Purpose

Fixes a regression where version-store directories (e.g. .stversions/a/b) can become read-only (0555), preventing Syncthing from creating nested version paths during later archive operations (#10532).

If a parent directory inside the version store loses owner-write permission, subsequent attempts to archive files into deeper paths fail with a permission error. This prevents further versions from being archived for affected folders.

This patch ensures version-store directories always remain owner-writable so nested paths can be created safely.

Changes Made

• When traversing existing directories in the version store, ensure they remain owner-writable.
• When creating new directories in the version store:
  • Preserve the original permission behaviour.
  • Explicitly ensure owner-write permission is present to avoid stale read-only states.
• Add a regression test reproducing the failure scenario where a parent directory inside the version store is read-only and prevents creation of nested paths.

The fix is intentionally minimal:

• It only adds the owner-write bit if missing.
• It preserves all other permission bits.
• It does not alter permission semantics outside the version store path.

Background

Recent permission synchronisation changes caused version-store directories to inherit restrictive permissions from the source folder. In cases where a source directory was read-only (e.g. 0555), the corresponding version-store directory could also become read-only.

Because the version store must create additional nested directories over time, this could leave it permanently unable to archive further versions.

This change ensures version-store directories remain owner-writable while preserving all other permission bits.

Testing

Added a regression test that:

• Creates a version-store parent directory with mode 0555.
• Attempts to archive a file requiring creation of a nested directory.
• Verifies:
  • The archive operation succeeds.
  • The nested directory is created.
  • The parent directory regains owner-write permission.

Additionally verified:

• Existing permission behaviour for normal archive cases remains unchanged.
• All versioner tests pass.