Plugin/Proxy - add new policy always_first to mimic windows dns resolvers (#1459)

* add new policy always_first to mimic windows dns resolvers
fill documentation, add UT and cleanup fmt

* change name of policy from always_first to first. Update docs

plugin/pkg/healthcheck/policy.go

@@ -27,6 +27,7 @@ func init() {
 	RegisterPolicy("random", func() Policy { return &Random{} })
 	RegisterPolicy("least_conn", func() Policy { return &LeastConn{} })
 	RegisterPolicy("round_robin", func() Policy { return &RoundRobin{} })
+	RegisterPolicy("first", func() Policy { return &First{} })
 }
 
 // Random is a policy that selects up hosts from a pool at random.
@@ -118,3 +119,19 @@ func (r *RoundRobin) Select(pool HostPool) *UpstreamHost {
 	}
 	return host
 }
+
+// First is a policy that selects always the first healthy host in the list order.
+type First struct{}
+
+// Select always the first that is not Down.
+func (r *First) Select(pool HostPool) *UpstreamHost {
+	for i := 0; i < len(pool); i++ {
+		host := pool[i]
+		if host.Down() {
+			continue
+		}
+		return host
+	}
+	// return the first one, anyway none is correct
+	return nil
+}

plugin/pkg/healthcheck/policy_test.go

@@ -32,8 +32,8 @@ func (r *customPolicy) Select(pool HostPool) *UpstreamHost {
 
 func testPool() HostPool {
 	pool := []*UpstreamHost{
-		{Name: workableServer.URL},         // this should resolve (healthcheck test)
-		{Name: "http://shouldnot.resolve"}, // this shouldn't
+		{Name: workableServer.URL},            // this should resolve (healthcheck test)
+		{Name: "http://shouldnot.resolve:85"}, // this shouldn't, especially on port other than 80
 		{Name: "http://C"},
 	}
 	return HostPool(pool)
@@ -136,3 +136,24 @@ func TestCustomPolicy(t *testing.T) {
 		t.Error("Expected custom policy host to be the first host.")
 	}
 }
+
+func TestFirstPolicy(t *testing.T) {
+	pool := testPool()
+	rrPolicy := &First{}
+	h := rrPolicy.Select(pool)
+	// First selected host is 1, because counter starts at 0
+	// and increments before host is selected
+	if h != pool[0] {
+		t.Error("Expected always first to be first host in the pool.")
+	}
+	h = rrPolicy.Select(pool)
+	if h != pool[0] {
+		t.Error("Expected always first to be first host in the pool, even in second call")
+	}
+	// set this first in pool as failed
+	pool[0].Fails = 1
+	h = rrPolicy.Select(pool)
+	if h != pool[1] {
+		t.Error("Expected first to be he second in pool if the first one is down.")
+	}
+}