RE: [PATCH] pipe: Make a partially-satisfied blocking read wait for more

[David Laight <David.Laight@ACULAB.COM>]

From: Linus Torvalds
> Sent: 24 June 2023 00:32
> 
> On Fri, 23 Jun 2023 at 16:08, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > In fact, I'd expect that patch to fail immediately on a perfectly
> > normal program that passes a token around by doing a small write to a
> > pipe, and have the "token reader" do a bigger write.
> 
> Bigger _read_, of course.
> 
> This might be hidden by such programs typically doing a single byte
> write and a single byte read, but I could easily imagine situations
> where people actually depend on the POSIX atomicity guarantees, ie you
> write a "token packet" that might be variable-sized, and the reader
> then just does a maximally sized read, knowing that it will get a full
> packet or nothing.

There are definitely programs that just do a large read in order
to consume all the single byte 'wakeup' writes.

(The 'must check' on these reads is a right PITA.)

They ought to set the pipe non-blocking, but I suspect many
don't - because it all works anyway.

	David

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