From ca108d109d55e927c292e3e558fc4f761a3a4e7c Mon Sep 17 00:00:00 2001
From: Lukasz Szaszkiewicz <lukasz.szaszkiewicz@gmail.com>
Date: Wed, 30 Jun 2021 11:04:44 +0200
Subject: [PATCH] readyz signals when the handler succeeds for the first time.

Co-authored-by: Dr. Stefan Schimanski <stefan.schimanski@gmail.com>
---
 .../apiserver/pkg/server/healthz/healthz.go   | 26 ++++++++++++++++---
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/staging/src/k8s.io/apiserver/pkg/server/healthz/healthz.go b/staging/src/k8s.io/apiserver/pkg/server/healthz/healthz.go
index e80b8501edf..eb288239ce5 100644
--- a/staging/src/k8s.io/apiserver/pkg/server/healthz/healthz.go
+++ b/staging/src/k8s.io/apiserver/pkg/server/healthz/healthz.go
@@ -140,6 +140,12 @@ func InstallReadyzHandler(mux mux, checks ...HealthChecker) {
 	InstallPathHandler(mux, "/readyz", checks...)
 }
 
+// InstallReadyzHandlerWithHealthyFunc is like InstallReadyzHandler, but in addition call firstTimeReady
+// the first time /readyz succeeds.
+func InstallReadyzHandlerWithHealthyFunc(mux mux, firstTimeReady func(), checks ...HealthChecker) {
+	InstallPathHandlerWithHealthyFunc(mux, "/readyz", firstTimeReady, checks...)
+}
+
 // InstallLivezHandler registers handlers for liveness checking on the path
 // "/livez" to mux. *All handlers* for mux must be specified in
 // exactly one call to InstallHandler. Calling InstallHandler more
@@ -154,6 +160,12 @@ func InstallLivezHandler(mux mux, checks ...HealthChecker) {
 // InstallPathHandler more than once for the same path and mux will
 // result in a panic.
 func InstallPathHandler(mux mux, path string, checks ...HealthChecker) {
+	InstallPathHandlerWithHealthyFunc(mux, path, nil, checks...)
+}
+
+// InstallPathHandlerWithHealthyFunc is like InstallPathHandler, but calls firstTimeHealthy exactly once
+// when the handler succeeds for the first time.
+func InstallPathHandlerWithHealthyFunc(mux mux, path string, firstTimeHealthy func(), checks ...HealthChecker) {
 	if len(checks) == 0 {
 		klog.V(5).Info("No default health checks specified. Installing the ping handler.")
 		checks = []HealthChecker{PingHealthz}
@@ -172,7 +184,7 @@ func InstallPathHandler(mux mux, path string, checks ...HealthChecker) {
 			/* component = */ "",
 			/* deprecated */ false,
 			/* removedRelease */ "",
-			handleRootHealth(name, checks...)))
+			handleRootHealth(name, firstTimeHealthy, checks...)))
 	for _, check := range checks {
 		mux.Handle(fmt.Sprintf("%s/%v", path, check.Name()), adaptCheckToHandler(check.Check))
 	}
@@ -209,8 +221,9 @@ func getExcludedChecks(r *http.Request) sets.String {
 }
 
 // handleRootHealth returns an http.HandlerFunc that serves the provided checks.
-func handleRootHealth(name string, checks ...HealthChecker) http.HandlerFunc {
-	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+func handleRootHealth(name string, firstTimeHealthy func(), checks ...HealthChecker) http.HandlerFunc {
+	var notifyOnce sync.Once
+	return func(w http.ResponseWriter, r *http.Request) {
 		excluded := getExcludedChecks(r)
 		// failedVerboseLogOutput is for output to the log.  It indicates detailed failed output information for the log.
 		var failedVerboseLogOutput bytes.Buffer
@@ -246,6 +259,11 @@ func handleRootHealth(name string, checks ...HealthChecker) http.HandlerFunc {
 			return
 		}
 
+		// signal first time this is healthy
+		if firstTimeHealthy != nil {
+			notifyOnce.Do(firstTimeHealthy)
+		}
+
 		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 		w.Header().Set("X-Content-Type-Options", "nosniff")
 		if _, found := r.URL.Query()["verbose"]; !found {
@@ -255,7 +273,7 @@ func handleRootHealth(name string, checks ...HealthChecker) http.HandlerFunc {
 
 		individualCheckOutput.WriteTo(w)
 		fmt.Fprintf(w, "%s check passed\n", name)
-	})
+	}
 }
 
 // adaptCheckToHandler returns an http.HandlerFunc that serves the provided checks.