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📝 WalkthroughWalkthroughThis PR implements automatic zero-copy screenshot buffers on GNU/Linux for Python 3.12+ by introducing buffer-protocol finalization to manage shared-memory slot lifecycles. The XShmGetImage backend is refactored from a single preallocated buffer to a thread-safe slot pool, with comprehensive tests validating both the core finalization mechanism and backend integration. ChangesZero-Copy Screenshot Buffers
🎯 4 (Complex) | ⏱️ ~60 minutes sequenceDiagram
participant Client as grab()
participant SHM as _acquire_shm_slot
participant XCB as _grab_xshmgetimage
participant BufferMod as finalizing_buffer
participant Pool as _release_shm_slot
Client->>SHM: request slot for monitor
SHM->>SHM: reuse or create slot
Client->>XCB: perform xcb.shm_get_image using slot
XCB->>BufferMod: wrap sliced memoryview with finalizer
BufferMod-->>Client: return memoryview
BufferMod->>Pool: call finalizer on release
Pool->>Pool: return slot to free-list or destroy
🚥 Pre-merge checks | ✅ 3 | ❌ 2❌ Failed checks (1 warning, 1 inconclusive)
✅ Passed checks (3 passed)
✏️ Tip: You can configure your own custom pre-merge checks in the settings. ✨ Finishing Touches🧪 Generate unit tests (beta)
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🧹 Nitpick comments (2)
src/tests/test_gnu_linux.py (2)
326-340: ⚡ Quick winMissing
displayfixture may cause test flakiness.This test uses
mss.MSS(backend="xshmgetimage")without explicitly providing a display, relying on theDISPLAYenvironment variable. Other SHM-related tests liketest_shm_availableexplicitly create apyvirtualdisplay.Display. This could cause the test to fail in CI environments without an X server, or behave inconsistently across different test runners.Consider using the
displayfixture or inlinepyvirtualdisplay.Displayfor consistency:-def test_finalizing_buffer_releases_shm_slot(monkeypatch: pytest.MonkeyPatch) -> None: +def test_finalizing_buffer_releases_shm_slot(display: str, monkeypatch: pytest.MonkeyPatch) -> None: """Verify that the returned buffer releases its SHM slot when finalized.""" - with mss.MSS(backend="xshmgetimage") as sct: + with mss.MSS(display=display, backend="xshmgetimage") as sct:🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the rest with a brief reason, keep changes minimal, and validate. In `@src/tests/test_gnu_linux.py` around lines 326 - 340, The test test_finalizing_buffer_releases_shm_slot relies on an external DISPLAY and can be flaky; ensure an X display is available by using the existing display fixture or creating a pyvirtualdisplay.Display() within the test before instantiating mss.MSS(backend="xshmgetimage") so that mss.MSS and its MSSImplXShmGetImage implementation always have a valid X server; wrap the mss.MSS(...) usage in the display context or add the display fixture to the test signature to make the environment consistent in CI.
343-359: ⚡ Quick winSame display fixture concern applies here.
This test also lacks an explicit display fixture, which could cause inconsistent behavior.
🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the rest with a brief reason, keep changes minimal, and validate. In `@src/tests/test_gnu_linux.py` around lines 343 - 359, The test test_exception_while_wrapping_finalizing_buffer_releases_shm_slot relies on the display environment but doesn't declare the display fixture; update the test to accept and use the display fixture (so the X server/display is explicitly provided) before creating mss.MSS and invoking sct.grab, ensuring consistent behavior across runs; keep the existing monkeypatch usage (spy_and_patch, sct._impl, and mss.linux.xshmgetimage.finalizing_buffer) otherwise unchanged.
🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
Nitpick comments:
In `@src/tests/test_gnu_linux.py`:
- Around line 326-340: The test test_finalizing_buffer_releases_shm_slot relies
on an external DISPLAY and can be flaky; ensure an X display is available by
using the existing display fixture or creating a pyvirtualdisplay.Display()
within the test before instantiating mss.MSS(backend="xshmgetimage") so that
mss.MSS and its MSSImplXShmGetImage implementation always have a valid X server;
wrap the mss.MSS(...) usage in the display context or add the display fixture to
the test signature to make the environment consistent in CI.
- Around line 343-359: The test
test_exception_while_wrapping_finalizing_buffer_releases_shm_slot relies on the
display environment but doesn't declare the display fixture; update the test to
accept and use the display fixture (so the X server/display is explicitly
provided) before creating mss.MSS and invoking sct.grab, ensuring consistent
behavior across runs; keep the existing monkeypatch usage (spy_and_patch,
sct._impl, and mss.linux.xshmgetimage.finalizing_buffer) otherwise unchanged.
