Feature #10200: Symbol list/count API with Symbol GC

• Status: Closed
• Assignee: matz (Yukihiro Matsumoto)
• Target version: 2.2.0

[ko1 (Koichi Sasada)]

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?

[ko1 (Koichi Sasada)]

• Description updated (diff)

[akr (Akira Tanaka)]

I feel ObjectSpace.count_objects can be extended to return number of symbols.

[ko1 (Koichi Sasada)]

Akira Tanaka wrote:

I feel ObjectSpace.count_objects can be extended to return number of symbols.

Make a new method? Or return a hash object with new types like T_SYMBOL_MORTAL and T_SYMBOL_IMMORTAL?

Now, ObjectSpace.count_objects returns a number of (1) + (2) - [number of statically created symbol (immediate symbol values)].

[akr (Akira Tanaka)]

New hash entries.

I'm not sure that "T_" prefix is appropriate here, though.

[matz (Yukihiro Matsumoto)]

This request is bit vague.

As a result of the developers meething on 2014-09-04, we will:

• keep Symbol.all_symbols as it is.
• remove Symbol.find(name).

Matz.

[matz (Yukihiro Matsumoto)]

• Status changed from Open to Closed