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

push!(c::AbstractChannel, v) = (put!(c, v); c)
popfirst!(c::AbstractChannel) = take!(c)

"""
    Channel{T=Any}(size::Int=0)

Constructs a `Channel` with an internal buffer that can hold a maximum of `size` 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()`: default constructor, equivalent to `Channel{Any}(0)`
* `Channel(Inf)`: equivalent to `Channel{Any}(typemax(Int))`
* `Channel(sz)`: equivalent to `Channel{Any}(sz)`

!!! compat "Julia 1.3"
    The default constructor `Channel()` and default `size=0` were added in Julia 1.3.
"""
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
    @atomic state::Symbol
    excp::Union{Exception, Nothing}      # exception to be thrown when state !== :open

    data::Vector{T}
    @atomic n_avail_items::Int           # Available items for taking, can be read without lock
    sz_max::Int                          # maximum size of channel

    function Channel{T}(sz::Integer = 0) 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}(), 0, 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=0) = Channel{Any}(sz)

# special constructors
"""
    Channel{T=Any}(func::Function, size=0; taskref=nothing, spawn=false)

Create a new task from `func`, bind it to a new channel of type
`T` and size `size`, and schedule the task, all in a single call.
The channel is automatically closed when the task terminates.

`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
the keyword argument `taskref`.

If `spawn = true`, the Task created for `func` may be scheduled on another thread
in parallel, equivalent to creating a task via [`Threads.@spawn`](@ref).

Return a `Channel`.

# Examples
```jldoctest
julia> chnl = Channel() do ch
           foreach(i -> put!(ch, i), 1:4)
       end;

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(taskref=taskref) do ch
           println(take!(ch))
       end;

julia> istaskdone(taskref[])
false

julia> put!(chnl, "Hello");
Hello

julia> istaskdone(taskref[])
true
```

!!! compat "Julia 1.3"
    The `spawn=` parameter was added in Julia 1.3. This constructor was added in Julia 1.3.
    In earlier versions of Julia, Channel used keyword arguments to set `size` and `T`, but
    those constructors are deprecated.

```jldoctest
julia> chnl = Channel{Char}(1, spawn=true) do ch
           for c in "hello world"
               put!(ch, c)
           end
       end
Channel{Char}(1) (2 items available)

julia> String(collect(chnl))
"hello world"
```
"""
function Channel{T}(func::Function, size=0; taskref=nothing, spawn=false) where T
    chnl = Channel{T}(size)
    task = Task(() -> func(chnl))
    task.sticky = !spawn
    bind(chnl, task)
    if spawn
        schedule(task) # start it on (potentially) another thread
    else
        yield(task) # immediately start it, yielding the current thread
    end
    isa(taskref, Ref{Task}) && (taskref[] = task)
    return chnl
end
Channel(func::Function, args...; kwargs...) = Channel{Any}(func, args...; kwargs...)

