Refactor environment parsing

Systemd environment variables LISTEN* are unset by default and saved for
future calls

anyhttp.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"sync"
 	"syscall"
 )
 
@@ -39,6 +40,39 @@ func NewUnixSocketConfig(socketPath string) UnixSocketConfig {
 	return usc
 }
 
+type sysdEnvData struct {
+	pid        int
+	fdNames    []string
+	fdNamesStr string
+	numFds     int
+}
+
+var sysdEnvParser = struct {
+	sysdOnce sync.Once
+	data     sysdEnvData
+	err      error
+}{}
+
+func parse() (sysdEnvData, error) {
+	p := &sysdEnvParser
+	p.sysdOnce.Do(func() {
+		p.data.pid, p.err = strconv.Atoi(os.Getenv("LISTEN_PID"))
+		if p.err != nil {
+			p.err = fmt.Errorf("invalid LISTEN_PID, err: %w", p.err)
+			return
+		}
+		p.data.numFds, p.err = strconv.Atoi(os.Getenv("LISTEN_FDS"))
+		if p.err != nil {
+			p.err = fmt.Errorf("invalid LISTEN_FDS, err: %w", p.err)
+			return
+		}
+		p.data.fdNamesStr = os.Getenv("LISTEN_FDNAMES")
+		p.data.fdNames = strings.Split(p.data.fdNamesStr, ":")
+
+	})
+	return p.data, p.err
+}
+
 // SysdConfig has the configuration for the socket activated fd
 type SysdConfig struct {
 	// Integer value starting at 0. Either index or name is required
@@ -54,7 +88,7 @@ type SysdConfig struct {
 // DefaultSysdConfig has the default values for SysdConfig
 var DefaultSysdConfig = SysdConfig{
 	CheckPID: true,
-	UnsetEnv: false,
+	UnsetEnv: true,
 }
 
 // NewSysDConfigWithFDIdx creates SysdConfig with defaults and fdIdx
@@ -112,53 +146,47 @@ func (s *SysdConfig) GetListener() (net.Listener, error) {
 		defer UnsetSystemdListenVars()
 	}
 
-	if s.CheckPID {
-		pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
-		if err != nil {
-			return nil, fmt.Errorf("invalid LISTEN_PID, err: %w", err)
-		}
-		if pid != os.Getpid() {
-			return nil, fmt.Errorf("unexpected PID, current:%v, LISTEN_PID: %v", os.Getpid(), pid)
-		}
-	}
-
-	numFds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
+	envData, err := parse()
 	if err != nil {
-		return nil, fmt.Errorf("invalid LISTEN_FDS, err: %w", err)
+		return nil, err
 	}
 
-	fdNames := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
+	if s.CheckPID {
+		if envData.pid != os.Getpid() {
+			return nil, fmt.Errorf("unexpected PID, current:%v, LISTEN_PID: %v", os.Getpid(), envData.pid)
+		}
+	}
 
 	if s.FDIndex != nil {
 		idx := *s.FDIndex
-		if idx < 0 || idx >= numFds {
-			return nil, fmt.Errorf("invalid fd index, expected between 0 and %v, got: %v", numFds, idx)
+		if idx < 0 || idx >= envData.numFds {
+			return nil, fmt.Errorf("invalid fd index, expected between 0 and %v, got: %v", envData.numFds, idx)
 		}
 		fd := StartFD + idx
-		if idx < len(fdNames) {
-			return makeFdListener(fd, fdNames[idx])
+		if idx < len(envData.fdNames) {
+			return makeFdListener(fd, envData.fdNames[idx])
 		}
 		return makeFdListener(fd, fmt.Sprintf("sysdfd_%d", fd))
 	}
 
 	if s.FDName != nil {
-		for idx, name := range fdNames {
+		for idx, name := range envData.fdNames {
 			if name == *s.FDName {
 				fd := StartFD + idx
 				return makeFdListener(fd, name)
 			}
 		}
-		return nil, fmt.Errorf("fdName not found: %q, LISTEN_FDNAMES:%q", *s.FDName, os.Getenv("LISTEN_FDNAMES"))
+		return nil, fmt.Errorf("fdName not found: %q, LISTEN_FDNAMES:%q", *s.FDName, envData.fdNamesStr)
 	}
 
 	return nil, errors.New("neither FDIndex nor FDName set")
 }
 
 // UnknownAddress Error is returned when address does not match any known syntax
-type UnknownAddress string
+type UnknownAddress struct{}
 
 func (u UnknownAddress) Error() string {
-	return fmt.Sprintf("unknown address: %q", string(u))
+	return "unknown address"
 }
 
 // GetListener gets a unix or systemd socket listener
@@ -182,15 +210,15 @@ func GetListener(addr string) (net.Listener, error) {
 		return sysdc.GetListener()
 	}
 
-	return nil, UnknownAddress(addr)
+	return nil, UnknownAddress{}
 }
 
 // ListenAndServe is the drop-in replacement for `http.ListenAndServe`.
 // Supports unix and systemd sockets in addition
 func ListenAndServe(addr string, h http.Handler) error {
 
 	listener, err := GetListener(addr)
-	if _, ok := err.(UnknownAddress); err != nil && !ok {
+	if _, isUnknown := err.(UnknownAddress); err != nil && !isUnknown {
 		return err
 	}
 
@@ -211,7 +239,7 @@ func ListenAndServe(addr string, h http.Handler) error {
 
 // UnsetSystemdListenVars unsets the LISTEN* environment variables so they are not passed to any child processes
 func UnsetSystemdListenVars() {
-	os.Unsetenv("LISTEN_PID")
-	os.Unsetenv("LISTEN_FDS")
-	os.Unsetenv("LISTEN_FDNAMES")
+	_ = os.Unsetenv("LISTEN_PID")
+	_ = os.Unsetenv("LISTEN_FDS")
+	_ = os.Unsetenv("LISTEN_FDNAMES")
 }