Feature #17284
Updated by ko1 (Koichi Sasada) over 3 years ago
For some reasons, we need to provide a way to make sharable Proc between ractors.
* (1) A block for the `Ractor.new`.
* (2) Send a proc between ractors.
* (3) A block for global callback methods: `define_method` ([Bug #17159]), `TracePoint`, ...
For (1), we use `Proc#isolate` (`isolate` is temporary name here) which prohibit to access outer variables.
```ruby
a = 1
Proc.new{
p a
}.isolate # => can not isolate a Proc because it accesses outer variables (a).
# error on `isolate` method call
```
There are no states to share, so it is okay.
For (2), `Proc#isolate` is one option because we can send parameters with an argument `call`.
But it should be a bit long.
```ruby
i, j, k = nil
pr = Proc.new do |i, j, k|
p i, j, k
end.isolate end
r = Ractor.new do |task, param|
task.call(*param)
end
r.send([pr, [i, j, k]])
```
For (3), maybe we need to make more flexible Proc which can *read* outer block parameter on that snapshot (discussed in #17159).
Now, I named it with `freeze`, because it seems frozen Proc.
```ruby
a = 1
# try to read, and returns old value (snapshot at `freeze`)
pr = Proc.new{
p a #=> 1
}
pr = pr.freeze
pr.call
a = 2
pr.call #=> 1
# try to write, and it is not allowed
pr2 = Proc.new{
a = 1
}
pr2 = pr.freeze
#=> can not freeze a Proc because it accesses outer variables (a). (ArgumentError)
```
To share the "frozen" Proc between ractors, outer values should be (deep) frozen. It means readable values (in above case, `a`) should be shareable.
Now we named it `Proc#shareable!`
```ruby
a = [1, [2, 3]]
pr = Proc.new{
p a.frozen? #=> true
}.shareable!
a[0] = 0 #=> frozen error
```
This ticket has three different variant of mutability and shareability for Proc.
| | outer lvar | shareable | freeze/making shareable other objects
|---------------|---------------|------------|------------------------------------------
|a. isolate | N/A | Yes | No
|b. freeze | allow to read | No | No
|c. shareable! | allow to read | Yes | Yes
I want to introduce functionality of `shareable!`, but not sure the Ruby-level API.
I think (b) `freeze` for this semantics is good name because it only allows to read-only local variables.
However, it is not enough to make a sharable Proc because read objects from the Proc should be also sharable.
Making `freeze` with (c) `shareable!` functionality is one idea, but I think `freeze` should not deep-freezing because it is very surprising that read objects become the sharable (== frozen) for usual Ruby users.
Maybe `Ractor.make_sharable(pr)` makes `pr` sharable is no surprise because it is good declaration the `pr` should be shareable, even if the read objects from `pr` become shareable (== frozen).
Removing (a) `isolate` and using (c) `shareable!` at `Ractor.new(&b)` is one idea, but I think it is surprising that they can access outer local variables, but the they can not access newly assigned variables as usual blocks.
```
a = 1
Ractor.new do
p a # only 1
end
a = 2
```
(a) `isolate` does not have such issue because all outer lvars accesses are not allowed == easy to understand, easy to debug.
In practice, accessing outer variables with multi-ractor program is very useful because we need to declare same local variables if we want to access them from different ractors.
The following example is from [Feature #17261]:
```ruby
tv1 = Thread::TVar.new(0)
tv2 = Thread::TVar.new(0)
r1 = Ractor.new tv1, tv2 do |tv1, tv2| # <-- here
loop do
Thread.atomically do
v1, v2 = tv1.value, tv2.value
raise if v1 != v2
end
end
end
```
With (c) `shareable!` semantics, it is easier to write:
```ruby
tv1 = Thread::TVar.new(0)
tv2 = Thread::TVar.new(0)
r1 = Ractor.new do
loop do
Thread.atomically do
v1, v2 = tv1.value, tv2.value
raise if v1 != v2
end
end
end
```
Above example is also enable to make more simple:
```ruby
i, j, k = nil
pr = Proc.new do
p i, j, k
end
r = Ractor.new do |task|
task.call
end
r.send(pr)
```
However, using this semantics (`shareable!`) can freeze extra-variables in accidents:
```ruby
a = [1, 2, 3]
Ractor.new do
do_something if a.length > 0
end
a << 4 # raises FrozenError
```
It is clear that there is a syntax or method to apply `shareable!` functionality.
```ruby
a = [1, 2, 3]
Ractor.new &(Ractor.make_shareable(Proc.new{ a.length ... })
```
It can be used with `define_method` which can invoke from ractors:
```ruby
define_method(name, Ractor.make_shareable(Proc.new{ ... }))`
```
But it is too long.
There are implementations for (a), (b) and (c), but the API is not fixed, so there is no PR now.
I'm thinking to introduce (c)'s feature in `Ractor.make_sharaeble(pr)`.
To use with `define_method`, maybe it should be more friendly. Ideally, new syntax is great.
There is no conclusion, and your comments are welcome.
Thanks,
Koichi