[TUHS] Excise process from a pipe

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > 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.
    > So now I have to regroup and figure out how to deal with that. My most
    > likely approach is to copy the inode data across

So I've had a good look at the pipe code, and it turns out that the simple
hack won't work, for two reasons.

First, the pipe on the _other_ side of the middle process is _also_ probably
in the middle of a write call, and so you can't snarf its inode out from
underneath it. (This whole problem reminds me of 'musical chairs' - I just
want the music to stop so everything will go quiet so I can move things
around! :-)

Second, if the process that wants to close down and do a splice is either the
start or end process, its neighbour is going to go from having a pipe to
having a plain file - and the pipe code knows the inode for a pipe has two
users, etc.

So 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, or ii) perhaps some sort of
non-local goto to forcefully back out the call to readp()/writep(), back to
the start of the read/write sequence.

(Simply terminating the read/write call will not work, I think, because that
will often, AFAICT, return with 0 bytes transferred, which will look like an
EOF, etc; so the I/O will have to be restarted.)


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.

Anyway, none of this is in any way a problem with the fundamental service
model - it's purely kernel implementation issues.

	Noel