[ruby-core:61803] [ruby-trunk - Bug #9683] Segmentation fault when using default proc feature in ruby Hash object

Issue #9683 has been updated by Sam Rawlins.

File 9683.patch added

Currently, trunk act's like Eran's example: `a[1]` first results in SystemStackError, then results  in segmentation fault. I've tracked the segfault down to vm.c, in `vm_exec()`:

    1327  vm_loop_start:
    1328      result = vm_exec_core(th, initial);
    1329      if ((state = th->state) != 0) {

I _do_ see that we get to line 1328, but I don't think we actually enter `vm_exec_core()`. My debugging abilities end there :(

In any case, maybe this should be solved like #9151 : cut it off and return nil if recursion is detected. I've attached a small patch with tests.

This patch prevents the simple SystemStackError case, and permits some legitimate recursion, like in Psych's `yaml_tree.rb`:

    @dispatch_cache = Hash.new do |h,klass|
      method = "visit_#{(klass.name || '').split('::').join('_')}"
      method = respond_to?(method) ? method : h[klass.superclass]
      raise(TypeError, "Can't dump #{target.class}") unless method
      h[klass] = method
    end

 It also permits creative recursive uses of the default block:

    fac = Hash.new {|h,k| h[k] = h[k-1]*k }; fac[1] = 1
    fac[10]  #=> 3628800 == 10 factorial
    fib = Hash.new {|h,k| h[k] = h[k-2] + h[k-1] }; fib[1] = fib[2] = 1
    fib[10]  #=> 55, the 10th Fibonacci number
    collatz = Hash.new {|h,k| h[k] = k.even? ? h[k/2] + 1 : h[3*k+1] + 1 }
    collatz[1] = collatz[2] = collatz[4] = 0
    collatz[7]  #=> 14, the # of Collatz sequence steps to get from 7 to the 1-4-2 loop

----------------------------------------
Bug #9683: Segmentation fault when using default proc feature in ruby Hash object
https://bugs.ruby-lang.org/issues/9683#change-46038

* Author: Eran Barak Levi
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
* ruby -v: ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin12.0]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------

➜  bruno-v2 rvm:(ruby-2.1.0) git:(master) ✗ uname -a
Darwin erans-mbp.intkontera.com 13.1.0 Darwin Kernel Version 13.1.0: Thu Jan 16 19:40:37 PST 2014; root:xnu-2422.90.20~2/RELEASE_X86_64 x86_64
➜  bruno-v2 rvm:(ruby-2.1.0) git:(master) ✗ rvm -v
rvm 1.25.19 (stable) by Wayne E. Seguin <[e-mail]>, Michal Papis <[e-mail]> [https://rvm.io/]
➜  bruno-v2 rvm:(ruby-2.1.0) git:(master) ✗ ruby -v                                                                            
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin12.0]
➜  bruno-v2 rvm:(ruby-2.1.0) git:(master) ✗ ruby -e "a = Hash.new {|h,k| h[k] += 1};a[1]" 2> output                      
[1]    24934 segmentation fault  ruby -e "a = Hash.new {|h,k| h[k] += 1};a[1]" 2> output

more information in the attached files

---Files--------------------------------
ruby_2014-03-27-153720_Erans-MacBook-Pro.crash (45.3 KB)
output (489 KB)
9683.patch (1.31 KB)


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