Load external .so plugins

Original patch written by sparrc, brought up to date by timidger.
Original from 92d8a2e message follows:

support for the Go 1.8 shared object feature of loading external
plugins.

this support relies on the developer defining a `Plugin` symbol in their
.so file that is a telegraf plugin interface.

So instead of the plugin developer "Adding" their own plugin to the
telegraf registry, telegraf loads the .so, looks up the Plugin symbol,
and then adds it if it finds it.

The name of the plugin is determined by telegraf, and is namespaced
based on the filename and path.

see #1717

cmd/telegraf/telegraf.go

@@ -10,20 +10,26 @@ import (
 	_ "net/http/pprof" // Comment this line to disable pprof endpoint.
 	"os"
 	"os/signal"
+	"path"
+	"path/filepath"
+	"plugin"
 	"runtime"
 	"strings"
 	"syscall"
 	"time"
 
+	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/agent"
 	"github.com/influxdata/telegraf/internal"
 	"github.com/influxdata/telegraf/internal/config"
 	"github.com/influxdata/telegraf/logger"
+	"github.com/influxdata/telegraf/plugins/aggregators"
 	_ "github.com/influxdata/telegraf/plugins/aggregators/all"
 	"github.com/influxdata/telegraf/plugins/inputs"
 	_ "github.com/influxdata/telegraf/plugins/inputs/all"
 	"github.com/influxdata/telegraf/plugins/outputs"
 	_ "github.com/influxdata/telegraf/plugins/outputs/all"
+	"github.com/influxdata/telegraf/plugins/processors"
 	_ "github.com/influxdata/telegraf/plugins/processors/all"
 	"github.com/kardianos/service"
 )
@@ -64,6 +70,8 @@ var fService = flag.String("service", "",
 var fServiceName = flag.String("service-name", "telegraf", "service name (windows only)")
 var fServiceDisplayName = flag.String("service-display-name", "Telegraf Data Collector Service", "service display name (windows only)")
 var fRunAsConsole = flag.Bool("console", false, "run as console application (windows only)")
+var fPlugins = flag.String("external-plugins", "",
+	"path to directory containing external plugins")
 
 var (
 	version string
@@ -111,6 +119,67 @@ func reloadLoop(
 	}
 }
 
+// loadExternalPlugins loads external plugins from shared libraries (.so, .dll, etc.)
+// in the specified directory.
+func loadExternalPlugins(rootDir string) error {
+	return filepath.Walk(rootDir, func(pth string, info os.FileInfo, err error) error {
+		// Stop if there was an error.
+		if err != nil {
+			return err
+		}
+
+		// Ignore directories.
+		if info.IsDir() {
+			return nil
+		}
+
+		// Ignore files that aren't shared libraries.
+		ext := strings.ToLower(path.Ext(pth))
+		if ext != ".so" && ext != ".dll" {
+			return nil
+		}
+
+		// name will be the path to the plugin file beginning at the root
+		// directory, minus the extension.
+		// ie, if the plugin file is /opt/telegraf-plugins/group1/foo.so, name
+		// will be "external/group1/foo"
+		name := strings.TrimPrefix(strings.TrimPrefix(pth, rootDir), string(os.PathSeparator))
+		name = strings.TrimSuffix(name, filepath.Ext(pth))
+		name = "external" + string(os.PathSeparator) + name
+
+		// Load plugin.
+		p, err := plugin.Open(pth)
+		if err != nil {
+			return fmt.Errorf("error loading [%s]: %s", pth, err)
+		}
+
+		s, err := p.Lookup("Plugin")
+		if err != nil {
+			fmt.Printf("ERROR Could not find 'Plugin' symbol in [%s]\n", pth)
+			return nil
+		}
+
+		switch tplugin := s.(type) {
+		case *telegraf.Input:
+			fmt.Printf("Adding external input plugin: %s\n", name)
+			inputs.Add(name, func() telegraf.Input { return *tplugin })
+		case *telegraf.Output:
+			fmt.Printf("Adding external output plugin: %s\n", name)
+			outputs.Add(name, func() telegraf.Output { return *tplugin })
+		case *telegraf.Processor:
+			fmt.Printf("Adding external processor plugin: %s\n", name)
+			processors.Add(name, func() telegraf.Processor { return *tplugin })
+		case *telegraf.Aggregator:
+			fmt.Printf("Adding external aggregator plugin: %s\n", name)
+			aggregators.Add(name, func() telegraf.Aggregator { return *tplugin })
+		default:
+			fmt.Printf("ERROR: 'Plugin' symbol from [%s] is not a telegraf interface, it has type: %T\n", pth, tplugin)
+		}
+
+		return nil
+	})
+}
+
 func runAgent(ctx context.Context,
 	inputFilters []string,
 	outputFilters []string,
@@ -279,6 +348,18 @@ func main() {
 		processorFilters = strings.Split(":"+strings.TrimSpace(*fProcessorFilters)+":", ":")
 	}
 
+	// Load external plugins, if requested.
+	if *fPlugins != "" {
+		pluginsDir, err := filepath.Abs(*fPlugins)
+		if err != nil {
+			log.Fatal(err.Error())
+		}
+		fmt.Printf("Loading external plugins from: %s\n", pluginsDir)
+		if err := loadExternalPlugins(*fPlugins); err != nil {
+			log.Fatal(err.Error())
+		}
+	}
+
 	if *pprofAddr != "" {
 		go func() {
 			pprofHostPort := *pprofAddr

internal/usage.go

@@ -16,6 +16,9 @@ The commands & flags are:
   --aggregator-filter <filter>   filter the aggregators to enable, separator is :
   --config <file>                configuration file to load
   --config-directory <directory> directory containing additional *.conf files
+  --external-plugins             directory containing *.so files, this directory will be
+                                 searched recursively. Any Plugin found will be loaded
+                                 and namespaced.
   --debug                        turn on debug logging
   --input-filter <filter>        filter the inputs to enable, separator is :
   --input-list                   print available input plugins.