files: await pending stream writes before closing the handle

[Christof Marti (chrmarti)]

Summary

Fix the buffered file-write race behind the macOS / Electron failure on #336992.

A stream can emit end or error while its asynchronous onData write is still pending. FileService previously settled the stream operation immediately and closed the provider handle before the write finished. This can leave the disk provider retrying a write against a closed handle, and can incorrectly report success when the pending write subsequently fails.

The CI log reports two teardown failures under ExtHostAuthentication / createIfNone - false, but the leaked cancellation listeners originate in the preceding file-service work (doWriteBuffer -> disk write retry -> timeout). The second failure is the console-output guard reacting to the leak report. The notebook-only changes in #336992 are unrelated.

Changes

• Serialize pending stream writes and await them before settling on end/error.
• Use one completion handler for stream end, stream error, and write failure; preserve the first observed error while waiting for pending writes, and stop resuming the stream afterward.
• Add deterministic deferred-write regressions covering successful completion, a late provider-write failure, a stream error, and a stream error followed by a write failure.

Incoming PR check

No overlapping fix found. In particular:

• authentication: release provider activation listeners #335659 fixes authentication provider-activation listener cleanup, not these file-write retries.
• Streams: settle pending writes when a writeable stream is destroyed #332731 settles producer backpressure promises on stream destruction, not pending consumer file writes.
• files: report copy progress #331673 adds file-copy progress; it does not address stream completion ordering.

Validation

• Before the fix: all 3 new regression cases fail; the handle closes before the write finishes, and the late write error is lost.
• The combined-failure regression failed against the initial fix and now passes, preserving the earlier stream error.
• After the fix: file-service and in-memory file-service unit suites: 33 passing.
• Disk file-service and extension-host authentication integration suites: 198 passing, 6 pending.
• npm run gulp compile-client: 0 errors.
• ESLint on both changed files, git diff --check, and commit hygiene: passed.

Local validation was on Windows. macOS-specific confirmation is left to PR CI.

[Copilot]

Copilot review overview

🟢 Approval recommended

The narrow lifecycle fix correctly preserves write errors and handle ordering with focused regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes a file-service race by waiting for asynchronous stream writes before closing provider handles.

Changes:

• Serializes pending buffered writes and delays stream settlement.
• Prevents resuming completed or failed streams.
• Adds regression tests for success, write failure, and stream failure.

File	Description
src/​vs/​platform/​files/​common/​fileService.ts	Awaits pending writes before resolving or rejecting.
src/​vs/​platform/​files/​test/​browser/​fileService.test.ts	Adds deterministic stream-write race coverage.

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