Make isempty(c::Channel) a non-mutating operation (JuliaLang#36641)

Previously, `isempty(c::Channel)` would fall back to `iterate(c) ===
nothing`, which actually consumed a value from the channel.  Instead,
let's just define it in terms of its internal datastructures.

base/channels.jl

@@ -419,6 +419,7 @@ 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)
+isempty(c::Channel) = isbuffered(c) ? isempty(c.data) : isempty(c.cond_put.waitq)
 
 lock(c::Channel) = lock(c.cond_take)
 unlock(c::Channel) = unlock(c.cond_take)

test/channels.jl

@@ -470,6 +470,45 @@ end
  @test_throws InvalidStateException Base.check_channel_state(c)
 end
 
+# PR #36641
+# Ensure that `isempty()` does not mutate a Channel's state:
+@testset "isempty(::Channel) mutation" begin
+ function isempty_timeout(c::Channel)
+ inner_c = Channel{Union{Bool,Nothing}}()
+ @async put!(inner_c, isempty(c))
+ @async begin
+ sleep(0.01)
+ if isopen(inner_c)
+ put!(inner_c, nothing)
+ end
+ end
+ result = take!(inner_c)
+ if result === nothing
+ error("isempty() timed out!")
+ end
+ return result
+ end
+ # First, with a non-buffered channel
+ c = Channel()
+ @test isempty_timeout(c)
+ t_put = @async put!(c, 1)
+ @test !isempty_timeout(c)
+ # check a second time to ensure `isempty(c)` didn't just consume the element.
+ @test !isempty_timeout(c)
+ @test take!(c) == 1
+ @test isempty_timeout(c)
+ wait(t_put)
+
+ # Next, with a buffered channel:
+ c = Channel(2)
+ @test isempty_timeout(c)
+ t_put = put!(c, 1)
+ @test !isempty_timeout(c)
+ @test !isempty_timeout(c)
+ @test take!(c) == 1
+ @test isempty_timeout(c)
+end
+
 # issue #12473
 # make sure 1-shot timers work
 let a = []