Read the CalDAV password from a file, not from argv (V-644)

mavcaldav took -pass and -render-pass as flag values, so enabling it would
have put his calendar password in `ps` inside the container, in the compose
file, and in shell history. mavpoll and mavmaild both read their secret from
a file for exactly that reason.

readSecret reads once at start, trims, and refuses an empty or missing file.
An empty file is a deployment mistake, not a password, and basic auth would
otherwise send "" and collect a 401 every poll. A rotated password means a
restart, which is cheaper than re-reading the credential every five minutes.

Nothing called the old flags: no compose service, no systemd unit, no test.
So they are replaced rather than kept beside the new ones.

cmd/mavcaldav/main.go

@@ -50,10 +50,10 @@ func run(args []string) error {
 	socket := fs.String("socket", "", "core IPC socket path (required)")
 	url := fs.String("url", "", "CalDAV calendar URL, e.g. http://localhost:5232/kami/personal (required)")
 	user := fs.String("user", "", "CalDAV basic-auth username (required)")
-	pass := fs.String("pass", "", "CalDAV basic-auth password (required)")
+	passFile := fs.String("pass-file", "", "file holding the CalDAV basic-auth password (required — never passed as a flag value)")
 	renderURL := fs.String("render-url", "", "CalDAV collection maven publishes her own reminders to; empty disables rendering")
 	renderUser := fs.String("render-user", "", "basic-auth username for -render-url (defaults to -user)")
-	renderPass := fs.String("render-pass", "", "basic-auth password for -render-url (defaults to -pass)")
+	renderPassFile := fs.String("render-pass-file", "", "file holding the password for -render-url (defaults to -pass-file)")
 	renderDur := fs.Duration("render-duration", calendar.DefaultReminderDuration, "how long a rendered reminder occupies")
 	interval := fs.Duration("interval", 5*time.Minute, "poll cadence")
 	timeout := fs.Duration("timeout", 10*time.Second, "per-request HTTP timeout")
@@ -63,13 +63,22 @@ func run(args []string) error {
 	if *socket == "" {
 		return fmt.Errorf("-socket is required")
 	}
-	if *url == "" || *user == "" || *pass == "" {
-		return fmt.Errorf("-url, -user, -pass are required")
+	if *url == "" || *user == "" || *passFile == "" {
+		return fmt.Errorf("-url, -user, -pass-file are required")
 	}
 	if err := checkRenderTarget([]string{*url}, *renderURL); err != nil {
 		return err
 	}
 
+	// The password is read from a file, never taken as a flag value: an argv
+	// secret is visible in `ps` to every user on the box and lands in the compose
+	// file and the shell history. Same rule mavmaild and mavpoll follow. Read
+	// once at start, so a rotated password means a restart.
+	pass, err := readSecret(*passFile)
+	if err != nil {
+		return err
+	}
+
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
@@ -85,17 +94,20 @@ func run(args []string) error {
 		http: hc,
 		url:  strings.TrimRight(*url, "/"),
 		user: *user,
-		pass: *pass,
+		pass: pass,
 	}
 
 	var rend *renderer
 	if *renderURL != "" {
-		ru, rp := *renderUser, *renderPass
+		ru, rp := *renderUser, pass
 		if ru == "" {
 			ru = *user
 		}
-		if rp == "" {
-			rp = *pass
+		if *renderPassFile != "" {
+			rp, err = readSecret(*renderPassFile)
+			if err != nil {
+				return err
+			}
 		}
 		rend = newRenderer(core, hc, *renderURL, ru, rp, *renderDur)
 		log.Printf("mavcaldav: rendering reminders to %s", *renderURL)
@@ -131,6 +143,21 @@ func run(args []string) error {
 // It takes the whole read set, not one URL. The guarantee in the package
 // comment is about every calendar maven reads, and a second read target added
 // later must not quietly fall outside the check.
+// readSecret reads one credential from a file and refuses an empty one. An
+// empty file is a deployment mistake, not a password, and CalDAV basic auth
+// would send it and get a 401 every poll.
+func readSecret(path string) (string, error) {
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return "", fmt.Errorf("read password file: %w", err)
+	}
+	secret := strings.TrimSpace(string(raw))
+	if secret == "" {
+		return "", fmt.Errorf("password file %s is empty", path)
+	}
+	return secret, nil
+}
+
 func checkRenderTarget(readURLs []string, renderURL string) error {
 	if renderURL == "" {
 		return nil

cmd/mavcaldav/main_test.go

@@ -5,12 +5,38 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"os"
+	"path/filepath"
 	"testing"
 	"time"
 
 	"github.com/kami/maven/internal/ipc"
 )
 
+// The password comes from a file so it never reaches argv. An empty or missing
+// file must fail at start rather than authenticate as "" against his calendar.
+func TestReadSecret(t *testing.T) {
+	dir := t.TempDir()
+	good := filepath.Join(dir, "ok")
+	if err := os.WriteFile(good, []byte("  hunter2\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if got, err := readSecret(good); err != nil || got != "hunter2" {
+		t.Fatalf("readSecret(good) = %q, %v; want \"hunter2\", nil", got, err)
+	}
+
+	empty := filepath.Join(dir, "empty")
+	if err := os.WriteFile(empty, []byte("\n \n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := readSecret(empty); err == nil {
+		t.Fatal("readSecret(empty) = nil error, want refusal")
+	}
+	if _, err := readSecret(filepath.Join(dir, "absent")); err == nil {
+		t.Fatal("readSecret(absent) = nil error, want refusal")
+	}
+}
+
 type fakeCore struct {
 	ipc.UnimplementedCoreAPI
 	facts    map[string]ipc.Fact // composite key "key|source" → Fact