duplicate stat() system calls on file tests

[bugfood]

The Problem

IO::Path methods to test file metadata (.l, .f, etc.) issue two stat()/lstat() system calls when one will suffice.

Expected Behavior

A single "foo".IO.l call will result in a single lstat().
A single "foo".IO.f call will result in a single stat().

Actual Behavior

A single "foo".IO.l call results in two calls to lstat().
A single "foo".IO.f call result in two calls to stat().

Steps to Reproduce

Prepare:

$ cd /tmp
$ touch foo

Perl 6 test, unexpected behavior:

$ strace -f -e trace=stat,lstat perl6 -e 'say "hi" if "foo".IO.l' 2>&1 | grep foo
[pid 10876] lstat("/tmp/foo", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
[pid 10876] lstat("/tmp/foo", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
$ strace -f -e trace=stat,lstat perl6 -e 'say "hi" if "foo".IO.f' 2>&1 | grep foo
[pid 10881] stat("/tmp/foo", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
[pid 10881] stat("/tmp/foo", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0

Perl 5 test, expected behavior:

$ strace -f -e trace=stat,lstat perl -e 'print "hi\n" if -l "foo"' 2>&1 | grep foo
lstat("foo", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
$ strace -f -e trace=stat,lstat perl -e 'print "hi\n" if -f "foo"' 2>&1 | grep foo
stat("foo", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0

One possible hint as to why this happens is that there is only a single call when the file is missing. It appears as if perl 6 is issuing one stat() to determine if the file exists and then another stat() to gather data for the user-requested test.

$ rm -f foo
$ strace -f -e trace=stat,lstat perl6 -e 'say "hi" if "foo".IO.f' 2>&1 | grep foo
[pid 10911] stat("/tmp/foo", 0x7ffd7d0e9810) = -1 ENOENT (No such file or directory)

Environment

• Operating system:

$ cat /etc/debian_version 
buster/sid

• Compiler version (perl6 -v):

$ perl6 -v
This is Rakudo version 2018.11 built on MoarVM version 2018.11
implementing Perl 6.d.

Thanks,
Corey

[cuonglm]

You can't do it, at least with current version of MoarVM https://github.com/MoarVM/MoarVM/blob/a5dd7652d4ca0ff67aac5c7809958e030abcbacf/src/io/fileops.c#L16

Basically, file stat is not expose directly to nqp side, you have to perform multiple nqp::stat calls to get the file information, mostly 2 (.l, .e...), .w even use 3.

[zoffixznet]

This was the never-implemented extra part of the IO Grant:
https://github.com/rakudo/rakudo/blob/master/docs/archive/2017-IO-Grant--Action-Plan.md#iopath-routines-that-involve-a-stat-call-issue-for-discussion

With a discussion that led to the proposal for nqp::statmulti op, to be used by methods that need more than one piece of stat info: zoffixznet/IOwesomeness#4 (also IRC discussion)

[bugfood]

Thanks for the clarifying information; I was not able to find the earlier discussions when I looked.

If the ideas are covered elsewhere, then I would understand if you close this bug as a duplicate.

Thanks,
Corey

[zoffixznet]

then I would understand if you close this bug as a duplicate.

I think it's good to have this ticket open. I'm not aware of any other ticket on this matter in the rakudo's repo.