18 Jul
2014
18 Jul
'14
3:33 p.m.
> the downstream process is in the middle of a read
call (waiting for
> more data to be put in the pipe), and it has already computed a pointer
> to the pipe's inode, and it's looping waiting for that inode to have
> data.
I think it would be necessary to make non-trivial
adjustments to the
pipe and file reading/writing code to make this work; either i) some
sort of flag bit to say 'you've been spliced, take appropriate action'
which the pipe code would have to check on being woken up, and then
back out to let the main file reading/writing code take another crack
at it
...
I'm not sure I want to do the work to make this actually work - it's
not clear if anyone is really that interested? And it's not something
that I'm interested in having for my own use.
So I decided that it was silly to put all that work into this, and not get it
to work. I did 'cut a corner', by not handling the case where it's the
first
or last process which is bailing (which requires a file-pipe splice, not a
pipe-pipe; the former is more complex); i.e. I was just doing a 'working proof
of concept', not a full implementation.
I used the 'flag bit on the inode' approach; the pipe-pipe case could be dealt
with entirely inside pipe.c/readp(). Here's the added code in readp() (at the
loop start):
if ((ip->i_flag & ISPLICE) != 0) {
closei(ip, 0);
ip = rp->f_inode;
}
It worked first time!
In more detail, I had written a 'splicetest' program that simply passed input
to its output, looking for a line with a single keyword ("stop"); at that
point, it did a splice() call and exited. When I did "cat input | splicetest
| cat > output", with appropriate test data in "input", all of the test
data
(less the "stop" line) appeared in the output file!
For the first time (AFAIK) a process succesfully departed a pipeline, which
continued to operate! So it is do-able. (If anyone has any interest in the
code, let me know.)
Noel