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# This file is a part of Julia. License is MIT: https://julialang.org/license
abstract type AbstractChannel{T} end
"""
Channel{T}(sz::Int)
Constructs a `Channel` with an internal buffer that can hold a maximum of `sz` objects
of type `T`.
[`put!`](@ref) calls on a full channel block until an object is removed with [`take!`](@ref).
`Channel(0)` constructs an unbuffered channel. `put!` blocks until a matching `take!` is called.
And vice-versa.
Other constructors:
* `Channel(Inf)`: equivalent to `Channel{Any}(typemax(Int))`
* `Channel(sz)`: equivalent to `Channel{Any}(sz)`
"""
mutable struct Channel{T} <: AbstractChannel{T}
cond_take::Condition # waiting for data to become available
cond_put::Condition # waiting for a writeable slot
state::Symbol
excp::Union{Exception, Nothing} # exception to be thrown when state != :open
data::Vector{T}
sz_max::Int # maximum size of channel
# Used when sz_max == 0, i.e., an unbuffered channel.
waiters::Int
takers::Vector{Task}
putters::Vector{Task}
function Channel{T}(sz::Float64) where T
if sz == Inf
Channel{T}(typemax(Int))
else
Channel{T}(convert(Int, sz))
end
end
function Channel{T}(sz::Integer) where T
if sz < 0
throw(ArgumentError("Channel size must be either 0, a positive integer or Inf"))
end
ch = new(Condition(), Condition(), :open, nothing, Vector{T}(), sz, 0)
if sz == 0
ch.takers = Vector{Task}()
ch.putters = Vector{Task}()
end
return ch
end
end
Channel(sz) = Channel{Any}(sz)
# special constructors
"""
Channel(func::Function; ctype=Any, csize=0, taskref=nothing)
Create a new task from `func`, bind it to a new channel of type
`ctype` and size `csize`, and schedule the task, all in a single call.
`func` must accept the bound channel as its only argument.
If you need a reference to the created task, pass a `Ref{Task}` object via
keyword argument `taskref`.
Return a `Channel`.
# Examples
```jldoctest
julia> chnl = Channel(c->foreach(i->put!(c,i), 1:4));
julia> typeof(chnl)
Channel{Any}
julia> for i in chnl
@show i
end;
i = 1
i = 2
i = 3
i = 4
```
Referencing the created task:
```jldoctest
julia> taskref = Ref{Task}();
julia> chnl = Channel(c->(@show take!(c)); taskref=taskref);
julia> istaskdone(taskref[])
false
julia> put!(chnl, "Hello");
take!(c) = "Hello"
julia> istaskdone(taskref[])
true
```
"""
function Channel(func::Function; ctype=Any, csize=0, taskref=nothing)
chnl = Channel{ctype}(csize)
task = Task(() -> func(chnl))
bind(chnl, task)
yield(task) # immediately start it
isa(taskref, Ref{Task}) && (taskref[] = task)
return chnl
end
closed_exception() = InvalidStateException("Channel is closed.", :closed)
isbuffered(c::Channel) = c.sz_max==0 ? false : true
function check_channel_state(c::Channel)
if !isopen(c)
c.excp !== nothing && throw(c.excp)
throw(closed_exception())
end
end
"""
close(c::Channel)
Close a channel. An exception is thrown by:
* [`put!`](@ref) on a closed channel.
* [`take!`](@ref) and [`fetch`](@ref) on an empty, closed channel.
"""
function close(c::Channel)
c.state = :closed
c.excp = closed_exception()
notify_error(c)
nothing
end
isopen(c::Channel) = (c.state == :open)
"""
bind(chnl::Channel, task::Task)
Associate the lifetime of `chnl` with a task.
`Channel` `chnl` is automatically closed when the task terminates.
Any uncaught exception in the task is propagated to all waiters on `chnl`.
The `chnl` object can be explicitly closed independent of task termination.
Terminating tasks have no effect on already closed `Channel` objects.
When a channel is bound to multiple tasks, the first task to terminate will
close the channel. When multiple channels are bound to the same task,
termination of the task will close all of the bound channels.
# Examples
```jldoctest
julia> c = Channel(0);
julia> task = @async foreach(i->put!(c, i), 1:4);
julia> bind(c,task);
julia> for i in c
@show i
end;
i = 1
i = 2
i = 3
i = 4
julia> isopen(c)
false
```
```jldoctest
julia> c = Channel(0);
julia> task = @async (put!(c,1);error("foo"));
julia> bind(c,task);
julia> take!(c)
1
julia> put!(c,1);
ERROR: foo
Stacktrace:
[...]
```
"""
function bind(c::Channel, task::Task)
ref = WeakRef(c)
register_taskdone_hook(task, tsk->close_chnl_on_taskdone(tsk, ref))
c
end
"""
channeled_tasks(n::Int, funcs...; ctypes=fill(Any,n), csizes=fill(0,n))
A convenience method to create `n` channels and bind them to tasks started
from the provided functions in a single call. Each `func` must accept `n` arguments
which are the created channels. Channel types and sizes may be specified via
keyword arguments `ctypes` and `csizes` respectively. If unspecified, all channels are
of type `Channel{Any}(0)`.
