bug#35497: [PATCH] Don't rewrite buffer contents after saving by rename

[Jonathan Tomer]

On Tue, Apr 30, 2019 at 2:21 PM Michael Albinus <michael.albinus@gmx.de> wrote:

Jonathan Tomer <jktomer@google.com> writes:

Hi Jonathan,

> I thought about checking that the inode number changes, but that
> wouldn't have caught this particular bug (where the file is renamed
> into place with the correct contents, and then rewritten in place
> again); indeed, that doesn't appear to be easily caught with any
> examination of the final state alone, since what we're looking for is
> to prove the *absence* of any write that fails to change the inode
> number. (Perhaps we could check that the modification time of the
> file, after write, is *less* than its inode change time, proving that
> there has been no ordinary write since the rename -- but in my
> experience, inode timestamps are not actually more reliable than
> inotify, and in particular this check is easily defeated by the
> mode-setting that happens after the write is complete, requiring care
> to make sure that save-buffer will not attempt to do so.)

I see. But pls keep in mind, that inotify is not the only file
notification backend. Currently, we have six different beasts for this.

I was a bit worried about that; I considered disabling the test when

file-notify--library is not inotify, but instead designed the test so that

*missed* notifications would not cause it to fail, and I believe the other

implementations at least should not record a spurious "changed" notification.

It's still not ideal but it is the best regression coverage I can think of here.

I don't have BSD or win32 machines available to test on, so if you can point

me to a way to test this change on those targets I'm happy to run the new

tests.

Updated patch incoming.