runtime: added counter to failpoint.terms field

Each time sleep is executed in actSleep counter is incremented. Added counter endpoint that gives number of sleeps executed on the current terms: GET pkg/failpoint/count
When DELETE /pkg/failpoint is executed, failpoint.terms is nilled, hence counter is reset.

Requested in reference to:
tests/linearizability: added sleep fail point injection #14796

Signed-off-by: Ramil Mirhasanov <ramil600@yahoo.com>

runtime/http.go

@@ -72,6 +72,13 @@ func (*httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 				lines[i] = fps[i] + "=" + s
 			}
 			w.Write([]byte(strings.Join(lines, "\n") + "\n"))
+		} else if strings.HasSuffix(key, "/count") {
+			fp := key[:len(key)-6]
+			count, err := StatusCount(fp)
+			if err != nil {
+				http.Error(w, "failed to GET: "+err.Error(), http.StatusNotFound)
+			}
+			w.Write([]byte(count))
 		} else {
 			status, err := Status(key)
 			if err != nil {

runtime/runtime.go

@@ -114,6 +114,25 @@ func Status(failpath string) (string, error) {
 	return t.desc, nil
 }
 
+// StatusCount gives the number of sleeps executed on the current terms
+func StatusCount(failpath string) (string, error) {
+	failpointsMu.RLock()
+	fp := failpoints[failpath]
+	failpointsMu.RUnlock()
+	if fp == nil {
+		return "", ErrNoExist
+	}
+	fp.mu.RLock()
+	t := fp.t
+	fp.mu.RUnlock()
+	if t == nil {
+		return "", ErrDisabled
+	}
+	count := fp.t.counter
+	return fmt.Sprint(count), nil
+
+}
+
 func List() []string {
 	failpointsMu.RLock()
 	ret := make([]string, 0, len(failpoints))

runtime/terms.go

@@ -41,6 +41,8 @@ type terms struct {
 
 	// mu protects the state of the terms chain
 	mu sync.Mutex
+	// counts amount of sleep executions
+	counter int
 }
 
 // term is an executable unit of the failpoint terms chain
@@ -310,6 +312,9 @@ func actSleep(t *term) interface{} {
 		return nil
 	}
 	time.Sleep(dur)
+	t.parent.mu.Lock()
+	t.parent.counter++
+	t.parent.mu.Unlock()
 	return nil
 }