[TUHS] Excise process from a pipe

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Jul 15 01:12:00 AEST 2014

    > From: Doug McIlroy <doug at cs.dartmouth.edu>

    > The spec below isn't hard: just hook two buffer chains together and
    > twiddle a couple of file desciptors.

How amusing! I was about to send a message with almost the exact same
description - it even had the exact same syntax for the splice() call! A
couple of points from my thoughts which were not covered in your message:

In thinking about how to implement it, I was thinking that if there was any
buffered data in an output pipe, that the process doing the splice() would
wait (inside the splice() system call) on all the buffered data being read by
the down-stream process.

The main point of this is for the case where the up-stream is the head of the
chain (i.e. it's reading from a file), where one more or less has to wait,
because one will want to set the down-streams' file descriptor to point to
the file - but one can't really do that until all the buffered data was
consumed (else it will be lost - one can't exactly put it into the file :-).

As a side-benefit, if one adopted that line, one wouldn't have to deal with
the case (in the middle of the chain) of a pipe-pipe splice with buffered
data in both pipes (where one would have to copy the data across); instead
one could just use the exact same code for both cases, and in that case the
wait would be until the down-stream pipe can simply be discarded.

One thing I couldn't decide is what to do if the upstream is a pipe with
buffered data, and the downstream is a file - does one discard the buffered
data, write it to the file, abort the system call so the calling process can
deal with the buffered data, or what? Perhaps there could be a flag argument
to control the behaviour in such cases.

Speaking of which, I'm not sure I quite grokked this:

    > If file descriptor fd0 is associated with a pipe and fd1 is not, then
    > fd1 is updated to reflect the effect of buffered data for fd0, and the
    > pipe's other descriptor is replaced with a duplicate of fd1.

But what happens to the data? Is it written to the file? (That's the
implication, but it's not stated directly.)

    > The same statement holds when "fd0" is exchanged with "fd1" and "write"
    > is exchanged with "read".

Ditto - what happens to the data? One can't simply stuff it into the input
file? I think the 'wait in the system call until it drains' approach is

Also, it seemed to me that the right thing to do was to bash the entry in the
system-wide file table (i.e. not the specific pointers in the u area). That
would automatically pick up any children.

Finally, there are 'potential' security issues (I say 'potential' because I'm
not sure they're really problems). For instance, suppose that an end process
(i.e. reading/writing a file) has access to that file (e.g. because it
executed a SUID program), but its neighbour process does not. If the end
process wants to go away, should the neighbour process be allowed access to
the file? A 'simple' implementation would do so (since IIRC file permissions
are only checked at open time, not read/write time).

I don't pretend that this is a complete list of issues - just what I managed
to think up while considering the new call.

    > For stdio, of course, one would need fsplice(3), which must flush the
    > in-process buffers--penance for stdio's original sin of said buffering.

Err, why is buffering data in the process a sin? (Or was this just a
humourous aside?)


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