Support getting listeners from systemd

Add a new module for getting listener sockets via systemd's file descriptor
passing (see sd_listen_fds(3) for more details), and make the main daemon use
it when "systemd" is given an address.

internal/systemd/systemd_test.go

@@ -0,0 +1,157 @@
+package systemd
+
+import (
+	"math/rand"
+	"net"
+	"os"
+	"strconv"
+	"testing"
+)
+
+func setenv(pid, fds string) {
+	os.Setenv("LISTEN_PID", pid)
+	os.Setenv("LISTEN_FDS", fds)
+}
+
+func TestEmptyEnvironment(t *testing.T) {
+	cases := []struct{ pid, fds string }{
+		{"", ""},
+		{"123", ""},
+		{"", "4"},
+	}
+	for _, c := range cases {
+		setenv(c.pid, c.fds)
+
+		if ls, err := Listeners(); ls != nil || err != nil {
+			t.Logf("Case: LISTEN_PID=%q  LISTEN_FDS=%q", c.pid, c.fds)
+			t.Errorf("Unexpected result: %v // %v", ls, err)
+		}
+	}
+}
+
+func TestBadEnvironment(t *testing.T) {
+	ourPID := strconv.Itoa(os.Getpid())
+	cases := []struct{ pid, fds string }{
+		{"a", "4"},
+		{ourPID, "a"},
+	}
+	for _, c := range cases {
+		setenv(c.pid, c.fds)
+
+		if ls, err := Listeners(); err == nil {
+			t.Logf("Case: LISTEN_PID=%q  LISTEN_FDS=%q", c.pid, c.fds)
+			t.Errorf("Unexpected result: %v // %v", ls, err)
+		}
+	}
+}
+
+func TestWrongPID(t *testing.T) {
+	// Find a pid != us. 1 should always work in practice.
+	pid := 1
+	for pid == os.Getpid() {
+		pid = rand.Int()
+	}
+
+	setenv(strconv.Itoa(pid), "4")
+	if _, err := Listeners(); err != PIDMismatch {
+		t.Errorf("Did not fail with PID mismatch: %v", err)
+	}
+}
+
+func TestNoFDs(t *testing.T) {
+	setenv(strconv.Itoa(os.Getpid()), "0")
+	if ls, err := Listeners(); len(ls) != 0 || err != nil {
+		t.Errorf("Got a non-empty result: %v // %v", ls, err)
+	}
+}
+
+// newListener creates a TCP listener.
+func newListener(t *testing.T) *net.TCPListener {
+	addr := &net.TCPAddr{
+		Port: 0,
+	}
+
+	l, err := net.ListenTCP("tcp", addr)
+	if err != nil {
+		t.Fatalf("Could not create TCP listener: %v", err)
+	}
+
+	return l
+}
+
+// listenerFd returns a file descriptor for the listener.
+// Note it is a NEW file descriptor, not the original one.
+func listenerFd(t *testing.T, l *net.TCPListener) int {
+	f, err := l.File()
+	if err != nil {
+		t.Fatalf("Could not get TCP listener file: %v", err)
+	}
+
+	return int(f.Fd())
+}
+
+func sameAddr(a, b net.Addr) bool {
+	return a.Network() == b.Network() && a.String() == b.String()
+}
+
+func TestOneSocket(t *testing.T) {
+	l := newListener(t)
+	firstFD = listenerFd(t, l)
+
+	setenv(strconv.Itoa(os.Getpid()), "1")
+
+	ls, err := Listeners()
+	if err != nil || len(ls) != 1 {
+		t.Fatalf("Got an invalid result: %v // %v", ls, err)
+	}
+
+	if !sameAddr(ls[0].Addr(), l.Addr()) {
+		t.Errorf("Listener 0 address mismatch, expected %#v, got %#v",
+			l.Addr(), ls[0].Addr())
+	}
+
+	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" {
+		t.Errorf("Failed to reset the environment")
+	}
+}
+
+func TestManySockets(t *testing.T) {
+	// Create two contiguous listeners.
+	// The test environment does not guarantee us that they are contiguous, so
+	// keep going until they are.
+	var l0, l1 *net.TCPListener
+	var f0, f1 int = -1, -3
+
+	for f0+1 != f1 {
+		// We have to be careful with the order of these operations, because
+		// listenerFd will create *new* file descriptors.
+		l0 = newListener(t)
+		l1 = newListener(t)
+		f0 = listenerFd(t, l0)
+		f1 = listenerFd(t, l1)
+		t.Logf("Looping for FDs: %d %d", f0, f1)
+	}
+
+	firstFD = f0
+
+	setenv(strconv.Itoa(os.Getpid()), "2")
+
+	ls, err := Listeners()
+	if err != nil || len(ls) != 2 {
+		t.Fatalf("Got an invalid result: %v // %v", ls, err)
+	}
+
+	if !sameAddr(ls[0].Addr(), l0.Addr()) {
+		t.Errorf("Listener 0 address mismatch, expected %#v, got %#v",
+			l0.Addr(), ls[0].Addr())
+	}
+
+	if !sameAddr(ls[1].Addr(), l1.Addr()) {
+		t.Errorf("Listener 1 address mismatch, expected %#v, got %#v",
+			l1.Addr(), ls[1].Addr())
+	}
+
+	if os.Getenv("LISTEN_PID") != "" || os.Getenv("LISTEN_FDS") != "" {
+		t.Errorf("Failed to reset the environment")
+	}
+}