channels.jl
# This file is a part of Julia. License is MIT: https://julialang.org/license
"""
AbstractChannel{T}
Representation of a channel passing objects of type `T`.
"""
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::Threads.Condition # waiting for data to become available
cond_wait::Threads.Condition # waiting for data to become maybe available
cond_put::Threads.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
function Channel{T}(sz::Integer) where T
if sz < 0
throw(ArgumentError("Channel size must be either 0, a positive integer or Inf"))
end
lock = ReentrantLock()
cond_put, cond_take = Threads.Condition(lock), Threads.Condition(lock)
cond_wait = (sz == 0 ? Threads.Condition(lock) : cond_take) # wait is distinct from take iff unbuffered
return new(cond_take, cond_wait, cond_put, :open, nothing, Vector{T}(), sz)
end
end
function Channel{T}(sz::Float64) where T
sz = (sz == Inf ? typemax(Int) : convert(Int, sz))
return Channel{T}(sz)
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)
excp = c.excp
excp !== nothing && throw(excp)
throw(closed_exception())
end
end
"""
close(c::Channel[, excp::Exception])
Close a channel. An exception (optionally given by `excp`), is thrown by:
* [`put!`](@ref) on a closed channel.
* [`take!`](@ref) and [`fetch`](@ref) on an empty, closed channel.
"""
function close(c::Channel, excp::Exception=closed_exception())
lock(c)
try
c.state = :closed
c.excp = excp
notify_error(c.cond_take, excp)
notify_error(c.cond_wait, excp)
notify_error(c.cond_put, excp)
finally
unlock(c)
end
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)
# TODO: implement "schedulewait" and deprecate taskdone_hook
#T = Task(() -> close_chnl_on_taskdone(task, c))
#schedulewait(task, T)
register_taskdone_hook(task, tsk -> close_chnl_on_taskdone(tsk, c))
return 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, c::Channel)
isopen(c) || return
cleanup = () -> try
isopen(c) || return
if istaskfailed(t)
excp = task_result(t)
if excp isa Exception
close(c, excp)
return
end
end
close(c)
return
finally
unlock(c)
end
if trylock(c)
# can't use `lock`, since attempts to task-switch to wait for it
# will just silently fail and leave us with broken state
cleanup()
else
# so schedule this to happen once we are finished destroying our task
# (on a new Task)
@async (lock(c); cleanup())
end
nothing
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.
!!! compat "Julia 1.1"
`v` now gets converted to the channel's type with [`convert`](@ref) as `put!` is called.
"""
function put!(c::Channel{T}, v) where T
check_channel_state(c)
v = convert(T, v)
return isbuffered(c) ? put_buffered(c, v) : put_unbuffered(c, v)
end
function put_buffered(c::Channel, v)
lock(c)
try
while length(c.data) == c.sz_max
check_channel_state(c)
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)
finally
unlock(c)
end
return v
end
function put_unbuffered(c::Channel, v)
lock(c)
taker = try
while isempty(c.cond_take.waitq)
check_channel_state(c)
notify(c.cond_wait)
wait(c.cond_put)
end
# unfair scheduled version of: notify(c.cond_take, v, false, false); yield()
popfirst!(c.cond_take.waitq)
finally
unlock(c)
end
# unfair version of: schedule(taker, v); yield()
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)
lock(c)
try
while isempty(c.data)
check_channel_state(c)
wait(c.cond_take)
end
return c.data[1]
finally
unlock(c)
end
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)
lock(c)
try
while isempty(c.data)
check_channel_state(c)
wait(c.cond_take)
end
v = popfirst!(c.data)
notify(c.cond_put, nothing, false, false) # notify only one, since only one slot has become available for a put!.
return v
finally
unlock(c)
end
end
popfirst!(c::Channel) = take!(c)
# 0-size channel
function take_unbuffered(c::Channel{T}) where T
lock(c)
try
check_channel_state(c)
notify(c.cond_put, nothing, false, false)
return wait(c.cond_take)::T
finally
unlock(c)
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.cond_put.waitq)
lock(c::Channel) = lock(c.cond_take)
unlock(c::Channel) = unlock(c.cond_take)
trylock(c::Channel) = trylock(c.cond_take)
function wait(c::Channel)
isready(c) && return
lock(c)
try
while !isready(c)
check_channel_state(c)
wait(c.cond_wait)
end
finally
unlock(c)
end
nothing
end
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()
end
end
end
IteratorSize(::Type{<:Channel}) = SizeUnknown()