kwargs in Array.each mutates argument

[amacleay]

Environment

From https://oss.sonatype.org/content/repositories/snapshots/org/jruby/jruby-dist/9000.dev-SNAPSHOT/jruby-dist-9000.dev-20140908.223539-1-bin.tar.gz

$ bash jruby-9000.dev-SNAPSHOT/bin/jruby --version
jruby 9000.dev-SNAPSHOT (2.1.2p142) 2014-09-04 177ed02 Java HotSpot(TM) 64-Bit Server VM 25.181-b13 on 1.8.0_181-b13 [linux-amd64]
09:56:01 ~/repro
$ uname -a
Linux amacleay-cht-tp 4.15.0-54-generic #58-Ubuntu SMP Mon Jun 24 10:55:24 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Script:

# $ cat repro.rb 
list = Array.new(200, { a: 1, b: 2, c: 3 })
list.each do |a:, b:, **|
  puts "#{a} #{b}"
end

Expected Behavior

It should print 1 2 200 times and exit happily, as does MRI:

$ rvm ruby-2.5.0 do ruby repro.rb | nl | tail -n1
   200  1 2

Actual Behavior

Runs once and then fails. Further investigation shows that the runtime is mutating the invocant to .each

$ bash jruby-9000.dev-SNAPSHOT/bin/jruby repro.rb 
1 2
ArgumentError: missing keyword: a
  (root) at repro.rb:2
    each at org/jruby/RubyArray.java:1546
  (root) at repro.rb:2

More recent releases run a variable number of iterations and then fail:
9.2.5.0:

$ rvm jruby-9.2.5.0 do ruby repro.rb | nl | tail -n1
ArgumentError: missing keyword: a
  <main> at repro.rb:2
    each at org/jruby/RubyArray.java:1792
  <main> at repro.rb:2
   177  1 2
10:03:08 ~/repro
$ rvm jruby-9.2.5.0 do ruby repro.rb | nl | tail -n1
ArgumentError: missing keyword: a
  <main> at repro.rb:2
    each at org/jruby/RubyArray.java:1792
  <main> at repro.rb:2
   143  1 2
10:03:13 ~/repro
$ rvm jruby-9.2.5.0 do ruby repro.rb | nl | tail -n1
ArgumentError: missing keyword: a
  <main> at repro.rb:2
    each at org/jruby/RubyArray.java:1792
  <main> at repro.rb:2
   142  1 2

9.2.7.0:

$ rvm jruby-9.2.7.0 do ruby repro.rb | nl | tail -n1
ArgumentError: missing keyword: a
  <main> at repro.rb:2
    each at org/jruby/RubyArray.java:1792
  <main> at repro.rb:2
   151  1 2
10:03:38 ~/repro
$ rvm jruby-9.2.7.0 do ruby repro.rb | nl | tail -n1
ArgumentError: missing keyword: a
  <main> at repro.rb:2
    each at org/jruby/RubyArray.java:1792
  <main> at repro.rb:2
   193  1 2
10:03:42 ~/repro
$ rvm jruby-9.2.7.0 do ruby repro.rb | nl | tail -n1
ArgumentError: missing keyword: a
  <main> at repro.rb:2
    each at org/jruby/RubyArray.java:1792
  <main> at repro.rb:2
   171  1 2

[enebo]

Just a quick note on this. It appears to be an issue with how kwargs work once we make it to a full build. If we disable full builds it works as expected:

jruby -Xjit.threshold=-1 --dev ../snippets/kwargs3.rb  | wc -l
200

I would predict the helper we use changes and it is not performing a dup somewhere.

[headius]

I did remove some dup'ing several months back, to reduce the cost of kwargs. Perhaps I removed too much.

[amacleay]

@enebo I can confirm that -Xjit.threshold=-1 --dev seems to solve the issue for 9.2.5.0 and 9.2.7.0, but I get the same bad behavior with the snapshot I pulled down.

[enebo]

There were two separate things at play and we shall see what impact there is here. fwiw, we never passed this in the JIT unless it was a method and not a block receiving the kwargs. So this exposed what I am guessing is a fairly uncommon combination? In any case it works now. Second, the full interpreter was not dup'ing the hash if we applied call protocol instructions and I honestly don't know why that would break anything. Combine this with the fact that the JIT does not have this check at all and I am guessing it probably does not matter.

As an aside I would like to add a prelude instr which is what frobnicate keywords does today but it will return something which can be a handle to processing keyword arguments without neccesarily performing any dupe of the incoming hash OR use a destructive hash for figuring out if all required kwargs were passed in. In cases where we know we need not do kwargs processing (like we realize we are passing in a literal hash to the call; then we will just noop the instr and replace it will some null object pattern for the operand we pass to the kwargs recv instrs.