Move responsibility of ls/inspect to volume driver

Makes `docker volume ls` and `docker volume inspect` ask the volume
drivers rather than only using what is cached locally.

Previously in order to use a volume from an external driver, one would
either have to use `docker volume create` or have a container that is
already using that volume for it to be visible to the other volume
API's.

For keeping uniqueness of volume names in the daemon, names are bound to
a driver on a first come first serve basis. If two drivers have a volume
with the same name, the first one is chosen, and a warning is logged
about the second one.

Adds 2 new methods to the plugin API, `List` and `Get`.
If a plugin does not implement these endpoints, a user will not be able
to find the specified volumes as well requests go through the drivers.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

plugins/discovery.go

@@ -25,6 +25,38 @@ func newLocalRegistry() localRegistry {
 	return localRegistry{}
 }
 
+// Scan scans all the plugin paths and returns all the names it found
+func Scan() ([]string, error) {
+	var names []string
+	if err := filepath.Walk(socketsPath, func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return nil
+		}
+
+		if fi.Mode()&os.ModeSocket != 0 {
+			name := strings.TrimSuffix(fi.Name(), filepath.Ext(fi.Name()))
+			names = append(names, name)
+		}
+		return nil
+	}); err != nil {
+		return nil, err
+	}
+
+	for _, path := range specsPaths {
+		if err := filepath.Walk(path, func(p string, fi os.FileInfo, err error) error {
+			if err != nil || fi.IsDir() {
+				return nil
+			}
+			name := strings.TrimSuffix(fi.Name(), filepath.Ext(fi.Name()))
+			names = append(names, name)
+			return nil
+		}); err != nil {
+			return nil, err
+		}
+	}
+	return names, nil
+}
+
 // Plugin returns the plugin registered with the given name (or returns an error).
 func (l *localRegistry) Plugin(name string) (*Plugin, error) {
 	socketpaths := pluginPaths(socketsPath, name, ".sock")