaskbarProgressChanged` | | | | x | | `_pfnShowWindowChanged` | | | | x | | `_pfnPlayMidiNote` | | | | x | | `_pfnCompletionsChanged` | | | | x | | `_renderSettings` | x | | x | x | | `_stateMachine` | x | | | x | | `_terminalInput` | x | | | x | | `_title` | x | | x | x | | `_startingTitle` | x | | x | | | `_startingTabColor` | x | | | | | `_defaultCursorShape` | x | | | x | | `_systemMode` | | x | x | x | | `_snapOnInput` | x | x | | | | `_altGrAliasing` | x | | | | | `_suppressApplicationTitle` | x | | | x | | `_trimBlockSelection` | x | | | | | `_autoMarkPrompts` | x | | | | | `_taskbarState` | x | | | x | | `_taskbarProgress` | x | | | x | | `_workingDirectory` | x | | | x | | `_fontInfo` | x | | x | | | `_selection` | x | x | x | x | | `_blockSelection` | x | x | x | | | `_wordDelimiters` | x | x | | | | `_multiClickSelectionMode` | x | x | x | | | `_selectionMode` | x | x | x | | | `_selectionIsTargetingUrl` | x | x | x | | | `_selectionEndpoint` | x | x | x | | | `_anchorInactiveSelectionEndpoint` | x | x | x | | | `_mainBuffer` | x | x | x | x | | `_altBuffer` | x | x | x | x | | `_mutableViewport` | x | | x | x | | `_scrollbackLines` | x | | | | | `_detectURLs` | x | | | | | `_altBufferSize` | x | x | x | x | | `_deferredResize` | x | | | x | | `_scrollOffset` | x | x | x | x | | `_patternIntervalTree` | x | x | x | x | | `_lastKeyEventCodes` | x | | | | | `_currentPromptState` | x | | | x | Only 7 members are specific to one thread and don't require locking. All other members require some for of locking to be safe for use. To address the issue this changeset adds `LockForReading/LockForWriting` calls everywhere `_terminal` is accessed in `ControlCore/HwndTerminal`. Additionally, to ensure these issues don't pop up anymore, it adds to all `Terminal` functions a debug assertion that the lock is being held. Finally, because this changeset started off rather modest, it contains changes that I initially made without being aware about the extent of the issue. It simplifies the access around `_patternIntervalTree` by making `_InvalidatePatternTree()` directly use that member. Furthermore, it simplifies `_terminal->SetCursorOn(!IsCursorOn())` to `BlinkCursor()`, allowing the code to be shared with `HwndTerminal`. Ideally `Terminal` should not be that much of a class so that we don't need such coarse locking. Splitting out selection and rendering state should allow deduplicating code with conhost and use finer locking. Closes #9617 ## Validation Steps Performed I tried to use as many Windows Terminal features as I could and fixed every occurrence of `_assertLocked()` failures.
claunia added the pull-request label 2026-01-31 09:42:51 +00:00
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