[PATCH] D28010: FileOutputBuffer: support non-mmap'able file.

[Rafael Avila de Espindola via llvm-commits]

Rui Ueyama <ruiu at google.com> writes:

> On Wed, Dec 21, 2016 at 9:06 AM, Rafael Avila de Espindola <
> rafael.espindola at gmail.com> wrote:
>
>> > +// Represents a non-memory-mapped file. We need this class
>> > +// because not all files are mmap'able. For example, /dev/null
>> > +// is not mmap'able.
>>
>> The comment is a bit confusing. The issue is that with the old code if
>> you tried to "-o /dev/null", we would not try to mmap /dev/null. We
>> would try to mmap a temp file and move it to /dev/null.
>>
>> Maybe the best way to do this is
>> * Check the file type as you did.
>> * Remember the result in a member variable.
>> * If it is regular, use fs::createUniqueFile.
>> * If it is not, use fs::createTemporaryFile, which can also be mmaped.
>> * In commit, do a move if the target is regular or write the contents if
>>   it is not.
>>
>
> If you want do the same thing until the final rename(2) or file copy, you
> need to create a temporary file in the same directory as the final
> destination file (because the temporary file needs to be in the same file
> system as the final output.) But that's not always doable. For example, if
> the final output is /dev/null, /dev/ is not writable. So I guess this patch
> is reasonable, no?

That is why I suggested using fs::createTemporaryFile for non regular
files. It will create a file in /tmp.

Cheers,
Rafael