On Wed, Nov 04, 2015 at 06:05:17PM -0800, Junio C Hamano wrote:
> I've always assumed that the original reason why we wanted to set
> the fd to nonblock was because poll(2) only tells us there is
> something to read (even a single byte), and the xread_nonblock()
> call strbuf_read_once() makes with the default size of 8KB is
> allowed to consume all available bytes and then get stuck waiting
> for the remainder of 8KB before returning.
>
> If the read(2) in xread_nonblock() always returns as soon as we
> receive as much as there is data available without waiting for any
> more, ignoring the size of the buffer (rather, taking the size of
> the buffer only as the upper bound), then there is no need for
> nonblock anywhere.
This latter paragraph was my impression of how pipe reading generally
worked, for blocking or non-blocking. That is, if there is data, both
cases return what we have (up to the length specified by the user), and
it is only when there is _no_ data that we might choose to block.
It's easy to verify experimentally. E.g.:
perl -e 'while(1) { syswrite(STDOUT, "a", 1); sleep(1); }' |
strace perl -e 'while(1) { sysread(STDIN, my $buf, 1024) }'
should show a series of 1-byte reads. But of course that only shows that
it works on my system[1], not everywhere.
POSIX implies it is the case in the definition of read[2] in two ways:
1. The O_NONBLOCK behavior for pipes is mentioned only when dealing
with empty pipes.
2. Later, it says:
The value returned may be less than nbyte if the number of bytes
left in the file is less than nbyte, if the read() request was
interrupted by a signal, or if the file is a pipe or FIFO or
special file and has fewer than nbyte bytes immediately available
for reading.
That is not explicit, but the "immediately" there seems to imply
it.
> So perhaps the original reasoning of doing nonblock was faulty, you
> are saying?
Exactly. And therefore a convenient way to deal with the portability
issue is to get rid of it. :)
-Peff
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