Return socket correctly in socket_reuse_port (#22017)

* Return socket correctly in socket_reuse_port
* Run reuseport tests only if SO_REUSEPORT is supported.
* Test fix - test for successful addition of workers in distributed test

Ref #22017
(cherry picked from commit d3bc329)

base/distributed/managers.jl

@@ -472,6 +472,7 @@ function socket_reuse_port()
  return TCPSocket()
  end
  is_apple() && bind_client_port(s)
+ return s
  else
  return TCPSocket()
  end

test/distributed_exec.jl

@@ -11,10 +11,9 @@ include("testenv.jl")
  end
 
 addprocs_with_testenv(4)
+@test nprocs() == 5
 
-# Test that the client port is reused. SO_REUSEPORT may not be supported on
-# all UNIX platforms, Linux kernels prior to 3.9 and older versions of OSX
-if is_unix()
+function reuseport_tests()
  # Run the test on all processes.
  results = asyncmap(procs()) do p
  remotecall_fetch(p) do
@@ -47,6 +46,19 @@ if is_unix()
  @test all(p -> p in results, procs())
 end
 
+# Test that the client port is reused. SO_REUSEPORT may not be supported on
+# all UNIX platforms, Linux kernels prior to 3.9 and older versions of OSX
+if is_unix()
+ # Run reuse client port tests only if SO_REUSEPORT is supported.
+ s = TCPSocket(delay = false)
+ is_linux() && Base.Distributed.bind_client_port(s)
+ if ccall(:jl_tcp_reuseport, Int32, (Ptr{Void},), s.handle) == 0
+ reuseport_tests()
+ else
+ info("SO_REUSEPORT is unsupported, skipping reuseport tests.")
+ end
+end
+
 id_me = myid()
 id_other = filter(x -> x != id_me, procs())[rand(1:(nprocs()-1))]