Add channel state checks to reduce exceptions

In channel iteration and in the `@sync` race check.

base/channels.jl

@@ -493,14 +493,18 @@ function show(io::IO, ::MIME"text/plain", c::Channel)
 end
 
 function iterate(c::Channel, state=nothing)
- try
- return (take!(c), nothing)
- catch e
- if isa(e, InvalidStateException) && e.state === :closed
- return nothing
- else
- rethrow()
+ 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
 

base/task.jl

@@ -424,19 +424,21 @@ function sync_end(c::Channel{Any})
  # Capture all waitable objects scheduled after the end of `@sync` and
  # include them in the exception. This way, the user can check what was
  # scheduled by examining at the exception object.
- local racy
- for r in c
- if !@isdefined(racy)
- racy = []
+ if isready(c)
+ local racy
+ for r in c
+ if !@isdefined(racy)
+ racy = []
+ end
+ push!(racy, r)
  end
- push!(racy, r)
- end
- if @isdefined(racy)
- if !@isdefined(c_ex)
- c_ex = CompositeException()
+ if @isdefined(racy)
+ if !@isdefined(c_ex)
+ c_ex = CompositeException()
+ end
+ # Since this is a clear programming error, show this exception first:
+ pushfirst!(c_ex, ScheduledAfterSyncException(racy))
  end
- # Since this is a clear programming error, show this exception first:
- pushfirst!(c_ex, ScheduledAfterSyncException(racy))
  end
 
  if @isdefined(c_ex)