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IO::Handle.read() won't return buffer sizes >= 100_000_000 #5218

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p6rt opened this issue Apr 7, 2016 · 4 comments
Open

IO::Handle.read() won't return buffer sizes >= 100_000_000 #5218

p6rt opened this issue Apr 7, 2016 · 4 comments

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@p6rt
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@p6rt p6rt commented Apr 7, 2016

Migrated from rt.perl.org#127854 (status was 'open')

Searchable as RT127854$

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@p6rt p6rt commented Apr 7, 2016

From james.neko@gmail.com

$ perl6 --version
This is Rakudo version 2016.03-98-g61d231c built on MoarVM version
2016.03-84-g4afd7b6
implementing Perl 6.c.

I recently attempted to grab a largeish (128MiB) Buf of pseudorandom data
from /dev/urandom, only to errors from read(). I found this surprising,
since the IO​::Handle.read() documentation doesn't specify any kind of
limits.

At first I started thinking, aha, it's calling C's read() and that's
limited to SSIZE_MAX or something. And if I were programming in C, it would
not surprise me that I can't ask for arbitrarily-large buffers. I assumed
Perl's implementation would just call the lower-level read() repeatedly and
then give me back a nice large Buf object. I've checked SSIZE_MAX on my
system though, and that's not quite it. After a binary search, I discovered
that the limit is 100_000_000 bytes. I don't know what's special about that
number.

Code to demonstrate​:-

$ perl6
To exit type 'exit' or '^D'

my $fh = open("/dev/zero", :r, :bin);
IO​::Handle<"/dev/zero".IO>(opened, at octet 0)
my $buf = $fh.read(100_000_000);
Out of range​: attempted to read 100000000 bytes from filehandle

Fun extra maybe-bug that has me questioning my sanity​:-

Obviously, $fh.read(99_999_999); does not produce the error message, and in
my original quest to find the limit managed to return a value in less than
30 seconds or so. But, being the responsible bug-reporter that I am, I made
sure to upgrade my version of rakudo by running 'rakudobrew build moar',
and checking to see if the behaviour was present on the latest version.
Except... attempting to read the large-but-still-valid buffer seems to take
forever now.

It's certainly testing my patience, and it's late and I'd rather submit
this report now and figure out just how many hours it's taking later. I've
built moar-2016.{02,01.1} and they also appear to be taking their sweet
time in returning this ~100MB buffer. The thing is... I can't reproduce the
"fast" experience I was getting previously. I'm pretty sure my perl6 was
reporting itself as moar-2015.12, the Christmas release, and yet if I check
that out specifically it still takes (figuratively) forever.

Anyway, if it's unreasonable to ask for such a large value, it's a
documentation bug; otherwise, perhaps Perl6 needs to do some magic behind
the scenes. It's certainly busy doing *something*.
Thanks.
-James

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@p6rt p6rt commented Jul 10, 2016

From @zoffixznet

Still present in Rakudo version 2016.06-154-g55c359e built on MoarVM version 2016.06-9-g8fc21d5

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@p6rt p6rt commented Apr 26, 2017

From @zoffixznet

On Thu, 07 Apr 2016 09​:57​:57 -0700, james.neko@​gmail.com wrote​:

$ perl6 --version
This is Rakudo version 2016.03-98-g61d231c built on MoarVM version
2016.03-84-g4afd7b6
implementing Perl 6.c.

I recently attempted to grab a largeish (128MiB) Buf of pseudorandom data
from /dev/urandom, only to errors from read(). I found this surprising,
since the IO​::Handle.read() documentation doesn't specify any kind of
limits.

At first I started thinking, aha, it's calling C's read() and that's
limited to SSIZE_MAX or something. And if I were programming in C, it would
not surprise me that I can't ask for arbitrarily-large buffers. I assumed
Perl's implementation would just call the lower-level read() repeatedly and
then give me back a nice large Buf object. I've checked SSIZE_MAX on my
system though, and that's not quite it. After a binary search, I discovered
that the limit is 100_000_000 bytes. I don't know what's special about that
number.

Code to demonstrate​:-

$ perl6
To exit type 'exit' or '^D'

my $fh = open("/dev/zero", :r, :bin);
IO​::Handle<"/dev/zero".IO>(opened, at octet 0)
my $buf = $fh.read(100_000_000);
Out of range​: attempted to read 100000000 bytes from filehandle

Fun extra maybe-bug that has me questioning my sanity​:-

Obviously, $fh.read(99_999_999); does not produce the error message, and in
my original quest to find the limit managed to return a value in less than
30 seconds or so. But, being the responsible bug-reporter that I am, I made
sure to upgrade my version of rakudo by running 'rakudobrew build moar',
and checking to see if the behaviour was present on the latest version.
Except... attempting to read the large-but-still-valid buffer seems to take
forever now.

It's certainly testing my patience, and it's late and I'd rather submit
this report now and figure out just how many hours it's taking later. I've
built moar-2016.{02,01.1} and they also appear to be taking their sweet
time in returning this ~100MB buffer. The thing is... I can't reproduce the
"fast" experience I was getting previously. I'm pretty sure my perl6 was
reporting itself as moar-2015.12, the Christmas release, and yet if I check
that out specifically it still takes (figuratively) forever.

Anyway, if it's unreasonable to ask for such a large value, it's a
documentation bug; otherwise, perhaps Perl6 needs to do some magic behind
the scenes. It's certainly busy doing *something*.
Thanks.
-James

The limit was artificial and got removed in rakudo/rakudo@756877e

Not going to add a test to regular stresstest, but wouldn't hurt to add one to the "dangerous/exotic" category of tests we were discussing awhile back.

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@p6rt p6rt commented Apr 26, 2017

The RT System itself - Status changed from 'new' to 'open'

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