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platform: use FILE_FLAG_WRITE_THROUGH on Windows for SyncFile data durability, fixes #9388#9389

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mr-raj12:win32-syncfile-write-through
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platform: use FILE_FLAG_WRITE_THROUGH on Windows for SyncFile data durability, fixes #9388#9389
mr-raj12 wants to merge 1 commit intoborgbackup:masterfrom
mr-raj12:win32-syncfile-write-through

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@mr-raj12 mr-raj12 commented Feb 21, 2026

Description

Implement Windows-native SyncFile using CreateFileW with FILE_FLAG_WRITE_THROUGH to bypass volatile write caches and write directly to persistent storage. This is the Windows equivalent of the macOS F_FULLFSYNC fix (#9383 / PR #9385).

Fixes #9388

Changes

  • src/borg/platform/windows.pyx — add SyncFile subclass using CreateFileW + FILE_FLAG_WRITE_THROUGH; falls back to base when an existing fd is provided (e.g. SaveFile)
  • src/borg/platform/__init__.py — import SyncFile from .windows instead of .base
  • src/borg/platform/base.py — update resolved TODO comment
  • src/borg/testsuite/platform/platform_test.py — add skipif_not_win32 marker
  • src/borg/testsuite/platform/windows_test.py — 6 tests (write/read, FileExistsError, text mode, fd fallback, sync, write-through flag verification)

Checklist

  • PR is against master (or maintenance branch if only applicable there)
  • New code has tests and docs where appropriate
  • Tests pass (run tox or the relevant test subset)
  • Commit messages are clean and reference related issues

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codecov bot commented Feb 21, 2026 •
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❌ 1 Tests Failed:

Tests completed Failed Passed Skipped
1681 1 1680 346
View the top 1 failed test(s) by shortest run time 
::src.borg.testsuite.platform.windows_test
Stack Traces | 0s run time 
ImportError while importing test module '.../testsuite/platform/windows_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
.../hostedtoolcache/Python/3.14.3.../x64/lib/python3.14/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.../testsuite/platform/windows_test.py:7: in <module>
    from ...platform import windows
E   ImportError: cannot import name 'windows' from 'borg.platform' (.../borg/platform/__init__.py)

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ThomasWaldmann
ThomasWaldmann requested changes Feb 22, 2026
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some minor things i found

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ThomasWaldmann commented Feb 22, 2026

Also:

Please update your local master branch and then rebase this branch onto that master and force push.

That should solve the CI test failures that are unrelated to your work.

@mr-raj12 mr-raj12 force-pushed the win32-syncfile-write-through branch from 59dcf0e to 5df3cfb Compare February 28, 2026 00:03
@mr-raj12 mr-raj12 mentioned this pull request Feb 28, 2026
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ThomasWaldmann
ThomasWaldmann reviewed Feb 28, 2026
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one idea about reducing these in-function imports.

i am not familiar with the windows api, did you check yourself that this win32 api usage is how it should be?

@ThomasWaldmann ThomasWaldmann added this to the 2.0.0b21 milestone Feb 28, 2026
@mr-raj12 mr-raj12 force-pushed the win32-syncfile-write-through branch from 5df3cfb to fcbd613 Compare March 1, 2026 10:24
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mr-raj12 commented Mar 1, 2026

one idea about reducing these in-function imports.

i am not familiar with the windows api, did you check yourself that this win32 api usage is how it should be?

Yes cross checked everything against the CreateFileW and _open_osfhandle docs

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ThomasWaldmann commented Mar 1, 2026

from ...platform import windows

SyncFile

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platform: implement Windows-native SyncFile using FILE_FLAG_WRITE_THROUGH

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