Give more details when plugin discovery fails for a legit reason.

Skip files the whose mime type is not application/x-sharedlib,
which would be the ones allowed to fail loading. All errors on
failed dlopen() attempts now cause the code to throw.

include/backends/backend_loader.hpp

@@ -21,6 +21,7 @@
 #include <string>
 #include <memory>
 #include <boost/utility/string_ref.hpp>
+#include <exception>
 
 namespace YAML {
 	class Node;
@@ -31,6 +32,11 @@ namespace dindb {
 
 	using BackendPtr = std::unique_ptr<dindb::Backend, void(*)(dindb::Backend*)>;
 
+	class SOLoadException : public std::runtime_error {
+	public:
+		explicit SOLoadException (std::string&& parMessage);
+	};
+
 	class BackendPlugin {
 	public:
 		BackendPlugin ( void );

src/backends/backend_loader.cpp

@@ -21,6 +21,9 @@
 #include "backends/backend_loader.hpp"
 #include "backends/exposed_functions.hpp"
 #include "backends/backend_version.hpp"
+#include "dindexer-machinery/scantask/mime.hpp"
+#include "dindexer-machinery/scantask/singlefile.hpp"
+#include "dindexer-machinery/recorddata.hpp"
 #include <dlfcn.h>
 #include <cassert>
 #include <functional>
@@ -72,17 +75,38 @@ namespace dindb {
 			auto get_version = reinterpret_cast<GetVersionFun>(dlsym(parSOHandle, "dindexer_backend_iface_version"));
 			return get_version();
 		}
+
+		std::string get_mime_type (const std::string& parPath) {
+			using mchlib::scantask::SingleFileTask;
+			using mchlib::scantask::Mime;
+			using std::shared_ptr;
+
+			shared_ptr<SingleFileTask> file(new SingleFileTask(parPath));
+			Mime mime(file, true);
+
+			const std::vector<mchlib::FileRecordData>& result = mime.get_or_create();
+			assert(result.size() == 1);
+			boost::string_ref retval = result.front().mime_type();
+			return std::string(retval.data(), retval.size());
+		}
 	} //unnamed namespace
 
+	SOLoadException::SOLoadException (std::string&& parMessage) :
+		std::runtime_error(std::move(parMessage))
+	{
+	}
+
 	std::string backend_name (const std::string& parSOPath) {
 		assert(not parSOPath.empty());
 		using SoHandle = std::unique_ptr<void, int(*)(void*)>;
+		if (get_mime_type(parSOPath) != "application/x-sharedlib")
+			return std::string();
 
 		auto handle = SoHandle(dlopen(parSOPath.c_str(), RTLD_LAZY), &dlclose);
 		if (handle)
 			return backend_name(handle.get());
 		else
-			return std::string();
+			throw SOLoadException(dlerror());
 	}
 
 	BackendPlugin::BackendPlugin() :

src/common/settings.cpp

@@ -49,13 +49,16 @@ namespace dinlib {
 		parOut.backend_name = settings["backend_name"].as<std::string>();
 		const std::string backend_settings_section = parOut.backend_name + "_settings";
 		if (settings[backend_settings_section]) {
+			try {
 				auto settings_node = settings[backend_settings_section];
 				const std::string plugin_path = find_plugin_by_name(split_and_trim(search_paths, ':'), parOut.backend_name);
-			if (plugin_path.empty())
-				throw std::runtime_error(std::string("Unable to find any suitable plugin with the specified name \"") + parOut.backend_name + "\"");
 				parOut.backend_plugin = dindb::BackendPlugin(plugin_path, &settings_node);
 				throw_if_plugin_failed(parOut.backend_plugin, plugin_path, parOut.backend_name);
 			}
+			catch (const dindb::SOLoadException& err) {
+				throw std::runtime_error(std::string("Unable to find any suitable plugin with the specified name \"") + err.what() + "\"");
+			}
+		}
 	}
 
 	namespace {