# This constructor is deprecated as of Julia v1.3, and should not be used.
# (Note that this constructor also matches `Channel(::Function)` w/out any kwargs, which is
# of course not deprecated.)
# We use `nothing` default values to check which arguments were set in order to throw the
# deprecation warning if users try to use `spawn=` with `ctype=` or `csize=`.
function Channel(func::Function; ctype=nothing, csize=nothing, taskref=nothing, spawn=nothing)
    # The spawn= keyword argument was added in Julia v1.3, and cannot be used with the
    # deprecated keyword arguments `ctype=` or `csize=`.
    if (ctype !== nothing || csize !== nothing) && spawn !== nothing
        throw(ArgumentError("Cannot set `spawn=` in the deprecated constructor `Channel(f; ctype=Any, csize=0)`. Please use `Channel{T=Any}(f, size=0; taskref=nothing, spawn=false)` instead!"))
    end
    # Set the actual default values for the arguments.
    ctype === nothing && (ctype = Any)
    csize === nothing && (csize = 0)
    spawn === nothing && (spawn = false)
    return Channel{ctype}(func, csize; taskref=taskref, spawn=spawn)
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)
        # if the monotonic load succeed, now do an acquire fence
        (@atomic :acquire c.state) === :open && concurrency_violation()
        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.excp = excp
        @atomic :release c.state = :closed
        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) = ((@atomic :monotonic 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: TaskFailedException
Stacktrace:
[...]
    nested task error: foo
[...]
```
"""
function bind(c::Channel, task::Task)
    T = Task(() -> close_chnl_on_taskdone(task, c))
    _wait2(task, T)
    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
    lock(c)
    try
        isopen(c) || return
        if istaskfailed(t)
            close(c, TaskFailedException(t))
            return
        end
        close(c)
    finally
        unlock(c)
    end
    nothing
end

struct InvalidStateException <: Exception
    msg::String
    state::Symbol
end
showerror(io::IO, ex::InvalidStateException) = print(io, "InvalidStateException: ", ex.msg)

"""
    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

# Atomically update channel n_avail, *assuming* we hold the channel lock.
function _increment_n_avail(c, inc)
    # We hold the channel lock so it's safe to non-atomically read and
    # increment c.n_avail_items
    newlen = c.n_avail_items + inc
    # Atomically store c.n_avail_items to prevent data races with other threads
    # reading this outside the lock.
    @atomic :monotonic c.n_avail_items = newlen
end

function put_buffered(c::Channel, v)
    lock(c)
    did_buffer = false
    try
        # Increment channel n_avail eagerly (before push!) to count data in the
        # buffer as well as offers from tasks which are blocked in wait().
        _increment_n_avail(c, 1)
        while length(c.data) == c.sz_max
            check_channel_state(c)
            wait(c.cond_put)
        end
        check_channel_state(c)
        push!(c.data, v)
        did_buffer = true
        # notify all, since some of the waiters may be on a "fetch" call.
        notify(c.cond_take, nothing, true, false)
    finally
        # Decrement the available items if this task had an exception before pushing the
        # item to the buffer (e.g., during `wait(c.cond_put)`):
        did_buffer || _increment_n_avail(c, -1)
        unlock(c)
    end
    return v
end

function put_unbuffered(c::Channel, v)
    lock(c)
    taker = try
        _increment_n_avail(c, 1)
        while isempty(c.cond_take.waitq)
            check_channel_state(c)
            notify(c.cond_wait)
            wait(c.cond_put)
        end
        check_channel_state(c)
        # unfair scheduled version of: notify(c.cond_take, v, false, false); yield()
        popfirst!(c.cond_take.waitq)
    finally
        _increment_n_avail(c, -1)
        unlock(c)
    end
    schedule(taker, v)
    yield()  # immediately give taker a chance to run, but don't block the current task
    return v
end

"""
    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)
        _increment_n_avail(c, -1)
        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

# 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
isempty(c::Channel) = n_avail(c) == 0
function n_avail(c::Channel)
    # Lock-free equivalent to `length(c.data) + length(c.cond_put.waitq)`
    @atomic :monotonic c.n_avail_items
end

lock(c::Channel) = lock(c.cond_take)
lock(f, c::Channel) = lock(f, 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), "(", c.sz_max, ")")

function show(io::IO, ::MIME"text/plain", c::Channel)
    show(io, c)
    if !(get(io, :compact, false)::Bool)
        if !isopen(c)
            print(io, " (closed)")
        else
            n = n_avail(c)
            if n == 0
                print(io, " (empty)")
            else
                s = n == 1 ? "" : "s"
                print(io, " (", n, " item$s available)")
            end
        end
    end
end

function iterate(c::Channel, state=nothing)
    if isopen(c) || isready(c)
        try
            return (take!(c), nothing)
        catch e
            if isa(e, InvalidStateException) && e.state === :closed
                return nothing
            else
                rethrow()
            end
        end
    else
        return nothing
    end
end

IteratorSize(::Type{<:Channel}) = SizeUnknown()