Fix health race (#645)

* Revert "middleware/proxy: Make Unhealthy a pointer (#615)"

This reverts commit acbf522ceb.

* middleware/proxy: add proper locking

This add the proper locking around `Unhealthy`.

middleware/proxy/proxy.go

@@ -3,6 +3,7 @@ package proxy
 
 import (
 	"errors"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -56,9 +57,10 @@ type UpstreamHost struct {
 	Name              string // IP address (and port) of this upstream host
 	Fails             int32
 	FailTimeout       time.Duration
-	Unhealthy         *bool
+	Unhealthy         bool
 	CheckDown         UpstreamHostDownFunc
 	WithoutPathPrefix string
+	checkMu           sync.Mutex
 }
 
 // Down checks whether the upstream host is down or not.
@@ -68,7 +70,7 @@ func (uh *UpstreamHost) Down() bool {
 	if uh.CheckDown == nil {
 		// Default settings
 		fails := atomic.LoadInt32(&uh.Fails)
-		return *uh.Unhealthy || fails > 0
+		return uh.Unhealthy || fails > 0
 	}
 	return uh.CheckDown(uh)
 }