Add support for reading MQTT password from file

Add `--mqtt.password-file` flag and `MQTT_PASSWORD_FILE` env var to allow
reading the MQTT password from a file (e.g. Docker secrets). When set,
the file contents overrides the --mqtt.password value.

Addresses diebietse/invertergui#46

cmd/invertergui/config.go

@@ -1,6 +1,10 @@
 package main
 
 import (
+	"fmt"
+	"os"
+	"strings"
+
 	"github.com/jessevdk/go-flags"
 )
 
@@ -21,6 +25,7 @@ type config struct {
 		Topic        string `long:"mqtt.topic" env:"MQTT_TOPIC" default:"invertergui/updates" description:"Set the MQTT topic updates published to."`
 		Username     string `long:"mqtt.username" env:"MQTT_USERNAME" default:"" description:"Set the MQTT username"`
 		Password     string `long:"mqtt.password" env:"MQTT_PASSWORD" default:"" description:"Set the MQTT password"`
+		PasswordFile string `long:"mqtt.password-file" env:"MQTT_PASSWORD_FILE" default:"" description:"Path to a file containing the MQTT password"`
 	}
 	Loglevel string `long:"loglevel" env:"LOGLEVEL" default:"info" description:"The log level to generate logs at. (\"panic\", \"fatal\", \"error\", \"warn\", \"info\", \"debug\", \"trace\")"`
 }
@@ -31,5 +36,30 @@ func parseConfig() (*config, error) {
 	if _, err := parser.Parse(); err != nil {
 		return nil, err
 	}
+	if err := resolvePasswordFile(conf); err != nil {
+		return nil, err
+	}
 	return conf, nil
 }
+
+func resolvePasswordFile(conf *config) error {
+	if conf.MQTT.PasswordFile != "" && conf.MQTT.Password != "" {
+		return fmt.Errorf("mqtt.password and mqtt.password-file are mutually exclusive")
+	}
+	if conf.MQTT.PasswordFile != "" {
+		password, err := readPasswordFile(conf.MQTT.PasswordFile)
+		if err != nil {
+			return err
+		}
+		conf.MQTT.Password = password
+	}
+	return nil
+}
+
+func readPasswordFile(path string) (string, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return "", fmt.Errorf("could not read MQTT password file: %w", err)
+	}
+	return strings.TrimRight(string(data), "\n\r"), nil
+}

cmd/invertergui/config_test.go

@@ -0,0 +1,107 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+const testInlineSecret = "inline-secret"
+
+func TestReadPasswordFile(t *testing.T) {
+	tests := []struct {
+		name     string
+		content  string
+		expected string
+	}{
+		{
+			name:     "plain password",
+			content:  "secret",
+			expected: "secret",
+		},
+		{
+			name:     "password with trailing newline",
+			content:  "secret\n",
+			expected: "secret",
+		},
+		{
+			name:     "password with trailing carriage return and newline",
+			content:  "secret\r\n",
+			expected: "secret",
+		},
+		{
+			name:     "empty file",
+			content:  "",
+			expected: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "password")
+			if err := os.WriteFile(path, []byte(tt.content), 0o600); err != nil {
+				t.Fatalf("failed to write test file: %v", err)
+			}
+
+			got, err := readPasswordFile(path)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got != tt.expected {
+				t.Errorf("got %q, want %q", got, tt.expected)
+			}
+		})
+	}
+}
+
+func TestReadPasswordFile_NotFound(t *testing.T) {
+	_, err := readPasswordFile("/nonexistent/path/password")
+	if err == nil {
+		t.Fatal("expected error for missing file, got nil")
+	}
+}
+
+func TestResolvePassword_MutuallyExclusive(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "password")
+	if err := os.WriteFile(path, []byte("secret"), 0o600); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	conf := &config{}
+	conf.MQTT.Password = testInlineSecret
+	conf.MQTT.PasswordFile = path
+
+	err := resolvePasswordFile(conf)
+	if err == nil {
+		t.Fatal("expected error when both mqtt.password and mqtt.password-file are set, got nil")
+	}
+}
+
+func TestResolvePassword_FromFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "password")
+	if err := os.WriteFile(path, []byte("file-secret\n"), 0o600); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	conf := &config{}
+	conf.MQTT.PasswordFile = path
+
+	if err := resolvePasswordFile(conf); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if conf.MQTT.Password != "file-secret" {
+		t.Errorf("got %q, want %q", conf.MQTT.Password, "file-secret")
+	}
+}
+
+func TestResolvePassword_NoFile(t *testing.T) {
+	conf := &config{}
+	conf.MQTT.Password = testInlineSecret
+
+	if err := resolvePasswordFile(conf); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if conf.MQTT.Password != testInlineSecret {
+		t.Errorf("got %q, want %q", conf.MQTT.Password, testInlineSecret)
+	}
+}