Feature: faster healthcheck, fix #283

internal/healthcheck/handler.go

@@ -1,21 +1,25 @@
 package healthcheck
 
 import (
-	"net"
+	"errors"
 	"net/http"
+	"sync"
 
 	"github.com/qdm12/golibs/logging"
 )
 
 type handler struct {
-	logger   logging.Logger
-	resolver *net.Resolver
+	logger      logging.Logger
+	healthErr   error
+	healthErrMu sync.RWMutex
 }
 
-func newHandler(logger logging.Logger, resolver *net.Resolver) http.Handler {
+var errHealthcheckNotRunYet = errors.New("healthcheck did not run yet")
+
+func newHandler(logger logging.Logger) *handler {
 	return &handler{
-		logger:   logger,
-		resolver: resolver,
+		logger:    logger,
+		healthErr: errHealthcheckNotRunYet,
 	}
 }
 
@@ -24,11 +28,22 @@ func (h *handler) ServeHTTP(responseWriter http.ResponseWriter, request *http.Re
 		http.Error(responseWriter, "method not supported for healthcheck", http.StatusBadRequest)
 		return
 	}
-	err := healthCheck(request.Context(), h.resolver)
-	if err != nil {
+	if err := h.getErr(); err != nil {
 		h.logger.Error(err)
 		http.Error(responseWriter, err.Error(), http.StatusInternalServerError)
 		return
 	}
 	responseWriter.WriteHeader(http.StatusOK)
 }
+
+func (h *handler) setErr(err error) {
+	h.healthErrMu.Lock()
+	defer h.healthErrMu.Unlock()
+	h.healthErr = err
+}
+
+func (h *handler) getErr() (err error) {
+	h.healthErrMu.RLock()
+	defer h.healthErrMu.RUnlock()
+	return h.healthErr
+}

internal/healthcheck/health.go

@@ -2,8 +2,46 @@ package healthcheck
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net"
+	"sync"
+	"time"
+)
+
+func (s *server) runHealthcheckLoop(ctx context.Context, wg *sync.WaitGroup) {
+	defer wg.Done()
+	for {
+		err := healthCheck(ctx, s.resolver)
+		s.handler.setErr(err)
+		if err != nil { // try again after 1 second
+			timer := time.NewTimer(time.Second)
+			select {
+			case <-ctx.Done():
+				if !timer.Stop() {
+					<-timer.C
+				}
+				return
+			case <-timer.C:
+			}
+			continue
+		}
+		// Success, check again in 10 minutes
+		const period = 10 * time.Minute
+		timer := time.NewTimer(period)
+		select {
+		case <-ctx.Done():
+			if !timer.Stop() {
+				<-timer.C
+			}
+			return
+		case <-timer.C:
+		}
+	}
+}
+
+var (
+	errNoIPResolved = errors.New("no IP address resolved")
 )
 
 func healthCheck(ctx context.Context, resolver *net.Resolver) (err error) {
@@ -12,9 +50,9 @@ func healthCheck(ctx context.Context, resolver *net.Resolver) (err error) {
 	ips, err := resolver.LookupIP(ctx, "ip", domainToResolve)
 	switch {
 	case err != nil:
-		return fmt.Errorf("cannot resolve github.com: %s", err)
+		return err
 	case len(ips) == 0:
-		return fmt.Errorf("resolved no IP addresses for %s", domainToResolve)
+		return fmt.Errorf("%w for %s", errNoIPResolved, domainToResolve)
 	default:
 		return nil
 	}

internal/healthcheck/server.go

@@ -16,26 +16,36 @@ type Server interface {
 }
 
 type server struct {
-	address string
-	logger  logging.Logger
-	handler http.Handler
+	address  string
+	logger   logging.Logger
+	handler  *handler
+	resolver *net.Resolver
 }
 
 func NewServer(address string, logger logging.Logger) Server {
+	healthcheckLogger := logger.WithPrefix("healthcheck: ")
 	return &server{
-		address: address,
-		logger:  logger.WithPrefix("healthcheck: "),
-		handler: newHandler(logger, net.DefaultResolver),
+		address:  address,
+		logger:   healthcheckLogger,
+		handler:  newHandler(healthcheckLogger),
+		resolver: net.DefaultResolver,
 	}
 }
 
 func (s *server) Run(ctx context.Context, wg *sync.WaitGroup) {
+	defer wg.Done()
+
+	internalWg := &sync.WaitGroup{}
+	internalWg.Add(1)
+	go s.runHealthcheckLoop(ctx, internalWg)
+
 	server := http.Server{
 		Addr:    s.address,
 		Handler: s.handler,
 	}
+	internalWg.Add(1)
 	go func() {
-		defer wg.Done()
+		defer internalWg.Done()
 		<-ctx.Done()
 		s.logger.Warn("context canceled: shutting down server")
 		defer s.logger.Warn("server shut down")
@@ -46,9 +56,12 @@ func (s *server) Run(ctx context.Context, wg *sync.WaitGroup) {
 			s.logger.Error("failed shutting down: %s", err)
 		}
 	}()
+
 	s.logger.Info("listening on %s", s.address)
 	err := server.ListenAndServe()
 	if err != nil && !errors.Is(ctx.Err(), context.Canceled) {
 		s.logger.Error(err)
 	}
+
+	internalWg.Wait()
 }