Marshal.load corrupts repeated object reference after Set appears earlier in payload #9405

Environment Information

JRuby:

$ jruby -v
jruby 10.1.0.0 (4.0.0) 2026-04-20 32f988b78c OpenJDK 64-Bit Server VM 26+35 on 26+35 +indy +jit [x86_64-linux]

MRI comparison:

ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x86_64-linux]

Expected Behavior

Marshal.load(Marshal.dump(obj)) should preserve repeated object references correctly. In the script below, loaded[2] should be "good".

Repro:

require 'set'

shared = 'good'
payload = [
  Set.new,
  ['bad', shared],
  shared,
]

loaded = Marshal.load(Marshal.dump(payload))

puts RUBY_DESCRIPTION
puts "expected: good"
puts "actual:   #{loaded[2].inspect}"
puts "ok: #{loaded[2] == 'good'}"

MRI output:

ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x86_64-linux]
expected: good
actual:   "good"
ok: true

Actual Behavior

JRuby 10.1.0.0 output:

jruby 10.1.0.0 (4.0.0) 2026-04-20 32f988b78c OpenJDK 64-Bit Server VM 26+35 on 26+35 +indy +jit [x86_64-linux]
expected: good
actual:   "bad"
ok: false

The third element of the top-level array becomes "bad" after the Marshal roundtrip. It should remain the same string object/value as shared, i.e. "good".

Some narrowing observations:

• No threading is involved.
• The issue reproduces with plain Marshal.dump / Marshal.load.
• Replacing Set.new with [] or {} avoids the problem.
• Using shared.dup for the third payload element avoids the problem.
• The failure seems to require a Set appearing before a nested array containing a shared string object that is referenced again later.

This was found while debugging Opal's compiler cache on JRuby 10.1.0.0 (a regression since JRuby 10.0.2.0), where a cached compiler's generated JavaScript string was replaced by a source-map mappings string after a Marshal roundtrip.

Ref: opal/opal#2787