feat(server): support listener passed by systemd for socket activation

This patch adds support for server to be used with systemd's
socket activation by expecting an open file descriptor to be
announced using environment variables.

To make use of this set either HTTPListenNetwork or
GRPCListenNetwork to "sd_listen_fd".

go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/armon/go-metrics v0.3.10
 	github.com/aws/aws-sdk-go v1.44.321
 	github.com/cespare/xxhash/v2 v2.3.0
+	github.com/coreos/go-systemd/v22 v22.5.0
 	github.com/cristalhq/hedgedhttp v0.9.1
 	github.com/davecgh/go-spew v1.1.1
 	github.com/facette/natsort v0.0.0-20181210072756-2cd4dd1e2dcb
@@ -61,7 +62,6 @@ require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/codahale/hdrhistogram v0.0.0-20161010025455-3a0bb77429bd // indirect
 	github.com/coreos/go-semver v0.3.0 // indirect
-	github.com/coreos/go-systemd/v22 v22.5.0 // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/fatih/color v1.13.0 // indirect
 	github.com/fsnotify/fsnotify v1.6.0 // indirect

server/server.go

@@ -38,6 +38,8 @@ import (
 	"github.com/grafana/dskit/log"
 	"github.com/grafana/dskit/middleware"
 	"github.com/grafana/dskit/signals"
+
+	"github.com/coreos/go-systemd/v22/activation"
 )
 
 // Listen on the named network
@@ -47,6 +49,8 @@ const (
 	DefaultNetwork = "tcp"
 	// NetworkTCPV4 for IPV4 only
 	NetworkTCPV4 = "tcp4"
+	// NetworkSystemdListenFd for using a passed open file descriptor
+	NetworkSystemdListenFd = "sd_listen_fd"
 )
 
 // SignalHandler used by Server.
@@ -248,6 +252,25 @@ func NewWithMetrics(cfg Config, metrics *Metrics) (*Server, error) {
 	return newServer(cfg, metrics)
 }
 
+func listen(network string, address string, port int, systemdListenFiles map[string][]net.Listener) (net.Listener, error) {
+	if network == "" {
+		network = DefaultNetwork
+	}
+
+	switch network {
+	case DefaultNetwork, NetworkTCPV4:
+		return net.Listen(network, net.JoinHostPort(address, strconv.Itoa(port)))
+	case NetworkSystemdListenFd:
+		if len(systemdListenFiles[address]) < 1 {
+			return nil, fmt.Errorf("expected at least one listener file descriptor named '%s'", address)
+		}
+
+		return systemdListenFiles[address][0], nil
+	default:
+		return nil, fmt.Errorf("cannot listen on unknown network '%s'", network)
+	}
+}
+
 func newServer(cfg Config, metrics *Metrics) (*Server, error) {
 	// If user doesn't supply a logging implementation, by default instantiate go-kit.
 	logger := cfg.Log
@@ -260,15 +283,17 @@ func newServer(cfg Config, metrics *Metrics) (*Server, error) {
 		gatherer = prometheus.DefaultGatherer
 	}
 
-	network := cfg.HTTPListenNetwork
-	if network == "" {
-		network = DefaultNetwork
+	systemdListenFiles, err := activation.ListenersWithNames()
+	if err != nil {
+		return nil, err
 	}
+
 	// Setup listeners first, so we can fail early if the port is in use.
-	httpListener, err := net.Listen(network, net.JoinHostPort(cfg.HTTPListenAddress, strconv.Itoa(cfg.HTTPListenPort)))
+	httpListener, err := listen(cfg.HTTPListenNetwork, cfg.HTTPListenAddress, cfg.HTTPListenPort, systemdListenFiles)
 	if err != nil {
 		return nil, err
 	}
+
 	httpListener = middleware.CountingListener(httpListener, metrics.TCPConnections.WithLabelValues("http"))
 	if cfg.HTTPLogClosedConnectionsWithoutResponse {
 		httpListener = middleware.NewZeroResponseListener(httpListener, level.Warn(logger))
@@ -279,11 +304,7 @@ func newServer(cfg Config, metrics *Metrics) (*Server, error) {
 		httpListener = netutil.LimitListener(httpListener, cfg.HTTPConnLimit)
 	}
 
-	network = cfg.GRPCListenNetwork
-	if network == "" {
-		network = DefaultNetwork
-	}
-	grpcListener, err := net.Listen(network, net.JoinHostPort(cfg.GRPCListenAddress, strconv.Itoa(cfg.GRPCListenPort)))
+	grpcListener, err := listen(cfg.GRPCListenNetwork, cfg.GRPCListenAddress, cfg.GRPCListenPort, systemdListenFiles)
 	if err != nil {
 		return nil, err
 	}