Make Socket hold a reference to its Context (JuliaInterop#229)

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

@@ -3,6 +3,7 @@ A ZMQ socket.
 """
 mutable struct Socket
     data::Ptr{Cvoid}
+    context::Context
     pollfd::FDWatcher
 
     @doc """
@@ -15,7 +16,7 @@ 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

@@ -14,7 +14,23 @@ 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)