Refuse invalid settings at start, health check follows PORT (closes #31)

PORT (1 to 65535) and XDG_DATA_HOME (an absolute path) are now checked in
the config package, which is built before the database opens, so a bad
value stops the start with an error and a non-zero exit instead of leaving
the daemon running without HTTP or putting the database somewhere else.
entrypoint.sh refuses a MALLOC_ARENA_MAX that is not a positive whole
number, since glibc ignores a bad one silently. The HEALTHCHECK probes the
port PORT names. The README corrects the Linux state directory, lists
XDG_DATA_HOME, and adds a "Running under upaas" section.

Model: opus-5-5

internal/config/config.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strconv"
 	"time"
 )
 
@@ -18,6 +19,12 @@ const (
 
 	// defaultRouteExpirationMinutes is the default route expiration timeout in minutes
 	defaultRouteExpirationMinutes = 5
+
+	// defaultPort is the HTTP port used when PORT is not set
+	defaultPort = 8080
+
+	// maxPort is the highest TCP port number
+	maxPort = 65535
 )
 
 // Config holds configuration for the entire application
@@ -25,6 +32,9 @@ type Config struct {
 	// StateDir is the directory for all application state (database, snapshots)
 	StateDir string
 
+	// Port is the TCP port the HTTP server listens on
+	Port int
+
 	// MaxRuntime is the maximum runtime (0 = run forever)
 	MaxRuntime time.Duration
 
@@ -43,8 +53,14 @@ func New() (*Config, error) {
 		return nil, fmt.Errorf("failed to determine state directory: %w", err)
 	}
 
+	port, err := getPort()
+	if err != nil {
+		return nil, err
+	}
+
 	return &Config{
 		StateDir:                    stateDir,
+		Port:                        port,
 		MaxRuntime:                  0,                                           // Run forever by default
 		EnableBatchedDatabaseWrites: true,                                        // Enable batching by default
 		RouteExpirationTimeout:      defaultRouteExpirationMinutes * time.Minute, // For active route monitoring
@@ -69,13 +85,20 @@ func getStateDirectory() (string, error) {
 		return filepath.Join(home, "Library", "Application Support", AppIdentifier), nil
 
 	case "linux", "freebsd", "openbsd", "netbsd":
+		// The XDG spec requires an absolute path; a relative one would put
+		// the database somewhere unexpected.
+		xdgData := os.Getenv("XDG_DATA_HOME")
+		if xdgData != "" && !filepath.IsAbs(xdgData) {
+			return "", fmt.Errorf("XDG_DATA_HOME must be an absolute path, got %q", xdgData)
+		}
+
 		// Unix-like: /var/lib/berlin.sneak.app.routewatch if root, else XDG_DATA_HOME
 		if os.Geteuid() == 0 {
 			return filepath.Join("/var/lib", AppIdentifier), nil
 		}
 
 		// Check XDG_DATA_HOME first
-		if xdgData := os.Getenv("XDG_DATA_HOME"); xdgData != "" {
+		if xdgData != "" {
 			return filepath.Join(xdgData, AppIdentifier), nil
 		}
 
@@ -92,6 +115,21 @@ func getStateDirectory() (string, error) {
 	}
 }
 
+// getPort returns the HTTP port from PORT, or defaultPort when PORT is not set
+func getPort() (int, error) {
+	value := os.Getenv("PORT")
+	if value == "" {
+		return defaultPort, nil
+	}
+
+	port, err := strconv.Atoi(value)
+	if err != nil || port < 1 || port > maxPort {
+		return 0, fmt.Errorf("PORT must be a whole number from 1 to %d, got %q", maxPort, value)
+	}
+
+	return port, nil
+}
+
 // EnsureDirectories creates all necessary directories if they don't exist
 func (c *Config) EnsureDirectories() error {
 	// Ensure state directory exists

