[thin_dump] --format custom=<shared lib>

Allow people to use their own emitters held in a shared library.

Put a trivial emitter in contrib/ as an example.

thin-provisioning/shared_library_emitter.cc

@@ -0,0 +1,29 @@
+#include "thin-provisioning/shared_library_emitter.h"
+
+#include <dlfcn.h>
+#include <stdexcept>
+
+using namespace std;
+using namespace thin_provisioning;
+
+//----------------------------------------------------------------
+
+emitter::ptr
+thin_provisioning::create_custom_emitter(string const &shared_lib, ostream &out)
+{
+	emitter::ptr (*create_fn)(ostream &out);
+	void *handle = dlopen(shared_lib.c_str(), RTLD_LAZY);
+	if (!handle)
+		throw runtime_error(dlerror());
+
+	dlerror();    // Clear any existing error
+	create_fn = reinterpret_cast<emitter::ptr (*)(ostream &)>(dlsym(handle, "create_emitter"));
+
+	char *error = dlerror();
+	if (error)
+		throw runtime_error(error);
+
+	return create_fn(out);
+}
+
+//----------------------------------------------------------------