weird `const_missing` exceptions in mrblib code (mruby 3.1.0 and above)

[kazuho]

I've been trying to use mruby-require with the recent versions of mruby (see iij/mruby-require#29) and have noticed one behavior that seems like an issue with mruby 3.1.0 and above.

When I run mruby-require of the PR linked above with mruby 3.0.0, everything works fine. But when I run it using mruby 3.1.0, a const_missing error is raised complaining that File.exist? cannot be found.

Steps to reproduce:

1. Checkout mruby 3.0 support iij/mruby-require#29.
2. Build mruby with the following configuration:

MRuby::Build.new do |conf|
  conf.toolchain :gcc
  conf.gembox "default"
  conf.gem "mruby-require"
end

3. Run the following script using mruby being built:

print "#{File.file?("/")}\n"
require "/nonexistent"

When I run this script using mruby 3.0.0 being built, the first line emits "false" (because / is a directory), then complains that /nonexistent cannot be loaded. Fine.
But when I run this script using mruby 3.1.0 being built, it raises a const_missing exception here.

Why I think this is a bug of mruby:

While this error is related to mruby-require, I'm reporting it here because I suspect this is a problem of mruby, due to the following reasons:

• the version of mruby-require being used does not have external dependency outside of mrbgems bundled by mruby
• the "gem_init" function of mruby-require just calls mrb_define_method and does nothing else, the problem that we see seems to happen before any logic of mruby-require written in C is being invoked

Therefore, I'm suspecting if there's an issue with the ruby code found in mrbgems having issues with accessing constants defined in other mrbgems.

[kazuho]

Did a bisect, it seems that this is a regression caused by 2d8f4fa #5272.

[matz]

Could you show us the exact error message from const_missing?

[kazuho]

@matz Of course. Thank you for looking into the issue.

When I run the ruby script above, I get:

% build/host/bin/mruby ~/tmp/defect.rb       
false
trace (most recent call last):
	[5] /Users/kazuho/tmp/defect.rb:2
	[4] /Users/kazuho/mydev/3p/mruby-require/mrblib/require.rb:40:in require
	[3] /Users/kazuho/mydev/3p/mruby/mrblib/enum.rb:78:in find
	[2] /Users/kazuho/mydev/3p/mruby/mrblib/array.rb:17:in each
	[1] /Users/kazuho/mydev/3p/mruby/mrblib/enum.rb:79:in detect
/Users/kazuho/mydev/3p/mruby-require/mrblib/require.rb:41:in require: undefined method 'const_missing' (NoMethodError)

And in fact, I've just found out that if I override const_missing and return the correct object,

module Kernel
  def const_missing(id)
    File
  end
end
print "#{File.file?("/")}\n"
require "/nonexistent"

then things work as expected:

% build/host/bin/mruby ~/tmp/defect.rb
false
trace (most recent call last):
	[1] /Users/kazuho/tmp/defect.rb:7
/Users/kazuho/mydev/3p/mruby-require/mrblib/require.rb:54:in require: cannot load such file -- /nonexistent (LoadError)

Therefore I think it could be the case that the script being loaded from mrblib is somehow not accessing the correct File object.

[matz]

Since I use master branch, the symptom was different from your report. But I believe I fixed the issue. You can cherrypick 92be161 to fix the issue for mruby 3.0.

[kazuho]

Thank you very much for the quick response! I'm afraid that the problem is still not fully fixed.

92be161 indeed is an improvement, I believe it fixes a TypeError that was raised at https://github.com/iij/mruby-require/blob/master/mrblib/require.rb#L6. FWIW, IIRC we started seeing this TypeError in mruby 3.0.0. I actually had a workaround for this, which simply removed the code for simplicity (iij/mruby-require@03fe79f).

However, that is not the only problem. Even with 92be161, I am still seeing a const_missing exception being raised at https://github.com/iij/mruby-require/blob/master/mrblib/require.rb#L51, i.e.:

    if ['/', '.'].include? path[0]
      path0 = filenames.find do |fname|
        File.file?(fname)    <--- HERE
      end

When doing git bisect, I know that 2d8f4fa introduced this problem. I anticipate that this problem could also be related to Proc, because we invoke File.extname just before entering this if and do block without having a problem (see https://github.com/iij/mruby-require/blob/master/mrblib/require.rb#L40).

I would appreciate it if you could take a look.

[dearblue]

The problem with #const_missing no longer exists after applying either of the following patches.

• diff --git a/include/mruby/proc.h b/include/mruby/proc.h
  index 9d197f48a..4f55c6ee6 100644
  --- a/include/mruby/proc.h
  +++ b/include/mruby/proc.h
  @@ -70,7 +70,7 @@ struct RProc {
   #define MRB_PROC_ENVSET 1024
   #define MRB_PROC_ENV_P(p) (((p)->flags & MRB_PROC_ENVSET) != 0)
   #define MRB_PROC_ENV(p) (MRB_PROC_ENV_P(p) ? (p)->e.env : NULL)
  -#define MRB_PROC_TARGET_CLASS(p) (MRB_PROC_ENV_P(p) ? (p)->e.env->c : (p)->e.target_class)
  +#define MRB_PROC_TARGET_CLASS(p) (MRB_PROC_ENV_P(p) ? (p)->e.env->c : (p)->e.target_class ? (p)->e.target_class : mrb->object_class)
   #define MRB_PROC_SET_TARGET_CLASS(p,tc) do {\
     if (MRB_PROC_ENV_P(p)) {\
       (p)->e.env->c = (tc);\

• diff --git a/src/variable.c b/src/variable.c
  index 68d3a388d..38d35f636 100644
  --- a/src/variable.c
  +++ b/src/variable.c
  @@ -828,6 +828,7 @@ mrb_vm_const_get(mrb_state *mrb, mrb_sym sym)
     proc = proc->upper;
     while (proc) {
       c2 = MRB_PROC_TARGET_CLASS(proc);
  +    if (!c2) c2 = mrb->object_class;
       if (c2 && iv_get(mrb, c2->iv, sym, &v)) {
         return v;
       }

I am currently checking to see which is preferred or if there are other good methods.

[dearblue]

I don't think this is very desirable, but for reference.
Disabling presym with MRuby::Build#disable_presym in build_config seems to work on mruby-3.1.0.