bug#37884: 27.0.50; Cannot write to a file in VirtualBox shared directory

[Paul Eggert]

On 10/30/19 9:25 AM, Eli Zaretskii wrote:

> Paul, do you
> see any downsides to the last change posted in this discussion?  IOW,
> could there be some situations where it could do any harm?

That change involves two extra system calls (one of which doesn't check 
the return value?!). In contrast, Bernardo's suggestion is simpler, 
involves no extra system calls (or machine instructions, for that 
matter), and has been tested. So I installed the attached patch to 
implement and document the workaround, and am optimistically closing the 
bug report.

The underlying problem is a significant bug in Virtual Box atop Windows 
10 Enterprise, a bug that surely breaks other applications (and could 
well break other parts of Emacs) that assume one can unlink a readonly 
file. Apparently the bug has been known for some time and a fix is 
available but the VirtualBox folks haven't gotten around to installing 
the fix:

https://www.virtualbox.org/ticket/4890
https://www.virtualbox.org/ticket/16463

If you're a VirtualBox user, I suggest that you ping the developers 
about fixing this, as one can't reasonably expect every application 
developer to work around their bug.

0001-Fix-bug-with-lock-file-on-nonstandard-filesystem.patch
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