Lock bit when opening file for writing

It might be a nice idea to have a lock attribute set on the file when it is open for writing, to protect against multiple instances writing to a file simultaneously.

This would protect against accidentally running two processes that write to the same file, which can cause corruption (this just happened), or even against two different people writing to the same file on a remote machine (on a common network share, for instance).

What do you think?

The issue does not name any files, tests, or entry points. First clarify the locking semantics, scope, and behavior for concurrent writers or remote shares, then locate the file-opening and write paths. Done requires an agreed design and tests demonstrating protection against simultaneous writes.

Written by the indexing model from the issue text.