ℹ️ Review info
⚙️ Run configuration
Configuration used: defaults
Review profile: CHILL
Plan: Pro
Run ID: 9d036e5d-8ca9-4f19-ad0b-b058d0ecde57
📒 Files selected for processing (10)
docs/source/release-history/v11.0.0.mddocs/source/usage.rstpyproject.tomlsrc/mss/base.pysrc/mss/buffer.pysrc/mss/linux/base.pysrc/mss/linux/xshmgetimage.pysrc/tests/test_buffer.pysrc/tests/test_gnu_linux.pysrc/tests/test_setup.py
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Pull request overview
This PR introduces infrastructure for zero-copy screenshot buffers on GNU/Linux (Python 3.12+), using Python 3.12’s buffer finalization hooks to safely recycle/cleanup shared-memory backed screenshot buffers. It also updates docs, packaging expectations, and adds tests around buffer lifetime and SHM slot lifecycle.
Changes:
- Add
mss.bufferwithfinalizing_buffer()to run finalizers when downstream buffer consumers release exported views (3.12+), with a copy-based fallback for older Python versions. - Refactor the X11
xshmgetimagebackend to manage a pool of SHM slots and return finalizing buffers (zero-copy where possible). - Add/expand tests and documentation; adjust test dependencies to install NumPy/Pillow on Python 3.12+.
Reviewed changes
Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| src/mss/buffer.py | New finalizing-buffer helper to support zero-copy buffers with correct lifetime finalization. |
| src/mss/linux/xshmgetimage.py | Introduces pooled SHM slots and returns finalizing buffers; adds close/cleanup logic. |
| src/mss/base.py | Widens backend grab() return type to Buffer to support memoryview-based paths. |
| src/mss/linux/base.py | Adds commentary about potential future zero-copy work for XGetImage path. |
| src/tests/test_buffer.py | New unit tests validating finalizer semantics across Python versions. |
| src/tests/test_gnu_linux.py | New/updated integration tests for SHM slot lifecycle and threaded release behavior. |
| src/tests/test_setup.py | Ensures new module/test files are included in sdist/wheel expectations. |
| pyproject.toml | Installs NumPy/Pillow for tests on Python 3.12+ (not just Linux/3.13). |
| docs/source/usage.rst | Documents automatic direct screenshot buffers and platform requirements. |
| docs/source/release-history/v11.0.0.md | Adds release notes entry describing the zero-copy feature and benchmark. |
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Summary ======= I’ve spoken at length about the importance of avoiding copies. This PR is to eliminate the remaining (CPU-side) copy: copying from the OS-supplied buffer to a Python byte / bytearray object. We introduce buffer lifetime tracking in MSS so backends can safely reclaim or reuse screenshot buffers when downstream consumers are truly done with them. On GNU/Linux, the XShmGetImage backend now uses that mechanism to enable zero-copy screenshot buffers on Python 3.12 and newer. Benchmark --------- I ran a benchmark on my home computer (Ryzen 7 2700X, B-450 chipset, DDR4-2133, RTX 3090). I captured 1000 iterations of 3840×2160 screenshots as quickly as possible while forcing all pixel data to be read (using a NumPy sum). I ran A/B tests of enabling or disabling the new feature, taking a best-of-three test. Capture time decreased from 22.64 ms to 18.59 ms per frame. Put differently, this is from 44 FPS to 54 FPS. This is approximately 18% faster. Why === Previously, backends had to copy screenshot data into fresh Python-owned buffers to avoid reusing memory that might still be referenced by NumPy, Pillow, or other buffer consumers. That copy cost is significant for large captures and high frame-rate use cases. What changed ============ Internal infrastructure ----------------------- This change keeps MSS user-facing behavior the same while improving backend memory handling and performance. - Added new buffer-finalization plumbing that lets backends attach a finalizer to screenshot buffer ownership. - Updated core typing/contracts so grab can return generic buffer-compatible objects. (The user-facing contracts were updated in BoboTiG#521 and others, but this updates the internal contracts.) - Expanded documentation and release notes to explain direct buffer behavior and platform/version scope. - Updated packaging tests and test dependencies to include new buffer and integration test coverage. XShmGetImage backend -------------------- - Reworked the GNU/Linux XShmGetImage backend to use a reusable SHM slot pool with dynamic growth and finalizer-driven slot return. - Added shutdown/cleanup safeguards so slot destruction and connection shutdown are coordinated safely, including finalizer interactions. - Kept fallback behavior intact when MIT-SHM is unavailable or unsuitable. Behavior by runtime =================== - Python 3.12+ on GNU/Linux XShm backend: - zero-copy buffer exposure from SHM-backed storage - SHM slot is released when downstream buffer users release it - Pre-3.12: - copy-based behavior is retained - finalization happens immediately after copy Testing ======= - Added focused unit tests for buffer-finalizer semantics, including fast and slow paths and downstream memoryview trees. - Added GNU/Linux backend lifecycle tests covering: - release on normal finalization - failure while wrapping finalizing buffers - finalization after close - pre-3.12 immediate-finalization behavior - dynamic SHM pool growth failure behavior - threaded release during close to validate shutdown race protections Notes for maintainers ===================== - This PR is intentionally backend-agnostic at the plumbing layer, with initial zero-copy adoption in GNU/Linux XShmGetImage. - The design keeps the existing user-facing API, while making buffer lifetime explicit for backend resource management.
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