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/plugins.go

@@ -108,6 +108,15 @@ func (p *Plugin) activateWithLock() error {
 	return nil
 }
 
+func (p *Plugin) implements(kind string) bool {
+	for _, driver := range p.Manifest.Implements {
+		if driver == kind {
+			return true
+		}
+	}
+	return false
+}
+
 func load(name string) (*Plugin, error) {
 	return loadWithRetry(name, true)
 }
@@ -166,11 +175,9 @@ func Get(name, imp string) (*Plugin, error) {
 	if err != nil {
 		return nil, err
 	}
-	for _, driver := range pl.Manifest.Implements {
-		logrus.Debugf("%s implements: %s", name, driver)
-		if driver == imp {
-			return pl, nil
-		}
+	if pl.implements(imp) {
+		logrus.Debugf("%s implements: %s", name, imp)
+		return pl, nil
 	}
 	return nil, ErrNotImplements
 }
@@ -179,3 +186,37 @@ func Get(name, imp string) (*Plugin, error) {
 func Handle(iface string, fn func(string, *Client)) {
 	extpointHandlers[iface] = fn
 }
+
+// GetAll returns all the plugins for the specified implementation
+func GetAll(imp string) ([]*Plugin, error) {
+	pluginNames, err := Scan()
+	if err != nil {
+		return nil, err
+	}
+
+	type plLoad struct {
+		pl  *Plugin
+		err error
+	}
+
+	chPl := make(chan plLoad, len(pluginNames))
+	for _, name := range pluginNames {
+		go func(name string) {
+			pl, err := loadWithRetry(name, false)
+			chPl <- plLoad{pl, err}
+		}(name)
+	}
+
+	var out []*Plugin
+	for i := 0; i < len(pluginNames); i++ {
+		pl := <-chPl
+		if pl.err != nil {
+			logrus.Error(err)
+			continue
+		}
+		if pl.pl.implements(imp) {
+			out = append(out, pl.pl)
+		}
+	}
+	return out, nil
+}