From: matz@...
Date: 2014-09-04T11:19:51+00:00
Subject: [ruby-core:64764] [ruby-trunk - Feature #10200] [Closed] Symbol list/count API with Symbol GC

Issue #10200 has been updated by Yukihiro Matsumoto.

Status changed from Open to Closed

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Feature #10200: Symbol list/count API with Symbol GC
https://bugs.ruby-lang.org/issues/10200#change-48654

* Author: Koichi Sasada
* Status: Closed
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* Category: core
* Target version: current: 2.2.0
----------------------------------------
# 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?




-- 
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