git » systemd » commit cd3225a

diff --git a/systemd.go b/systemd.go
index a7188b8..b931cf8 100644
--- a/systemd.go
+++ b/systemd.go
@@ -9,6 +9,7 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"sync"
 	"syscall"
 )
 
@@ -21,10 +22,24 @@ var (
 	firstFD = 3
 )
 
-// Listeners creates a slice net.Listener from the file descriptors passed
-// by systemd, via the LISTEN_FDS environment variable.
-// See sd_listen_fds(3) and sd_listen_fds_with_names(3) for more details.
-func Listeners() (map[string][]net.Listener, error) {
+// Keep a single global map of listeners, to avoid repeatedly parsing which
+// can be problematic (see parse).
+var listeners map[string][]net.Listener
+var parseError error
+var mutex sync.Mutex
+
+// parse listeners, updating the global state.
+// This function messes with file descriptors and environment, so it is not
+// idempotent and must be called only once. For the callers' convenience, we
+// save the listeners map globally and reuse it on the user-visible functions.
+func parse() {
+	mutex.Lock()
+	defer mutex.Unlock()
+
+	if listeners != nil {
+		return
+	}
+
 	pidStr := os.Getenv("LISTEN_PID")
 	nfdsStr := os.Getenv("LISTEN_FDS")
 	fdNamesStr := os.Getenv("LISTEN_FDNAMES")
@@ -32,32 +47,36 @@ func Listeners() (map[string][]net.Listener, error) {
 
 	// Nothing to do if the variables are not set.
 	if pidStr == "" || nfdsStr == "" {
-		return nil, nil
+		return
 	}
 
 	pid, err := strconv.Atoi(pidStr)
 	if err != nil {
-		return nil, fmt.Errorf(
+		parseError = fmt.Errorf(
 			"error converting $LISTEN_PID=%q: %v", pidStr, err)
+		return
 	} else if pid != os.Getpid() {
-		return nil, ErrPIDMismatch
+		parseError = ErrPIDMismatch
+		return
 	}
 
 	nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
 	if err != nil {
-		return nil, fmt.Errorf(
+		parseError = fmt.Errorf(
 			"error reading $LISTEN_FDS=%q: %v", nfdsStr, err)
+		return
 	}
 
 	// We should have as many names as we have descriptors.
 	// Note that if we have no descriptors, fdNames will be [""] (due to how
 	// strings.Split works), so we consider that special case.
 	if nfds > 0 && (fdNamesStr == "" || len(fdNames) != nfds) {
-		return nil, fmt.Errorf(
+		parseError = fmt.Errorf(
 			"Incorrect LISTEN_FDNAMES, have you set FileDescriptorName?")
+		return
 	}
 
-	listeners := map[string][]net.Listener{}
+	listeners = map[string][]net.Listener{}
 
 	for i := 0; i < nfds; i++ {
 		fd := firstFD + i
@@ -69,8 +88,9 @@ func Listeners() (map[string][]net.Listener, error) {
 		sysName := fmt.Sprintf("[systemd-fd-%d-%v]", fd, name)
 		lis, err := net.FileListener(os.NewFile(uintptr(fd), sysName))
 		if err != nil {
-			return nil, fmt.Errorf(
+			parseError = fmt.Errorf(
 				"Error making listener out of fd %d: %v", fd, err)
+			return
 		}
 
 		listeners[name] = append(listeners[name], lis)
@@ -81,6 +101,25 @@ func Listeners() (map[string][]net.Listener, error) {
 	os.Unsetenv("LISTEN_PID")
 	os.Unsetenv("LISTEN_FDS")
 	os.Unsetenv("LISTEN_FDNAMES")
+}
 
-	return listeners, nil
+// Listeners returns a map of listeners for the file descriptors passed by
+// systemd via environment variables.
+//
+// It returns a map of the form (file descriptor name -> slice of listeners).
+//
+// The file descriptor name comes from the "FileDescriptorName=" option in the
+// systemd socket unit. Multiple socket units can have the same name, hence
+// the slice of listeners for each name.
+//
+// Ideally you should not need to call this more than once. If you do, the
+// same listeners will be returned, as repeated calls to this function will
+// return the same results: the parsing is done only once, and the results are
+// saved and reused.
+//
+// See sd_listen_fds(3) and sd_listen_fds_with_names(3) for more details on
+// how the passing works.
+func Listeners() (map[string][]net.Listener, error) {
+	parse()
+	return listeners, parseError
 }
diff --git a/systemd_test.go b/systemd_test.go
index 226b124..6819ef4 100644
--- a/systemd_test.go
+++ b/systemd_test.go
@@ -13,6 +13,8 @@ func setenv(pid, fds string, names ...string) {
 	os.Setenv("LISTEN_PID", pid)
 	os.Setenv("LISTEN_FDS", fds)
 	os.Setenv("LISTEN_FDNAMES", strings.Join(names, ":"))
+	listeners = nil
+	parseError = nil
 }
 
 func TestEmptyEnvironment(t *testing.T) {