http(feat): add fallback executor support

This allows _trying_ to execute a command on a daemon, failing, and falling back
on another executor.

http/client.go

@@ -27,6 +27,7 @@ type client struct {
 	httpClient    *http.Client
 	ua            string
 	apiPrefix     string
+	fallback      cmds.Executor
 }
 
 type ClientOpt func(*client)
@@ -43,6 +44,15 @@ func ClientWithAPIPrefix(apiPrefix string) ClientOpt {
 	}
 }
 
+// ClientWithFallback adds a fallback executor to the client.
+//
+// Note: This may run the PreRun function twice.
+func ClientWithFallback(exe cmds.Executor) ClientOpt {
+	return func(c *client) {
+		c.fallback = exe
+	}
+}
+
 func NewClient(address string, opts ...ClientOpt) cmds.Executor {
 	if !strings.HasPrefix(address, "http://") {
 		address = "http://" + address
@@ -79,6 +89,10 @@ func (c *client) Execute(req *cmds.Request, re cmds.ResponseEmitter, env cmds.En
 	res, err := c.send(req)
 	if err != nil {
 		if isConnRefused(err) {
+			if c.fallback != nil {
+				// XXX: this runs the PreRun twice
+				return c.fallback.Execute(req, re, env)
+			}
 			err = fmt.Errorf("cannot connect to the api. Is the daemon running? To run as a standalone CLI command remove the api file in `$IPFS_PATH/api`")
 		}
 		return err