Fix FS storage races

[stanislav-shchetinin]

Changelog entry

...

Changelog category

• Not for changelog (changelog entry is not required)

Description for reviewers

• убрал гонку в PutObject (второй поток мог перетереть файл пока в него писал первый);
• добавил fsync для директорий.

[github-actions]

⚪ 2026-02-18 18:18:14 UTC Pre-commit check linux-x86_64-release-asan for 7d68433 has started.
⚪ 2026-02-18 18:18:33 UTC Artifacts will be uploaded here
⚪ 2026-02-18 18:20:44 UTC ya make is running...
🟡 2026-02-18 19:44:57 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
17120	17082	0	21	7	10

🟢 2026-02-18 19:45:09 UTC Build successful.
🟢 2026-02-18 19:45:40 UTC ydbd size 3.9 GiB changed* by +7.7 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: dfec5e2	merge: 7d68433	diff	diff %
ydbd size	4 188 796 920 Bytes	4 188 804 768 Bytes	+7.7 KiB	+0.000%
ydbd stripped size	1 567 485 568 Bytes	1 567 488 000 Bytes	+2.4 KiB	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2026-02-18 18:18:22 UTC Pre-commit check linux-x86_64-relwithdebinfo for 7d68433 has started.
⚪ 2026-02-18 18:18:40 UTC Artifacts will be uploaded here
⚪ 2026-02-18 18:21:01 UTC ya make is running...
🟡 2026-02-18 20:22:56 UTC Some tests failed, follow the links below. Going to retry failed tests...

Details

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
46543	43031	0	4	3494	14

⚪ 2026-02-18 20:23:19 UTC ya make is running... (failed tests rerun, try 2)
🟢 2026-02-18 20:25:47 UTC Tests successful.

Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
55 (only retried tests)	55	0	0	0	0

🟢 2026-02-18 20:25:54 UTC Build successful.
🟢 2026-02-18 20:26:18 UTC ydbd size 2.4 GiB changed* by +4.6 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: dfec5e2	merge: 7d68433	diff	diff %
ydbd size	2 572 231 544 Bytes	2 572 236 208 Bytes	+4.6 KiB	+0.000%
ydbd stripped size	542 282 952 Bytes	542 283 784 Bytes	+832 Bytes	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[ydbot]

Run Extra Tests

Run additional tests for this PR. You can customize:

• Test Size: small, medium, large (default: all)
• Test Targets: any directory path (default: ydb/)
• Sanitizers: ASAN, MSAN, TSAN
• Coredumps: enable for debugging (default: off)
• Additional args: custom ya make arguments

[github-actions]

🟢 2026-02-18 18:21:34 UTC The validation of the Pull Request description is successful.

[Copilot]

Pull request overview

This PR aims to eliminate file-system storage race conditions in the FS external storage wrapper, improving durability guarantees for object writes.

Changes:

• Reworked PutObject to avoid a truncate/write race by reusing the multipart session locking/truncation pattern.
• Added directory fsync (via Flush() on the parent directory fd) after object creation and after multipart completion rename.
• Improved logging for failed cleanup during AbortMultipartUpload.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

session is taken by reference (auto& session = it->second) and then ActiveUploads.erase(it) is called before session.File.Close(). Erasing the map element destroys the session, so the subsequent session.File.Close() is a use-after-free/UB. Close the file (and finish any other session access) before erasing, or move the TFile out before erase.

Suggested change

	ActiveUploads.erase(it);
	session.File.Close();
	session.File.Close();
	ActiveUploads.erase(it);

[Copilot]

FsyncParentDir opens a directory with TFile(parent.GetPath(), RdOnly | Seq) and calls Flush() (fsync). On Windows, TFileHandle uses CreateFileW without FILE_FLAG_BACKUP_SEMANTICS, so opening a directory path will fail and make Put/Complete operations error out. Consider guarding this code for non-Windows platforms or implementing directory-open with the required Windows flags (or using a dedicated helper in util).

Suggested change

	static void FsyncParentDir(const TString& filePath) {
	TFsPath parent = TFsPath(filePath).Parent();
	TFile dirFd(parent.GetPath(), RdOnly | Seq);
	dirFd.Flush();
	static void FsyncParentDir(const TString& filePath) {
	#if defined(_win_)
	(void)filePath;
	#else
	TFsPath parent = TFsPath(filePath).Parent();
	TFile dirFd(parent.GetPath(), RdOnly | Seq);
	dirFd.Flush();
	#endif