Extracting underlying C context object

You can now use the Context:lightuserdata() method to get a light
userdata pointer to the underlying ZeroMQ C context object.  On the
receiving end, the zmq.init() function can now take in a light
userdata object, which creates a new Context object wrapper for an
existing C context object.

The main use case for this is to provide an easy way to pass a ZeroMQ
context to another Lua interpreter.  The two interpreters (which
presumably would be running in separate threads) can then use ZeroMQ
sockets from this context to communicate with each other.

In this use case, we have to be sure not call zmq_term in the Context
__gc metamethod, since both Lua interpreters would try to do this.
Internally, we keep an additional boolean field that indicates whether
a Context object is responsible for terminating the context.  When
created with a light userdata, this will be false.  When created
normally, the default is true, but can be overridden.

zmq.c

@@ -33,7 +33,10 @@
 #define MT_ZMQ_CONTEXT "MT_ZMQ_CONTEXT"
 #define MT_ZMQ_SOCKET  "MT_ZMQ_SOCKET"
 
-typedef struct { void *ptr; } zmq_ptr;
+typedef struct {
+    void *ptr;
+    int should_free;
+} zmq_ptr;
 
 static int Lzmq_version(lua_State *L)
 {
@@ -66,16 +69,35 @@ static int Lzmq_version(lua_State *L)
 
 static int Lzmq_init(lua_State *L)
 {
-    int io_threads = luaL_checkint(L, 1);
     zmq_ptr *ctx = lua_newuserdata(L, sizeof(zmq_ptr));
+    luaL_getmetatable(L, MT_ZMQ_CONTEXT);
+    lua_setmetatable(L, -2);
+
+    if (lua_islightuserdata(L, 1)) {
+        // Treat a light userdata as a raw ZMQ context object, which
+        // we'll silently wrap.  (And we won't automatically call term
+        // on it.)
+
+        ctx->ptr = lua_touserdata(L, 1);
+        ctx->should_free = 0;
+        return 1;
+    }
+
+    int io_threads = luaL_checkint(L, 1);
+
     ctx->ptr = zmq_init(io_threads);
 
     if (!ctx->ptr) {
         zmq_return_error();
     }
 
-    luaL_getmetatable(L, MT_ZMQ_CONTEXT);
-    lua_setmetatable(L, -2);
+    // toboolean defaults to false, but we want a missing param #2
+    // to mean true
+    if (lua_isnil(L, 2)) {
+        ctx->should_free = 1;
+    } else {
+        ctx->should_free = lua_toboolean(L, 2);
+    }
 
     return 1;
 }
@@ -97,6 +119,23 @@ static int Lzmq_term(lua_State *L)
     return 1;
 }
 
+static int Lzmq_ctx_gc(lua_State *L)
+{
+    zmq_ptr *ctx = luaL_checkudata(L, 1, MT_ZMQ_CONTEXT);
+    if (ctx->should_free) {
+        return Lzmq_term(L);
+    } else {
+        return 0;
+    }
+}
+
+static int Lzmq_ctx_lightuserdata(lua_State *L)
+{
+    zmq_ptr *ctx = luaL_checkudata(L, 1, MT_ZMQ_CONTEXT);
+    lua_pushlightuserdata(L, ctx->ptr);
+    return 1;
+}
+
 static int Lzmq_socket(lua_State *L)
 {
     zmq_ptr *ctx = luaL_checkudata(L, 1, MT_ZMQ_CONTEXT);
@@ -338,7 +377,8 @@ static const luaL_reg zmqlib[] = {
 };
 
 static const luaL_reg ctxmethods[] = {
-    {"__gc",       Lzmq_term},
+    {"__gc",       Lzmq_ctx_gc},
+    {"lightuserdata", Lzmq_ctx_lightuserdata},
     {"term",       Lzmq_term}, 
     {"socket",     Lzmq_socket},
     {NULL,         NULL}