[ruby-core:81397] [Ruby trunk Feature#13378] Eliminate 4 of 8 syscalls when requiring file by absolute path

Issue #13378 has been updated by naruse (Yui NARUSE).


> -VALUE rb_find_file_safe(VALUE, int);
> +VALUE rb_find_file_safe(VALUE, int, int);

When you are CRuby developer, all functions declared under include/**/*.h are considered public C API.
(Note that if you are C extension developer, some of such APIs are experimental or private...)

Therefore rb_find_file_safe() should be kept as is.
You should declare rb_find_file_safe_with_defer_load_check or something in internal.h (not include/ruby/intern.h).

Or no one seems to use the function by GitHub search https://github.com/search?utf8=%E2%9C%93&q=rb_find_file_safe++extension%3Ac+path%3Aext&type=Code
we can simply change the prototype and move to internal.h.

>            if ((fd = rb_cloexec_open(fname, mode, 0)) < 0) {
> -               rb_load_fail(fname_v, strerror(errno));
> +               goto fail;
>             }

This seems to need `e = errno;` before goto fail;

>     int fd = 0;

It should be -1 because 0 is still valid fd even though it is STDIN.

>   if (fd > 0) (void)close(fd);

ditto.


Additionally, if you have tests to confirm the behavior, could you add them to test/ruby/test_require.rb?

----------------------------------------
Feature #13378: Eliminate 4 of 8 syscalls when requiring file by absolute path
https://bugs.ruby-lang.org/issues/13378#change-65109

* Author: burke (Burke Libbey)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Don't open file twice when specified by absolute path.

When invoking `require '/a.rb'` (i.e. via an absolute path), ruby generates this sequence of syscalls:

    open    /a.rb
    fstat64 /a.rb
    close   /a.rb
    open    /a.rb
    fstat64 /a.rb
    fstat64 /a.rb
    read    /a.rb
    close   /a.rb

It is apparent that the only inherently necessary members of this sequence are:

    open    /a.rb
    fstat64 /a.rb
    read    /a.rb
    close   /a.rb

(the fstat64 isn't *obviously* necessary, but it does serve a purpose and probably shouldn't be removed).

The first open/fstat64/close is used to check whether the file is loadable. This is important when scanning the `$LOAD_PATH`, since it is used to determine when a file has been found. However, when we've already unambiguously identified a file before invoking `require`, this serves no inherent purpose, since we can move whatever work is happening as a result of that `fstat64` into the second open/close sequence.

This change bypasses the first open/fstat64/close in the case of an absolute path to `require`. It also removes one of the doubled-up `fstat64` calls later in the sequence. As a result, the number of syscalls to require a file changes:

* From 8 to 4 when specified by absolute path;
* From 5+3n to 4+3n otherwise *(where n is the number of `$LOAD_PATH` items scanned)*.

In future work, it would be possible to re-use the file descriptor opened while searching the `$LOAD_PATH` without the close/open sequence, but this would cause some ugly layering issues.

---

*We intend to use this in conjunction with something like https://github.com/shopify/bootscale, which pre-resolves required features to absolute paths before calling `require`. This change reduces our total number of filesystem accesses by 13% during application boot.*

*Various notes and rationale at http://notes.burke.libbey.me/ruby-require-optimization*

---Files--------------------------------
0001-reduce-syscalls-on-require.patch (7.56 KB)
0001-reduce-syscalls-on-require-fixed.patch (6.94 KB)


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