lib: add plugin support

This enables loading plugins from RCLONE_PLUGIN_PATH if set.

lib/plugin/package.go

@@ -0,0 +1,16 @@
+// Package plugin implements loading out-of-tree storage backends
+// using https://golang.org/pkg/plugin/ on Linux and macOS.
+//
+// If the $RCLONE_PLUGIN_PATH is present, any Go plugins in that dir
+// named like librcloneplugin_NAME.so will be loaded.
+//
+// To create a plugin, write the backend package like it was in-tree
+// but set the package name to "main". Then, build the plugin with
+//
+//     go build -buildmode=plugin -o librcloneplugin_NAME.so
+//
+// where NAME equals the plugin's fs.RegInfo.Name.
+package plugin
+
+// Build for plugin for unsupported platforms to stop go complaining
+// about "no buildable Go source files".

lib/plugin/plugin.go

@@ -0,0 +1,41 @@
+// +build darwin linux
+
+package plugin
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"plugin"
+	"strings"
+)
+
+func init() {
+	dir := os.Getenv("RCLONE_PLUGIN_PATH")
+	if dir == "" {
+		return
+	}
+	// Get file names of plugin dir
+	listing, err := ioutil.ReadDir(dir)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "Failed to open plugin directory:", err)
+	}
+	// Enumerate file names, load valid plugins
+	for _, file := range listing {
+		// Match name
+		fileName := file.Name()
+		if !strings.HasPrefix(fileName, "librcloneplugin_") {
+			continue
+		}
+		if !strings.HasSuffix(fileName, ".so") {
+			continue
+		}
+		// Try to load plugin
+		_, err := plugin.Open(filepath.Join(dir, fileName))
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Failed to load plugin %s: %s\n",
+				fileName, err)
+		}
+	}
+}