internal/config/config_test.go

@@ -0,0 +1,62 @@
+package config
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestNewReadsPort(t *testing.T) {
+	tests := map[string]int{
+		"":      defaultPort,
+		"1":     1,
+		"9090":  9090,
+		"65535": 65535,
+	}
+
+	for value, want := range tests {
+		t.Run(value, func(t *testing.T) {
+			t.Setenv("PORT", value)
+			t.Setenv("XDG_DATA_HOME", "")
+
+			cfg, err := New()
+			if err != nil {
+				t.Fatalf("New() with PORT=%q: %v", value, err)
+			}
+
+			if cfg.Port != want {
+				t.Errorf("New() with PORT=%q: Port = %d, want %d", value, cfg.Port, want)
+			}
+		})
+	}
+}
+
+func TestNewRefusesInvalidPort(t *testing.T) {
+	for _, value := range []string{"0", "65536", "-1", "http", "80.5"} {
+		t.Run(value, func(t *testing.T) {
+			t.Setenv("PORT", value)
+			t.Setenv("XDG_DATA_HOME", "")
+
+			if _, err := New(); err == nil {
+				t.Errorf("New() with PORT=%q returned no error", value)
+			}
+		})
+	}
+}
+
+func TestNewRefusesRelativeXDGDataHome(t *testing.T) {
+	if runtime.GOOS == "darwin" {
+		t.Skip("macOS does not read XDG_DATA_HOME")
+	}
+
+	t.Setenv("PORT", "")
+
+	t.Setenv("XDG_DATA_HOME", "relative/path")
+	if _, err := New(); err == nil {
+		t.Error("New() with a relative XDG_DATA_HOME returned no error")
+	}
+
+	t.Setenv("XDG_DATA_HOME", "/var/lib")
+	if _, err := New(); err != nil {
+		t.Errorf("New() with XDG_DATA_HOME=/var/lib: %v", err)
+	}
+}

internal/routewatch/app_integration_test.go

@@ -450,7 +450,7 @@ func TestRouteWatchLiveFeed(t *testing.T) {
 	}
 
 	// Create server
-	srv := server.New(mockDB, s, logger)
+	srv := server.New(mockDB, s, logger, cfg)
 
 	// Create RouteWatch with 5 second limit
 	deps := Dependencies{

internal/server/handlers_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"git.eeqj.de/sneak/routewatch/internal/config"
 	"git.eeqj.de/sneak/routewatch/internal/database"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
 	"git.eeqj.de/sneak/routewatch/internal/metrics"
@@ -38,7 +39,7 @@ func (d blockingStatsDB) GetStatsContext(_ context.Context) (database.Stats, err
 func TestStatsHandlersDoNotLeakOnTimeout(t *testing.T) {
 	release := make(chan struct{})
 	db := blockingStatsDB{release: release}
-	s := New(db, streamer.New(logger.New(), metrics.New()), logger.New())
+	s := New(db, streamer.New(logger.New(), metrics.New()), logger.New(), &config.Config{})
 
 	handlers := map[string]http.HandlerFunc{
 		"status.json": s.handleStatusJSON(),

internal/server/server.go

@@ -4,9 +4,10 @@ package server
 import (
 	"context"
 	"net/http"
-	"os"
+	"strconv"
 	"time"
 
+	"git.eeqj.de/sneak/routewatch/internal/config"
 	"git.eeqj.de/sneak/routewatch/internal/database"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
 	"git.eeqj.de/sneak/routewatch/internal/streamer"
@@ -33,16 +34,18 @@ type Server struct {
 	db         database.Store
 	streamer   *streamer.Streamer
 	logger     *logger.Logger
+	port       int
 	srv        *http.Server
 	asnFetcher ASNFetcher
 }
 
 // New creates a new HTTP server
-func New(db database.Store, streamer *streamer.Streamer, logger *logger.Logger) *Server {
+func New(db database.Store, streamer *streamer.Streamer, logger *logger.Logger, cfg *config.Config) *Server {
 	s := &Server{
 		db:       db,
 		streamer: streamer,
 		logger:   logger,
+		port:     cfg.Port,
 	}
 
 	s.setupRoutes()
@@ -52,11 +55,6 @@ func New(db database.Store, streamer *streamer.Streamer, logger *logger.Logger)
 
 // Start starts the HTTP server
 func (s *Server) Start() error {
-	port := os.Getenv("PORT")
-	if port == "" {
-		port = "8080"
-	}
-
 	const (
 		readHeaderTimeout = 40 * time.Second
 		readTimeout       = 60 * time.Second
@@ -65,7 +63,7 @@ func (s *Server) Start() error {
 	)
 
 	s.srv = &http.Server{
-		Addr:              ":" + port,
+		Addr:              ":" + strconv.Itoa(s.port),
 		Handler:           s.router,
 		ReadHeaderTimeout: readHeaderTimeout,
 		ReadTimeout:       readTimeout,
@@ -73,7 +71,7 @@ func (s *Server) Start() error {
 		IdleTimeout:       idleTimeout,
 	}
 
-	s.logger.Info("Starting HTTP server", "port", port, "addr", s.srv.Addr)
+	s.logger.Info("Starting HTTP server", "port", s.port, "addr", s.srv.Addr)
 
 	// Start in goroutine but log when actually listening
 	go func() {