Implement and export isfull (#53159)

This PR implements `isfull(c::Channel)`. 

It calls `n_avail(c) ≥ c.sz_max` in all cases. The original
implementation was inspired by [this
comment](https://discourse.julialang.org/t/function-to-check-if-channel-is-full/44795/3?u=thelatekronos),
and therefore had a special case for unbuffered channels, which fell
back to `isready`. I opted against this behaviour, because it fails to
respect that an unbuffered channel is always full, in two important
senses:
1) The number of elements available is greater than or equal the
capacity
    2) A call to `put!` will block

With the current implementation, the behaviour is simply understood and
summarized in all cases by the start of the docstring:
> Determines whether a `Channel` is full, in the 
sense that calling `put!(c, some_value)` will block.

Shoutout to @SamuraiAku for their work in
#40720, which helped me a lot on
thinking this through, and remembering to change all relevant files. In
particular, the detail around how `c.cond_take.waitq` may result in
immediate unblocking, which is a really important caveat on a function
that may be used to check if `put!`ing will block. However, for buffered
channels, `isfull` is extremely close to `putwillblock` from #40720
(just a little better, with >= instead of ==), and for unbuffered
channels it does not make much sense to see if `put!`ing will block.

This PR is created based on
[this](#22863 (comment))
"call to action".

Checklist:
- [x] Entry in news
- [x] Docstring with example
- [x] Export function
- [x] Mention in manual
- [x] Entry in
[docs-reference](https://docs.julialang.org/en/v1/base/parallel/)

---------

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

NEWS.md

@@ -36,6 +36,7 @@ New library functions
 ---------------------
 
 * `logrange(start, stop; length)` makes a range of constant ratio, instead of constant step ([#39071])
+* The new `isfull(c::Channel)` function can be used to check if `put!(c, some_value)` will block. ([#53159])
 
 New library features
 --------------------

base/channels.jl

@@ -519,7 +519,7 @@ end
 Determines whether a [`Channel`](@ref) has a value stored in it.
 Returns immediately, does not block.
 
-For unbuffered channels returns `true` if there are tasks waiting on a [`put!`](@ref).
+For unbuffered channels, return `true` if there are tasks waiting on a [`put!`](@ref).
 
 # Examples
 
@@ -559,6 +559,47 @@ function n_avail(c::Channel)
     @atomic :monotonic c.n_avail_items
 end
 
+"""
+    isfull(c::Channel)
+
+Determines if a [`Channel`](@ref) is full, in the sense
+that calling `put!(c, some_value)` would have blocked.
+Returns immediately, does not block.
+
+Note that it may frequently be the case that `put!` will
+not block after this returns `true`. Users must take
+precautions not to accidentally create live-lock bugs
+in their code by calling this method, as these are
+generally harder to debug than deadlocks. It is also
+possible that `put!` will block after this call
+returns `false`, if there are multiple producer
+tasks calling `put!` in parallel.
+
+# Examples
+
+Buffered channel:
+```jldoctest
+julia> c = Channel(1); # capacity = 1
+
+julia> isfull(c)
+false
+
+julia> put!(c, 1);
+
+julia> isfull(c)
+true
+```
+
+Unbuffered channel:
+```jldoctest
+julia> c = Channel(); # capacity = 0
+
+julia> isfull(c) # unbuffered channel is always full
+true
+```
+"""
+isfull(c::Channel) = n_avail(c) ≥ c.sz_max
+
 lock(c::Channel) = lock(c.cond_take)
 lock(f, c::Channel) = lock(f, c.cond_take)
 unlock(c::Channel) = unlock(c.cond_take)

base/exports.jl

@@ -719,6 +719,7 @@ export
 # channels
     take!,
     put!,
+    isfull,
     isready,
     fetch,
     bind,

doc/src/base/parallel.md

@@ -62,6 +62,7 @@ Base.Channel
 Base.Channel(::Function)
 Base.put!(::Channel, ::Any)
 Base.take!(::Channel)
+Base.isfull(::Channel)
 Base.isready(::Channel)
 Base.fetch(::Channel)
 Base.close(::Channel)

doc/src/manual/asynchronous-programming.md

@@ -194,10 +194,11 @@ A channel can be visualized as a pipe, i.e., it has a write end and a read end :
     to the maximum number of elements that can be held in the channel at any time. For example, `Channel(32)`
     creates a channel that can hold a maximum of 32 objects of any type. A `Channel{MyType}(64)` can
     hold up to 64 objects of `MyType` at any time.
-  * If a [`Channel`](@ref) is empty, readers (on a [`take!`](@ref) call) will block until data is available.
-  * If a [`Channel`](@ref) is full, writers (on a [`put!`](@ref) call) will block until space becomes available.
+  * If a [`Channel`](@ref) is empty, readers (on a [`take!`](@ref) call) will block until data is available (see [`isempty`](@ref)).
+  * If a [`Channel`](@ref) is full, writers (on a [`put!`](@ref) call) will block until space becomes available (see [`isfull`](@ref)).
   * [`isready`](@ref) tests for the presence of any object in the channel, while [`wait`](@ref)
     waits for an object to become available.
+  * Note that if another task is currently waiting to `put!` an object into a channel, a channel can have more items available than its capacity.
   * A [`Channel`](@ref) is in an open state initially. This means that it can be read from and written to
     freely via [`take!`](@ref) and [`put!`](@ref) calls. [`close`](@ref) closes a [`Channel`](@ref).
     On a closed [`Channel`](@ref), [`put!`](@ref) will fail. For example:

test/channels.jl

@@ -40,6 +40,8 @@ end
     c = Channel()
     @test eltype(c) == Any
     @test c.sz_max == 0
+    @test isempty(c) == true  # Nothing in it
+    @test isfull(c) == true   # But no more room
 
     c = Channel(1)
     @test eltype(c) == Any
@@ -49,6 +51,11 @@ end
     @test isready(c) == false
     @test eltype(Channel(1.0)) == Any
 
+    c = Channel(1)
+    @test isfull(c) == false
+    put!(c, 1)
+    @test isfull(c) == true
+
     c = Channel{Int}(1)
     @test eltype(c) == Int
     @test_throws MethodError put!(c, "Hello")