Add a passthrough directory feature. (#76)

This is only compiled in when a `dev` build tag is specified (see #75)
for safety. Attempting to use the flag without that tag at build time
will fail. However, it is always available within the codebase itself so
that tests can be written against this functionality in a programmatic
way.

Manual testing was performed; no plans to add automated ones as this
isn't production-necessary nor exported functionality.

internal/cmd/commands/controller/controller.go

@@ -21,6 +21,8 @@ import (
 var _ cli.Command = (*Command)(nil)
 var _ cli.CommandAutocomplete = (*Command)(nil)
 
+var devOnlyControllerFlags = func(*Command, *base.FlagSet) {}
+
 type Command struct {
 	*base.Server
 
@@ -37,12 +39,13 @@ type Command struct {
 	flagConfig                         string
 	flagLogLevel                       string
 	flagLogFormat                      string
+	flagCombineLogs                    bool
 	flagDev                            bool
 	flagDevAdminPassword               string
 	flagDevControllerAPIListenAddr     string
 	flagDevControllerClusterListenAddr string
 	flagDevOrgId                       string
-	flagCombineLogs                    bool
+	flagDevPassthroughDirectory        string
 }
 
 func (c *Command) Synopsis() string {
@@ -106,19 +109,17 @@ func (c *Command) Flags() *base.FlagSets {
 	})
 
 	f.StringVar(&base.StringVar{
-		Name:    "dev-org-id",
-		Target:  &c.flagDevOrgId,
-		Default: "",
-		EnvVar:  "WATCHTWER_DEV_ORG_ID",
+		Name:   "dev-org-id",
+		Target: &c.flagDevOrgId,
+		EnvVar: "WATCHTWER_DEV_ORG_ID",
 		Usage: "Auto-created organization ID. This only applies when running in \"dev\" " +
 			"mode.",
 	})
 
 	f.StringVar(&base.StringVar{
-		Name:    "dev-admin-password",
-		Target:  &c.flagDevAdminPassword,
-		Default: "",
-		EnvVar:  "WATCHTWER_DEV_ADMIN_PASSWORD",
+		Name:   "dev-admin-password",
+		Target: &c.flagDevAdminPassword,
+		EnvVar: "WATCHTWER_DEV_ADMIN_PASSWORD",
 		Usage: "Initial admin password. This only applies when running in \"dev\" " +
 			"mode.",
 	})
@@ -138,12 +139,13 @@ func (c *Command) Flags() *base.FlagSets {
 	})
 
 	f.BoolVar(&base.BoolVar{
-		Name:    "combine-logs",
-		Target:  &c.flagCombineLogs,
-		Default: false,
-		Usage:   "If set, both startup information and logs will be sent to stdout. If not set (the default), startup information will go to stdout and logs will be sent to stderr.",
+		Name:   "combine-logs",
+		Target: &c.flagCombineLogs,
+		Usage:  "If set, both startup information and logs will be sent to stdout. If not set (the default), startup information will go to stdout and logs will be sent to stderr.",
 	})
 
+	devOnlyControllerFlags(c, f)
+
 	return set
 }
 
@@ -315,6 +317,7 @@ func (c *Command) ParseFlagsAndConfig(args []string) int {
 		if c.flagDevOrgId != "" {
 			c.Config.DefaultOrgId = c.flagDevOrgId
 		}
+		c.Config.PassthroughDirectory = c.flagDevPassthroughDirectory
 
 		for _, l := range c.Config.Listeners {
 			if len(l.Purpose) != 1 {