Write to pipe log using vmsplice()

[edsiper]

Linux kernel provides a syscall named vmsplice() which allows to write an
iov struct with zero copy to a pipe, we must use vmsplice() instead of
writev() for that procedure.

[charlie_brown]

I was skimming through the code and it came to my attention the following commented snippet:

        /* for some reason, vmsplice is not working as expected for us, 
         * maybe we need to fix something here, at the moment
         * we will keep using writev to push the iovec struct to the pipe
         *
         *
         * n = vmsplice(fd, 
         *            (const struct iovec *) mk_io->io, 
         *            mk_io->iov_idx, 
         *            SPLICE_F_GIFT);
         * return n;
         */

1) vmsplice's man page states that when the SPLICE_F_GIFT flag is present, the memory pointed by each iovec's iov_base MUST be aligned to page boundary:

"Data must also be properly page aligned, both in memory and length."

which, if one dumps the iov_base addresses at run-time, they're not.

2) BUT if we remove SPLICE_F_GIFT, the logger writes to the files with interleaved garbage from the response headers; I haven't checked why.

[davidlohr]

This code has been determined to be unapplicable and has already been removed.