common/httpserver: listen on an abstract Unix socket

And make healthcheck command use it by default. This makes the
healthcheck command works whatever port the user has configured for the
HTTP service.

common/httpserver/root.go

@@ -32,9 +32,10 @@ type Component struct {
 	t      tomb.Tomb
 	config Configuration
 
-	mux     *http.ServeMux
-	metrics metrics
-	address net.Addr
+	mux         *http.ServeMux
+	metrics     metrics
+	address     net.Addr
+	serviceName string
 
 	// GinRouter is the router exposed for /api
 	GinRouter  *gin.Engine
@@ -47,14 +48,15 @@ type Dependencies struct {
 }
 
 // New creates a new HTTP component.
-func New(r *reporter.Reporter, configuration Configuration, dependencies Dependencies) (*Component, error) {
+func New(r *reporter.Reporter, serviceName string, configuration Configuration, dependencies Dependencies) (*Component, error) {
 	c := Component{
 		r:      r,
 		d:      &dependencies,
 		config: configuration,
 
-		mux:       http.NewServeMux(),
-		GinRouter: gin.New(),
+		mux:         http.NewServeMux(),
+		serviceName: serviceName,
+		GinRouter:   gin.New(),
 	}
 	c.initMetrics()
 	c.d.Daemon.Track(&c.t, "common/http")
@@ -104,11 +106,6 @@ func (c *Component) AddHandler(location string, handler http.Handler) {
 
 // Start starts the HTTP component.
 func (c *Component) Start() error {
-	if c.config.Listen == "" {
-		return nil
-	}
-
-	c.r.Info().Msg("starting HTTP component")
 	var err error
 	c.cacheStore, err = c.config.Cache.Config.New()
 	if err != nil {
@@ -116,43 +113,83 @@ func (c *Component) Start() error {
 	}
 	server := &http.Server{Handler: c.mux}
 
-	// Most of the time, if we have an error, it's here!
-	c.r.Info().Str("listen", c.config.Listen).Msg("starting HTTP server")
-	listener, err := net.Listen("tcp", c.config.Listen)
-	if err != nil {
-		return fmt.Errorf("unable to listen to %v: %w", c.config.Listen, err)
-	}
-	c.address = listener.Addr()
-	server.Addr = listener.Addr().String()
-
-	// Start serving requests
-	c.t.Go(func() error {
-		if err := server.Serve(listener); err != http.ErrServerClosed {
-			c.r.Err(err).Str("listen", c.config.Listen).Msg("unable to start HTTP server")
-			return fmt.Errorf("unable to start HTTP server: %w", err)
+	for _, lc := range []struct {
+		network string
+		address string
+		fatal   bool
+	}{
+		// The regular one.
+		{
+			network: "tcp",
+			address: c.config.Listen,
+			fatal:   true,
+		},
+		// Using abstract unix sockets for healthchecking
+		{
+			network: "unix",
+			address: "@akvorado",
+			fatal:   false,
+		},
+		{
+			network: "unix",
+			address: fmt.Sprintf("@akvorado/%s", c.serviceName),
+			fatal:   false,
+		},
+	} {
+		if lc.address == "" {
+			continue
+		}
+		if lc.network == "unix" && runtime.GOOS != "linux" {
+			continue
 		}
-		return nil
-	})
 
-	// Gracefully stop when asked to
+		// Most of the time, if we have an error, it's here!
+		c.r.Info().Str("listen", lc.address).Msg("starting HTTP server")
+		listener, err := net.Listen(lc.network, lc.address)
+		if err != nil {
+			if lc.fatal {
+				return fmt.Errorf("unable to listen to %v: %w", c.config.Listen, err)
+			}
+			c.r.Info().Err(err).Msg("cannot start HTTP server")
+			continue
+		}
+		if lc.network == "tcp" {
+			c.address = listener.Addr()
+		}
+		server.Addr = listener.Addr().String()
+
+		// Serve requests
+		c.t.Go(func() error {
+			if err := server.Serve(listener); err != http.ErrServerClosed {
+				c.r.Err(err).Str("listen", lc.address).Msg("unable to start HTTP server")
+				return fmt.Errorf("unable to start HTTP server: %w", err)
+			}
+			return nil
+		})
+		// Gracefully stop when asked to
+		c.t.Go(func() error {
+			<-c.t.Dying()
+			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+			defer cancel()
+			if err := server.Shutdown(ctx); err != nil {
+				c.r.Err(err).Msg("unable to shutdown HTTP server")
+				return fmt.Errorf("unable to shutdown HTTP server: %w", err)
+			}
+			return nil
+		})
+	}
+
+	// In case we have no server
 	c.t.Go(func() error {
 		<-c.t.Dying()
-		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-		defer cancel()
-		if err := server.Shutdown(ctx); err != nil {
-			c.r.Err(err).Msg("unable to shutdown HTTP server")
-			return fmt.Errorf("unable to shutdown HTTP server: %w", err)
-		}
 		return nil
 	})
+
 	return nil
 }
 
 // Stop stops the HTTP component
 func (c *Component) Stop() error {
-	if c.config.Listen == "" {
-		return nil
-	}
 	c.r.Info().Msg("stopping HTTP component")
 	defer c.r.Info().Msg("HTTP component stopped")
 	c.t.Kill(nil)