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

[Rafael Avila de Espindola via llvm-dev]

Currently a power failure or other hard crash can cause lld leave a temporary
file around. The same is true for other llvm tools.

As an example, put a breakpoint in Writer.cpp:236 ("writeBuildId()") and
restart the run a few times. You will get

t.tmp43a735a  t.tmp4deeabb  t.tmp9bacdd3  t.tmpe4115c4  t.tmpeb01fff

The same would happen if there was a fatal error between the
FileOutputBuffer creation and commit. I don't think that is a code path
where that is possible right now, but it would be an easy thing to miss
in a code review.

I was hopping the OS could help us manage the temporary file so that
there was no way to accidentally leave it behind.

On linux there is O_TMPFILE, which allows us to create a file with no
name in the file system. A name can be given with linkat. Unfortunately
we can't use

linkat(fd, "", AT_FDCWD, "destination", AT_EMPTY_PATH)

Without special permissions and have instead to depend on proc:

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

Another annoyance is that linkat will not replace the destination and
renameat2 doesn't support AT_EMPTY_PATH. The result is that we have to
use unlink+linkat and loop.

On windows there is FILE_FLAG_DELETE_ON_CLOSE, but there seems to be no
way to cancel it. If a file is created with it I can rename it, but it
is still deleted in the end.

I couldn't find any support for this on FreeBSD.

This suggest that we cannot just have a createUniqueEntity that returs
just an FD. We will need a class that stores a temporary name too for
the systems where we cannot give a filename to a FD.

I have attached the patch I got so far, but given the existing
restrictions I would say it is not worth it.

It will try to just make it more obvious that lld's FileOutputBuffer is
deleted on all code paths.

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Cheers,
Rafael