Add token endpoint

This change adds a github registration endpoint that instances can use
to fetch a github registration token.

This change also invalidates disables access to an instance to the token
and status updates endpoints once the instance transitions from
"pending" or "installing" to any other state.

config/config.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"log"
 	"net"
+	"net/url"
 	"os"
 	"path/filepath"
 	"time"
@@ -169,8 +170,11 @@ type Default struct {
 	// ConfigDir is the folder where the runner may save any aditional files
 	// or configurations it may need. Things like auto-generated SSH keys that
 	// may be used to access the runner instances.
-	ConfigDir   string `toml:"config_dir,omitempty" json:"config-dir,omitempty"`
+	ConfigDir string `toml:"config_dir,omitempty" json:"config-dir,omitempty"`
+	// CallbackURL is the URL where the instances can send back status reports.
 	CallbackURL string `toml:"callback_url" json:"callback-url"`
+	// TokenURL is the URL where instances can fetch a github runner registration token.
+	TokenURL string `toml:"token_url" json:"token-url"`
 	// LogFile is the location of the log file.
 	LogFile           string `toml:"log_file,omitempty" json:"log-file"`
 	EnableLogStreamer bool   `toml:"enable_log_streamer"`
@@ -181,6 +185,17 @@ func (d *Default) Validate() error {
 		return fmt.Errorf("missing callback_url")
 	}
 
+	_, err := url.Parse(d.CallbackURL)
+	if err != nil {
+		return errors.Wrap(err, "validating callback_url")
+	}
+
+	if d.TokenURL != "" {
+		if _, err := url.Parse(d.TokenURL); err != nil {
+			return errors.Wrap(err, "validating token_url")
+		}
+	}
+
 	if d.ConfigDir == "" {
 		return fmt.Errorf("config_dir cannot be empty")
 	}