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Feature #10200

Updated by ko1 (Koichi Sasada) almost 8 years ago

# Abstract 

 We need to consider specification of "Symbol.all_symbols" method because of Symbol GC. 

 # Backgraound 

 Symbol.all_symbols returns an array includes all symbols in this Ruby interpreter process. 

 ``` 
 "a#{1+2}b".to_sym 
 p Symbol.all_symbols.last #=> :a3b. Order of this array is implementation dependent. 
 ``` 

 However, Ruby 2.2 will introduce [symbol GC] (https://bugs.ruby-lang.org/issues/9634). 
 With symbol GC, dynamically created symbols can be collected like this: 

 ``` 
 "a#{1+2}b".to_sym 
 p Symbol.all_symbols.last #=> :a3b 
 GC.start 
 p Symbol.all_symbols.last #=> :$-a    <- :a3b is collected 
 ``` 

 Symbol class has another API Symbol.find() to get a symbol from a corresponding string object like that: 

 ``` 
 str = "a#{1+2}b" 
 str.to_sym 
 p Symbol.all_symbols.last #=> :a3b 
 p Symbol.find(str)          #=> :a3b 
 GC.start 
 p Symbol.all_symbols.last #=> :$-a    <- :a3b is collected 
 p Symbol.find(str)          #=> nil 
 ``` 

 Symbol GC separate all symbols into two types (because of implementaion details): 

 * (1) Collecatable symbols 
 * (2) Uncollectable symbols (we can not free even if there are no reference to these symbols) 

 Now, Symbol.all_symbols returns (1) + (2). 

 Symbol.all_symbols and Symbol.find methods assume that all symbols are immortal (assume only (2)). However, this assumption is changed ((1) is added). 

 [Symbol.count] (https://bugs.ruby-lang.org/issues/9963) is proposed to count (2) symbols. 
 Now, we don't have any way to count (2), because Symbol.all_symbols.size returns (1) and (2) symbols. 

 # Discussion 

 Maybe there are several possibility: 

 (a) No change (Symbol.all_symbols and Symbol.find treat with (1) + (2) symbols) 
 (b) Symbol.all_symbols and Symbol.find treat with (2) symbols 
 (c) Add new parameter to Symbol.all_symbols and Symbol.find to specify (2) or (1)+(2)). 

 (b) and (c) is reasonable for recent usage for these API, to findout immortal objects. 
 However, Symbol GC reduces danger of DoS attack with huge number of immortal objects. 

 Thoughts? Thouts? 

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