Make Socket hold a reference to its Context

This prevents the `Context` from being garbage collected while the `Socket` is
still alive, otherwise the `Socket` would be closed by the `Context`'s finalizer
while the `Socket` is potentially still in use.

src/socket.jl

@@ -19,6 +19,7 @@ Do-block constructor.
 """
 mutable struct Socket
  data::Ptr{Cvoid}
+ context::Context
  pollfd::FDWatcher
 
  """
@@ -31,7 +32,7 @@ mutable struct Socket
  if p == C_NULL
  throw(StateError(jl_zmq_error_str()))
  end
- socket = new(p)
+ socket = new(p, ctx)
  setfield!(socket, :pollfd, FDWatcher(fd(socket), #=readable=#true, #=writable=#false))
  finalizer(close, socket)
  push!(getfield(ctx, :sockets), WeakRef(socket))

test/runtests.jl

@@ -13,7 +13,23 @@ using ZMQ, Test
  @test_throws StateError Socket(ctx, PUB)
 end
 
+# This test is in its own function to keep it simple and try to trick Julia into
+# thinking it can safely GC the Context.
+function context_gc_test()
+ ctx = Context()
+ s = Socket(ctx, PUB)
+
+ # Force garbage collection to attempt to delete ctx
+ GC.gc()
+
+ # But it shouldn't be garbage collected since the socket should have a
+ # reference to it, so the socket should still be open.
+ @test isopen(s)
+end
+
 @testset "ZMQ sockets" begin
+ context_gc_test()
+
  s=Socket(PUB)
  @test s isa Socket
  ZMQ.close(s)