[Bug] Fix singleton class lookup bug when singleton class is replaced on the instance #3088
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src/main/java/org/truffleruby/language/objects/MetaClassNode.java
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…e instance. Co-authored-by: Kevin Menard <[email protected]> Co-authored-by: Peter Arato <[email protected]> Co-authored-by: Benoit Daloze <[email protected]>
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IO#reopenis one of those few examples (maybe the only one) where the caller object's singleton class is replaced:https://github.com/oracle/truffleruby/blob/master/src/main/ruby/truffleruby/core/io.rb#L1901
This means, methods defined only on an IO object's singleton class won't be available after calling
#reopenon it.However, currently in TruffleRuby fetching the singleton class for an object is cached against the object - which makes an assumption that the same object must have the same singleton class during its lifetime. As the beginning states, this is not the case for IO objects currently.
An example where this occurs on production: ActiveSupport's test runner contains an IO capture mechanism: https://github.com/rails/rails/blob/0e99d0893b0e98c626a3c7d8972eea22d29c9d25/activesupport/lib/active_support/testing/stream.rb#L23-L38 This effectively erases
STDOUT's singleton class. Any extension onSTDOUT's singleton class, eg with this public gem could cause a lookup of the old singleton class function.The actual break on production was a bit more nuanced:
IO#writemethod with a new proxy method, both on the singleton class (to enhance the functionality ofSTDOUT.write).ActiveSupport's#capturemechanism, which deleted the singleton classSTDOUT.writewas - due to the bug - directed to the old metaclass'.writemethod, which failed to call the old alias.To sum up the bug's effect, after replacing a singleton class: