[ruby-core:69599] [Ruby trunk - Bug #11264] [Open] Memory leak in JSON stdlib ext (JSON generation)

[luke.gru@gmail.com]

Issue #11264 has been reported by Luke Gruber.

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Bug #11264: Memory leak in JSON stdlib ext (JSON generation)
https://bugs.ruby-lang.org/issues/11264

* Author: Luke Gruber
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: 2.2-head
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
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Hi,

I'm not sure if this is a bug, or just undocumented behaviour, but here's a script to reproduce the memory leak:

------------------------------
require 'json'

class MyClass
  def to_json(*)
    "a" * 1048576 # 1 megabytes of chars
  end
end

class MyOther
  def to_json(*)
    raise "OMG"
  end
end

1000.times do |i| # will leak up to ~ 4 gigs
  puts i
  JSON.dump([MyClass.new, MyClass.new, MyClass.new, MyOther.new]) rescue nil
end
------------------------------

What's happening is that the C extension is iterating over the array to eventually dump it out to JSON. It's going through the array in order, appending to the fbuffer as needed. The problem is that that the API extension point of adding a to_json method to a class (or object), without wrapping the code in some sort of 'begin...rescue , free(buffer), re-raise' block results in the buffer never being freed. Normally this isn't too bad, except if a lot of data was appended to the buffer before the error got raised.

To test it against normal behaviour in the above script, take out the offending MyOther.new in the array. It should run much more smoothly this way :)

Note that since the fbuffers aren't GC marked (not that they should be), it isn't possible to trace this leak using GC.stat.

Once again, not sure if this is a bug or if we should never raise errors from custom to_json methods (ie: always wrap them in a begin... rescue block.

Thanks,

I also reported this to the JSON gem maintainer here: https://github.com/flori/json/issues/251



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