[native] Skip F_FULLFSYNC on macOS for network file systems by Copilot · Pull Request #130779 · dotnet/runtime

Copilot · 2026-07-15T13:16:32Z

On macOS, F_FULLFSYNC on network file systems (NFS, SMB, CIFS, SMB2) can return success while silently discarding pending writes, causing data corruption. The existing fsync fallback only triggers on failure, so the data loss goes undetected.

Changes in src/native/libs/System.Native/pal_io.c:

Added IsNetworkFileSystem(int fileDescriptor) — macOS-only static helper that calls fstatfs and reuses the existing FileSystemNameSupportsLocking logic to identify NFS/CIFS/SMB/SMB2 mounts. Conservatively returns 1 (is network FS) when fstatfs fails, to skip F_FULLFSYNC and avoid data loss.
Modified SystemNative_FSync to skip F_FULLFSYNC entirely when IsNetworkFileSystem returns true, going directly to fsync. Behavior on local file systems is unchanged.

This mirrors the approach already used by CanLockTheFile / SystemNative_FileSystemSupportsLocking, which skip advisory locking on the same set of network file systems for the same category of reasons.

Don't use F_FULLFSYNC on macOS for network file systems (NFS, SMB, CIFS, SMB2) as it may cause pending writes to be discarded. Use the same detection logic already used by the locking code (fstatfs + FileSystemNameSupportsLocking). Fixes #124722 Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

On fstatfs failure, return 1 (treat as network FS) to safely skip F_FULLFSYNC and avoid potential data loss. Also improve comment clarity on return value logic. Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

azure-pipelines · 2026-07-15T13:17:19Z

Azure Pipelines: Successfully started running 3 pipeline(s). 12 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

adamsitnik

@copilot please address my feedback

dotnet-policy-service · 2026-07-15T13:29:51Z

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

…pal_io.c Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

adamsitnik

@copilot address my feedback

…ckTheFile in managed code Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

…ix WASM FSync signature Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

azure-pipelines · 2026-07-15T16:37:13Z

Azure Pipelines: Successfully started running 3 pipeline(s). 12 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

Copilot

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

adamsitnik

@copilot address feedback

…avior Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

adamsitnik

LGTM

adamsitnik · 2026-07-16T12:48:27Z

@jozkee the PR is ready for review, PTAL. #124722 is the context

jozkee · 2026-07-18T00:02:04Z

+            get
+            {
+                NullableBool useFullFsync = _useFullFsync;
+                if (useFullFsync == NullableBool.Undefined && !IsClosed && OperatingSystem.IsApplePlatform())


SystemNative_FSync uses TARGET_OSX only.

Suggested change

if (useFullFsync == NullableBool.Undefined && !IsClosed && OperatingSystem.IsApplePlatform())

if (useFullFsync == NullableBool.Undefined && !IsClosed && OperatingSystem.IsMacOS())

jozkee · 2026-07-18T00:07:51Z

+                NullableBool useFullFsync = _useFullFsync;
+                if (useFullFsync == NullableBool.Undefined && !IsClosed && OperatingSystem.IsApplePlatform())
+                {
+                    _useFullFsync = useFullFsync = Interop.Sys.FileSystemSupportsLocking(this, Interop.Sys.LockOperations.LOCK_SH, accessWrite: true)


The PR reuses SystemNative_FileSystemSupportsLocking as a proxy for "is this a network FS," but that function's contract is about locking and its result depends on lockOperation/accessWrite, so it can't simply be renamed to IsNetworkFileSystem. Consider extracting the shared network-FS detection (the FileSystemNameSupportsLocking nfs/cifs/smb/smb2 check) into a dedicated helper.

jozkee · 2026-07-18T00:15:52Z

I tried to reproduce #124722 on a macOS client (macOS 26.5, arm64) connecting to a Samba server running in WSL2, but was not able to trigger the truncation.

What I tried:

The exact repro from the issue (16 × 64 MB, FileShare.None + Flush(flushToDisk: true) ), plus larger sizes (256 MB × 16 and 1 GB × 4) and multiple runs.
Sequential, async, and fully-parallel (concurrent) writes.
Verifying final file sizes three ways to rule out the macOS SMB client's metadata cache: post-close stat , a full byte-for-byte read-back from a separate process, and a force-unmount/remount followed by a fresh stat .

In every case all files came out at their exact expected size — 0 truncated.

adamsitnik · 2026-07-18T08:53:37Z

I tried to reproduce #124722 on a macOS client (macOS 26.5, arm64) connecting to a Samba server running in WSL2, but was not able to trigger the truncation.

What I tried:

The exact repro from the issue (16 × 64 MB, FileShare.None + Flush(flushToDisk: true) ), plus larger sizes (256 MB × 16 and 1 GB × 4) and multiple runs.

Sequential, async, and fully-parallel (concurrent) writes.

Verifying final file sizes three ways to rule out the macOS SMB client's metadata cache: post-close stat , a full byte-for-byte read-back from a separate process, and a force-unmount/remount followed by a fresh stat .

In every case all files came out at their exact expected size — 0 truncated.

@sschultze is there something more specific required to reproduce the problem (#124722)?

Copilot AI and others added 2 commits July 15, 2026 13:14

Copilot AI self-assigned this Jul 15, 2026

Copilot AI review requested due to automatic review settings July 15, 2026 13:16

Copilot AI assigned adamsitnik Jul 15, 2026

Copilot AI removed the request for review from Copilot July 15, 2026 13:16

Copilot created this pull request from a session on behalf of adamsitnik July 15, 2026 13:17 View session

Copilot finished work on behalf of adamsitnik July 15, 2026 13:17

Copilot AI requested a review from adamsitnik July 15, 2026 13:17

adamsitnik requested changes Jul 15, 2026

View reviewed changes

Comment thread src/native/libs/System.Native/pal_io.c Outdated

adamsitnik linked an issue Jul 15, 2026 that may be closed by this pull request

FileStream.Flush(flushToDisk: true) causes corrupt (truncated) files to be written on macOS + SMB share #124722

Open

Copilot started work on behalf of adamsitnik July 15, 2026 13:27 View session

adamsitnik added this to the 11.0.0 milestone Jul 15, 2026

adamsitnik added the area-System.IO label Jul 15, 2026

Move IsNetworkFileSystem check to managed SafeFileHandle, cache in Init

cc4b633

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot July 15, 2026 13:46

Address code review feedback: improve comments in SafeFileHandle and …

c914251

…pal_io.c Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from adamsitnik and Copilot and removed request for Copilot July 15, 2026 13:48

Copilot finished work on behalf of adamsitnik July 15, 2026 13:49

adamsitnik requested changes Jul 15, 2026

View reviewed changes

Copilot started work on behalf of adamsitnik July 15, 2026 14:26 View session

Address review feedback: remove native IsNetworkFileSystem, use CanLo…

b4094fc

…ckTheFile in managed code Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot July 15, 2026 15:10

Copilot finished work on behalf of adamsitnik July 15, 2026 15:11

Address review feedback: lazy UseFullFSync for all Apple platforms, f…

011f209

…ix WASM FSync signature Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from adamsitnik and Copilot and removed request for Copilot July 15, 2026 16:32

Copilot finished work on behalf of adamsitnik July 15, 2026 16:33

Copilot AI temporarily deployed to copilot-pat-pool July 15, 2026 16:36 Inactive

adamsitnik marked this pull request as ready for review July 15, 2026 16:36

Copilot AI review requested due to automatic review settings July 15, 2026 16:36

Copilot started reviewing on behalf of adamsitnik July 15, 2026 16:37 View session

Copilot AI temporarily deployed to copilot-pat-pool July 15, 2026 16:37 Inactive

Copilot AI reviewed Jul 15, 2026

View reviewed changes

Comment thread src/libraries/System.Private.CoreLib/src/Microsoft/Win32/SafeHandles/SafeFileHandle.Unix.cs

Comment thread src/native/libs/System.Native/pal_io.c

adamsitnik reviewed Jul 15, 2026

View reviewed changes

Comment thread src/libraries/System.Private.CoreLib/src/Microsoft/Win32/SafeHandles/SafeFileHandle.Unix.cs

Comment thread src/native/libs/System.Native/pal_io.c

Copilot started work on behalf of adamsitnik July 15, 2026 16:50 View session

Update SafeFileHandle F_FULLFSYNC comments for conservative probe beh…

f16793c

…avior Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot July 15, 2026 17:21

Copilot finished work on behalf of adamsitnik July 15, 2026 17:24

Copilot AI requested a review from adamsitnik July 15, 2026 17:24

Copilot AI temporarily deployed to copilot-pat-pool July 15, 2026 17:50 Inactive

adamsitnik approved these changes Jul 16, 2026

View reviewed changes

adamsitnik requested a review from jozkee July 16, 2026 12:48

jozkee reviewed Jul 18, 2026

View reviewed changes

	if (useFullFsync == NullableBool.Undefined && !IsClosed && OperatingSystem.IsApplePlatform())
	if (useFullFsync == NullableBool.Undefined && !IsClosed && OperatingSystem.IsMacOS())

Uh oh!

Conversation

Copilot AI commented Jul 15, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

azure-pipelines Bot commented Jul 15, 2026

Uh oh!

adamsitnik left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

dotnet-policy-service Bot commented Jul 15, 2026

Uh oh!

adamsitnik left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

azure-pipelines Bot commented Jul 15, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

Uh oh!

Uh oh!

adamsitnik left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

adamsitnik left a comment

Choose a reason for hiding this comment

Uh oh!

adamsitnik commented Jul 16, 2026

Uh oh!

jozkee Jul 18, 2026

Choose a reason for hiding this comment

Uh oh!

jozkee Jul 18, 2026

Choose a reason for hiding this comment

Uh oh!

jozkee commented Jul 18, 2026

Uh oh!

adamsitnik commented Jul 18, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Copilot AI commented Jul 15, 2026 •

edited

Loading