Add Base.isdone(itr::EachLine, state...) (#39562)

With this method, `isempty` won't consume values from the iterator (See #27412).

Author: cmcaine

base/io.jl

@@ -1023,6 +1023,8 @@ eltype(::Type{<:EachLine}) = String
 
 IteratorSize(::Type{<:EachLine}) = SizeUnknown()
 
+isdone(itr::EachLine, state...) = eof(itr.stream)
+
 struct ReadEachIterator{T, IOT <: IO}
     stream::IOT
 end

test/read.jl

@@ -617,3 +617,12 @@ let p = Pipe()
     wait(t)
     close(p)
 end
+
+@testset "issue #27412" begin
+    itr = eachline(IOBuffer("a"))
+    @test !isempty(itr)
+    # check that the earlier isempty did not consume the iterator
+    @test !isempty(itr)
+    first(itr) # consume the iterator
+    @test  isempty(itr) # now it is empty
+end