dynamic loading of mapnik datasource plugins should now work on linux…

… - workaround for lack of ability to set dlopen flags in node process to RTLD_NOW is to preload libmapnik.so symbols by having mapnik.node load itself using the RTLD_NOW flag - yes this is nuts and will hopefully be removed soon by fix upstream in node, see: https://github.com/ry/node/issues/issue/436

mapnik/index.js

@@ -17,6 +17,21 @@ function warning(issue,manual_function)
     console.log(msg);
 }
 
+// hack to force libmapnik symbols to be loaded with RTLD_NOW|RTLD_GLOBAL
+// so that datasource plugins will have access to symbols since node.cc (process.dlopen)
+// uses RTLD_LAZY in its call to dlopen and has no mechanism to set the RTLD_NOW flag
+// not needed on darwin because mapnik input plugins are directly linked to libmapnik
+if (process.platform != 'darwin') {
+    var mapnik_node = path.join(__dirname,'_mapnik.node');
+    if (!path.existsSync(mapnik_node))
+        console.log(mapnik_node + ' does not exist, loading plugins will fail on linux');
+
+    var loaded = mapnik.make_mapnik_symbols_visible(mapnik_node);
+
+    if (!loaded)
+        console.log('Warning, attempt to pre-load mapnik symbols did not work, see FAQ for potential solutions');
+}
+
 if (settings.paths.fonts && path.existsSync(settings.paths.fonts)) {
    mapnik.register_fonts(settings.paths.fonts);
 } else {

src/_mapnik.cc

@@ -6,6 +6,9 @@
 #include <node_buffer.h>
 #include <node_version.h>
 
+/* dlopen(), dlsym() */
+#include <dlfcn.h> 
+
 // mapnik
 #include <mapnik/map.hpp>
 #include <mapnik/projection.hpp>
@@ -534,6 +537,23 @@ class Map: ObjectWrap
 
 Persistent<FunctionTemplate> Map::m_template;
 
+static Handle<Value> make_mapnik_symbols_visible(const Arguments& args)
+{
+  if (args.Length() != 1 || !args[0]->IsString())
+    ThrowException(Exception::TypeError(
+      String::New("first argument must be a path to a directory holding _mapnik.node")));
+  String::Utf8Value filename(args[0]->ToString());
+  void *handle = dlopen(*filename, RTLD_NOW|RTLD_GLOBAL);
+  if (handle == NULL) {
+      return False();
+  }
+  else
+  {
+      dlclose(handle);
+      return True();
+  }
+}
+
 static Handle<Value> register_datasources(const Arguments& args)
 {
   if (args.Length() != 1 || !args[0]->IsString())
@@ -707,6 +727,7 @@ extern "C" {
   static void init (Handle<Object> target)
   {
     // module level functions
+    NODE_SET_METHOD(target, "make_mapnik_symbols_visible", make_mapnik_symbols_visible);
     NODE_SET_METHOD(target, "register_datasources", register_datasources);
     NODE_SET_METHOD(target, "register_fonts", register_fonts);