Bug #22307: Ruby::Box: assignments to C-backed global variables do not reach C

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 4.1.0dev (2026-09-10T03:20:50Z master fe58143f12) +YJIT +MN +PRISM [arm64-darwin27]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[hsbt (Hiroshi SHIBATA)]

Under RUBY_BOX=1, assigning a global variable backed by a C variable only updates a per-box table. The setter never runs and the interpreter keeps reading the C variable. This happens in the main box too.

$ RUBY_BOX=1 ruby --disable-gems -e '$/ = "!"; p "hello!".chomp, [$/, $-0]; $/ = 1; $-a = 1; p $stdout.equal?(STDOUT)'
"hello!"
["!", "\n"]
false

Without RUBY_BOX, chomp returns "hello", $-0 follows $/, and $/ = 1 raises TypeError. 4.0.6 behaves the same.

gvar_use_box_tbl (variable.c:1025) sends every variable in a user box, the main box included, through box->gvar_tbl unless it is marked box_dynamic or box_ready. The first read clones the getter's value into the table and later writes stay there. So $/ and $-0 get separate slots for one rb_rs, $stdout becomes a copy of STDOUT, and validating setters never run.

So far this has been fixed one variable at a time with those two marks, in #21940 ($_), #21933 ($~) and #21991 ($!, $@). The open #22280, #22282 and #21867 have PRs that do the same. https://github.com/ruby/ruby/pull/18711 covers the rest in the main box by skipping the table for any variable whose setter is not the plain Ruby one, so the main box behaves like Ruby without boxes.

Optional boxes keep the table, and TestBox#test_global_variables asserts that $-0 and $, set in one box stay there. That holds only for Ruby code reading the variable. C code in every box reads the one C variable.

$ RUBY_BOX=1 ruby --disable-gems -e 'p Ruby::Box.new.eval(%q{$/ = "!"; ["hello!".chomp, $/, $-0]})'
["hello!", "!", "\n"]

I would like a decision on what these variables mean in an optional box. The first option keeps the box-local copy and accepts that the interpreter ignores it, which is where #18711 stops. The second shares them across all boxes like box_dynamic and changes test_global_variables. The third makes the C side per box, so every reader of rb_rs and the like looks it up through the current box.

[hsbt (Hiroshi SHIBATA)]

• Related to Misc #22275: Ruby::Box support plan for RubyGems and Bundler added

[hsbt (Hiroshi SHIBATA)]

• Related to Bug #21940: Ruby::Box: `$_` returns stale value due to gvar_tbl caching added

[hsbt (Hiroshi SHIBATA)]

• Related to Bug #21933: Ruby::Box: named capture local variable can become nil after non-matching lines added

[hsbt (Hiroshi SHIBATA)]

• Related to Bug #21991: `$!` stays as the first exception in Ruby Box added

[hsbt (Hiroshi SHIBATA)]

• Related to Bug #22280: Ruby::Box breaks $? after Kernel#system / IO.popen added

[hsbt (Hiroshi SHIBATA)]

• Related to Bug #22282: $VERBOSE and $DEBUG assignments have no effect under RUBY_BOX=1 added

[hsbt (Hiroshi SHIBATA)]

• Related to Bug #21867: enabling Ruby::Box changes puts/warn to no longer use $stdout/$stderr added