Re: [PATCH bisected regression] input_available_p() sometimes says 'no' when it should say 'yes'

Michael Matz
2015-05-04T16:56:55Z

Hi,

On Mon, 4 May 2015, Peter Hurley wrote:

> I think it would be a shame if ptrace() usage forced a whole class of
> i/o to be synchronous.

I think Neils patch doesn't do that, does it? If it has an indication
that in fact some data must be there but isn't it pulls it, otherwise it's
the same as after your patch/current state?

(Leaving the debugger question to Nic; but I guess it's similar interface
like gdb. Once you come back into the debugger (breakpoint hit) it looks
once for input on the tubes of the debuggee and then enters a prompt; it
doesn't continue looking for input until you continue the debuggee (1).
ptys would be used because it's Cobol, the programs contain data entry
masks presumably needing a real tty, not just pipes. That usecase would
be broken now; the tty provided by the debugger doesn't reflect the real
screen that the debuggee actually generated before the breakpoint. Note
how pipes in my test program _are_ synchronuous in this sense, just ptys
aren't.)

Ciao,
Michael.

(1) And in a single threaded debugger (no matter if the debuggee is
multithreaded) it would be awkward to implement. After read returns 0
you'd again call poll, which indicates data is there, and read again.
You repeat that until $SOMEWHEN. But when is it enough? Two loops, 10,
1000? To not sit in a tight loop you'd also add some nanosleeps, but that
would add unnecessary lags.

Basically, whenever poll indicates that read won't block then it should
also return some data, not 0, if at all reasonably implementable; i.e.
some progress should be guaranteed. I realize that this isn't always the
case, but here it is. In code, this loop:

while (poll ([fd, POLLIN], 0) == 1)
// So, read won't block, yippie
if (read (fd, ...) == 0)
continue;

shouldn't become a tight loop, without the read making progress but the
kernel continuously stating "yep, there's data available", until some
random point in the future.