[llvm-dev] Experiment on how to improve our temporary file handing.

[Rafael Avila de Espindola via llvm-dev]

Konstantin Belousov <kostikbel at gmail.com> writes:

> On Mon, Nov 13, 2017 at 01:46:32PM -0800, Davide Italiano wrote:
>> On Mon, Nov 13, 2017 at 11:48 AM, Rafael Avila de Espindola
>> <rafael.espindola at gmail.com> wrote:
>> > Davide Italiano <davide.italiano at gmail.com> writes:
>> >
>> >>> I couldn't find any support for this on FreeBSD.
>> >>>
>> >>
>> >> AFAIK FreeBSD supports some variant of /proc that should map Linux
>> >> (although the mapping isn't 1:1).
>> >> Does it lack support for this? Thanks for looking into this, BTW!
>> >
>> > O_TMPFILE is the main thing that seems to be missing.
>> >
>> 
>> It doesn't seem unreasonable to ask FreeBSD to implement this (at
>> least for ufs/zfs), maybe Ed/Kostik have thoughts about it.
>
> Can you please explain, how exactly O_TMPFILE functionality you asking
> for relates to the /proc ? Do you want to re-link O_TMPFILE-opened
> inodes into named files ?

The relation to proc seems to be a linux peculiarity.

What we want in the end is to implement "-o foo" in lld with the
following guarantees:

1 foo is only created with the final content.
2 We don't leave a temporary file behind on a power failure.
3 Less critical but nice: if foo already exists, it is replaced
  atomically.

Using mkstemp + rename we get 1 and 3.

On linux we can use O_TMPFILE, but the only way for a non privileged
process to then give a name to that fd is:

linkat(AT_FDCWD, "/proc/self/fd/<num>", AT_FDCWD, "destination",
       AT_SYMLINK_FOLLOW)

which has two problem: It requires proc to be mounted and it is not
atomic if the destination already exists.

A system with O_TMPFILE which allows some form of renameat to
atomically give that fd a name would be perfect.

Cheers,
Rafael