config: add support for environment variables

CLI flags are still respected.

Env vars will be prefixed with DROPLET_AGENT_ ; for example, DROPLET_AGENT_DEBUG maps to the --debug CLI flag.

This makes it easier to drive config with systemd drop-in files with the `Environment=` directive, for example.

go.mod

@@ -5,11 +5,11 @@ go 1.22
 require (
 	github.com/fsnotify/fsnotify v1.7.0
 	github.com/opencontainers/selinux v1.11.0
+	github.com/peterbourgon/ff/v3 v3.4.0
+	go.uber.org/mock v0.4.0
 	golang.org/x/crypto v0.16.0
 	golang.org/x/net v0.19.0
 	golang.org/x/sync v0.5.0
 	golang.org/x/sys v0.15.0
 	golang.org/x/time v0.3.0
 )
-
-require go.uber.org/mock v0.4.0

go.sum

@@ -2,6 +2,8 @@ github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nos
 github.com/fsnotify/fsnotify v1.7.0/go.mod h1:40Bi/Hjc2AVfZrqy+aj+yEI+/bRxZnMJyTJwOpGvigM=
 github.com/opencontainers/selinux v1.11.0 h1:+5Zbo97w3Lbmb3PeqQtpmTkMwsW5nRI3YaLpt7tQ7oU=
 github.com/opencontainers/selinux v1.11.0/go.mod h1:E5dMC3VPuVvVHDYmi78qvhJp8+M586T4DlDRYpFkyec=
+github.com/peterbourgon/ff/v3 v3.4.0 h1:QBvM/rizZM1cB0p0lGMdmR7HxZeI/ZrBWB4DqLkMUBc=
+github.com/peterbourgon/ff/v3 v3.4.0/go.mod h1:zjJVUhx+twciwfDl0zBcFzl4dW8axCRyXE/eKY9RztQ=
 go.uber.org/mock v0.4.0 h1:VcM4ZOtdbR4f6VXfiOpwpVJDL6lCReaZ6mw31wqh7KU=
 go.uber.org/mock v0.4.0/go.mod h1:a6FSlNadKUHUa9IP5Vyt1zh4fC7uAwxMutEAscFbkZc=
 golang.org/x/crypto v0.16.0 h1:mMMrFzRSCF0GvB7Ne27XVtVAaXLrPmgPC7/v0tkwHaY=

internal/config/config.go

@@ -2,7 +2,13 @@
 
 package config
 
-import "time"
+import (
+	"flag"
+	"os"
+	"time"
+
+	"github.com/peterbourgon/ff/v3"
+)
 
 const (
 	AppFullName  = "DigitalOcean Droplet Agent (code name: DOTTY)"
@@ -26,12 +32,20 @@ type Conf struct {
 
 // Init initializes the agent's configuration
 func Init() *Conf {
-	cliArgs := parseCLIArgs()
-	return &Conf{
-		UseSyslog:                   cliArgs.useSyslog,
-		DebugMode:                   cliArgs.debugMode,
-		CustomSSHDPort:              cliArgs.sshdPort,
-		CustomSSHDCfgFile:           cliArgs.sshdCfgFile,
+	cfg := Conf{
 		AuthorizedKeysCheckInterval: backgroundJobInterval,
 	}
+
+	fs := flag.NewFlagSet("droplet-agent", flag.ExitOnError)
+
+	fs.BoolVar(&cfg.UseSyslog, "syslog", false, "Use syslog service for logging")
+	fs.BoolVar(&cfg.DebugMode, "debug", false, "Turn on debug mode")
+	fs.IntVar(&cfg.CustomSSHDPort, "sshd_port", 0, "The port sshd is binding to")
+	fs.StringVar(&cfg.CustomSSHDCfgFile, "sshd_config", "", "The location of sshd_config")
+
+	ff.Parse(fs, os.Args[1:],
+		ff.WithEnvVarPrefix("DROPLET_AGENT"),
+	)
+
+	return &cfg
 }