Bug #17268: special global variables which can be accessed from ractors

• Status: Closed
• Assignee: -
• Target version: -
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[ko1 (Koichi Sasada)]

Ractors can't access global variables, but some special global variables should be accessed.
There are several types.

Proposal

(1) Read-only global variables

     # process-local (readonly): other commandline parameters
     '$-p' => $-p,
     '$-l' => $-l,
     '$-a' => $-a,

     # process-local (readonly): getpid
     '$$'  => $$,

(2) scope local variables

     # thread local: process result
     '$?'  => $?,

     # scope local: match
     '$~'  => $~.inspect,
     '$&'  => $&,
     '$`'  => $`,
     '$\''  => $',
     '$+'  => $+,
     '$1'  => $1,

     # scope local: last line
     '$_' => $_,

     # scope local: last backtrace
     '$@' => $@,
     '$!' => $!,

(3) Ractor local variables

     # ractor-local (derived from created ractor): debug
     '$DEBUG' => $DEBUG,
     '$-d' => $-d,

     # ractor-local (derived from created ractor): verbose
     '$VERBOSE' => $VERBOSE,
     '$-w' => $-w,
     '$-W' => $-W,
     '$-v' => $-v,

     # ractor local: stdin, out, err
     '$stdin'  => $stdin.inspect,
     '$stdout' => $stdout.inspect,
     '$stderr' => $stderr.inspect,

Implementation: https://github.com/ruby/ruby/pull/3670
I'll merge it soon.

Discussion

• only accessible from main ractor?
  • $0:
  • ARGV, ARGF, $.
• only accessible from main ractor because they will be obsolete
  • $, $/ $; $\
• So difficult: $" / $LOADED_FEATURES and $: / $LOAD_PATH

[ko1 (Koichi Sasada)]

• Description updated (diff)

[ko1 (Koichi Sasada)]

• Status changed from Open to Closed

Applied in changeset git|99310e3eb56fbc85bb45119285812eb959448d0c.

---

Some global variables can be accessed from ractors

Some global variables should be used from non-main Ractors.
[Bug #17268]

     # ractor-local (derived from created ractor): debug
     '$DEBUG' => $DEBUG,
     '$-d' => $-d,

     # ractor-local (derived from created ractor): verbose
     '$VERBOSE' => $VERBOSE,
     '$-w' => $-w,
     '$-W' => $-W,
     '$-v' => $-v,

     # process-local (readonly): other commandline parameters
     '$-p' => $-p,
     '$-l' => $-l,
     '$-a' => $-a,

     # process-local (readonly): getpid
     '$$'  => $$,

     # thread local: process result
     '$?'  => $?,

     # scope local: match
     '$~'  => $~.inspect,
     '$&'  => $&,
     '$`'  => $`,
     '$\''  => $',
     '$+'  => $+,
     '$1'  => $1,

     # scope local: last line
     '$_' => $_,

     # scope local: last backtrace
     '$@' => $@,
     '$!' => $!,

     # ractor local: stdin, out, err
     '$stdin'  => $stdin.inspect,
     '$stdout' => $stdout.inspect,
     '$stderr' => $stderr.inspect,

[Dan0042 (Daniel DeLorme)]

Why can't ractors access global variables? Shouldn't it be like constants, where a frozen/shareable object can be accessed?

$x = [1,2,3]
Ractor.new{ $x }.take  # I understand that this can't work
$x = true
Ractor.new{ $x }.take  # but why is this not ok?

[Eregon (Benoit Daloze)]

I think this issue is mostly about "special" global variables, not so much about "regular" global variables.

I wonder if "regular" global variables should be Ractor-local, or behave like constants.
Since most of the above became Ractor-local (instead of per-Process before) or a narrower scope, I think Ractor-local would make most sense.
The only special variables that are still process-local are read-only, which means it's the same as if they were Ractor-local read-only.

[Dan0042 (Daniel DeLorme)]

Eregon (Benoit Daloze) wrote in #note-4:

I wonder if "regular" global variables should be Ractor-local, or behave like constants.

I wouldn't mind either way. Although if Ractor-local I would hope they are derived from created ractor, like $DEBUG and $VERBOSE.

[ko1 (Koichi Sasada)]

The proposal is accepted.

I wonder if "regular" global variables should be Ractor-local, or behave like constants.

This is rejected at first by Matz (my original proposal is making all $gvars are ractor-local), to avoid confusion.