command/agent: HTTP Check: Create httpClient in Start()

For long (>10s) interval checks the http timeout is 10s, otherwise thetimeout is the interval. This means that a check *should* return
before the next check begins.

command/agent/agent.go

@@ -6,7 +6,6 @@ import (
 	"io"
 	"log"
 	"net"
-	"net/http"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -688,9 +687,6 @@ func (a *Agent) AddCheck(check *structs.HealthCheck, chkType *CheckType, persist
 				HTTP:     chkType.HTTP,
 				Interval: chkType.Interval,
 				Logger:   a.logger,
-				httpClient: &http.Client{
-					Timeout: chkType.Interval,
-				},
 			}
 			http.Start()
 			a.checkHTTPs[check.CheckID] = http

command/agent/check.go

@@ -280,6 +280,18 @@ type CheckHTTP struct {
 func (c *CheckHTTP) Start() {
 	c.stopLock.Lock()
 	defer c.stopLock.Unlock()
+
+	if c.httpClient == nil {
+		// For long (>10s) interval checks the http timeout is 10s, otherwise the
+		// timeout is the interval. This means that a check *should* return
+		// before the next check begins.
+		if c.Interval < 10*time.Second {
+			c.httpClient = &http.Client{Timeout: c.Interval}
+		} else {
+			c.httpClient = &http.Client{Timeout: 10 * time.Second}
+		}
+	}
+
 	c.stop = false
 	c.stopCh = make(chan struct{})
 	go c.run()