Feature #10200
Updated by ko1 (Koichi Sasada) over 10 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?