Returns a tuple, `(Array{Channel}, Array{Task})`, of the created channels and tasks.
"""
function channeled_tasks(n::Int, funcs...; ctypes=fill(Any,n), csizes=fill(0,n))
@assert length(csizes) == n
@assert length(ctypes) == n
chnls = map(i -> Channel{ctypes[i]}(csizes[i]), 1:n)
tasks = Task[ Task(() -> f(chnls...)) for f in funcs ]
# bind all tasks to all channels and schedule them
foreach(t -> foreach(c -> bind(c, t), chnls), tasks)
foreach(schedule, tasks)
yield() # Allow scheduled tasks to run
return (chnls, tasks)
end
function close_chnl_on_taskdone(t::Task, ref::WeakRef)
if ref.value !== nothing
c = ref.value
!isopen(c) && return
if istaskfailed(t)
c.state = :closed
c.excp = task_result(t)
notify_error(c)
else
close(c)
end
end
end
struct InvalidStateException <: Exception
msg::AbstractString
state::Symbol
end
"""
put!(c::Channel, v)
Append an item `v` to the channel `c`. Blocks if the channel is full.
For unbuffered channels, blocks until a [`take!`](@ref) is performed by a different
task.
"""
function put!(c::Channel, v)
check_channel_state(c)
isbuffered(c) ? put_buffered(c,v) : put_unbuffered(c,v)
end
function put_buffered(c::Channel, v)
while length(c.data) == c.sz_max
wait(c.cond_put)
end
push!(c.data, v)
# notify all, since some of the waiters may be on a "fetch" call.
notify(c.cond_take, nothing, true, false)
v
end
function put_unbuffered(c::Channel, v)
if length(c.takers) == 0
push!(c.putters, current_task())
c.waiters > 0 && notify(c.cond_take, nothing, false, false)
try
wait()
catch ex
filter!(x->x!=current_task(), c.putters)
rethrow(ex)
end
end
taker = popfirst!(c.takers)
yield(taker, v) # immediately give taker a chance to run, but don't block the current task
return v
end
push!(c::Channel, v) = put!(c, v)
"""
fetch(c::Channel)
Wait for and get the first available item from the channel. Does not
remove the item. `fetch` is unsupported on an unbuffered (0-size) channel.
"""
fetch(c::Channel) = isbuffered(c) ? fetch_buffered(c) : fetch_unbuffered(c)
function fetch_buffered(c::Channel)
wait(c)
c.data[1]
end
fetch_unbuffered(c::Channel) = throw(ErrorException("`fetch` is not supported on an unbuffered Channel."))
"""
take!(c::Channel)
Remove and return a value from a [`Channel`](@ref). Blocks until data is available.
For unbuffered channels, blocks until a [`put!`](@ref) is performed by a different
task.
"""
take!(c::Channel) = isbuffered(c) ? take_buffered(c) : take_unbuffered(c)
function take_buffered(c::Channel)
wait(c)
v = popfirst!(c.data)
notify(c.cond_put, nothing, false, false) # notify only one, since only one slot has become available for a put!.
v
end
popfirst!(c::Channel) = take!(c)
# 0-size channel
function take_unbuffered(c::Channel{T}) where T
check_channel_state(c)
push!(c.takers, current_task())
try
if length(c.putters) > 0
let refputter = Ref(popfirst!(c.putters))
return Base.try_yieldto(refputter) do putter
# if we fail to start putter, put it back in the queue
putter === current_task || pushfirst!(c.putters, putter)
end::T
end
else
return wait()::T
end
catch ex
filter!(x->x!=current_task(), c.takers)
rethrow(ex)
end
end
"""
isready(c::Channel)
Determine whether a [`Channel`](@ref) has a value stored to it. Returns
immediately, does not block.
For unbuffered channels returns `true` if there are tasks waiting
on a [`put!`](@ref).
"""
isready(c::Channel) = n_avail(c) > 0
n_avail(c::Channel) = isbuffered(c) ? length(c.data) : length(c.putters)
wait(c::Channel) = isbuffered(c) ? wait_impl(c) : wait_unbuffered(c)
function wait_impl(c::Channel)
while !isready(c)
check_channel_state(c)
wait(c.cond_take)
end
nothing
end
function wait_unbuffered(c::Channel)
c.waiters += 1
try
wait_impl(c)
finally
c.waiters -= 1
end
nothing
end
function notify_error(c::Channel, err)
notify_error(c.cond_take, err)
notify_error(c.cond_put, err)
# release tasks on a `wait()/yieldto()` call (on unbuffered channels)
if !isbuffered(c)
waiters = filter!(t->(t.state == :runnable), vcat(c.takers, c.putters))
foreach(t->schedule(t, err; error=true), waiters)
end
end
notify_error(c::Channel) = notify_error(c, c.excp)
eltype(::Type{Channel{T}}) where {T} = T
show(io::IO, c::Channel) = print(io, "$(typeof(c))(sz_max:$(c.sz_max),sz_curr:$(n_avail(c)))")
function iterate(c::Channel, state=nothing)
try
return (take!(c), nothing)
catch e
if isa(e, InvalidStateException) && e.state==:closed
return nothing
else
rethrow(e)
end
end
end
IteratorSize(::Type{<:Channel}) = SizeUnknown()