proxy: Add the first policy (#1513)

* Add the first policy which sends the request to the first available host

* Make the error message clear. As we expect the second not first upstream
host.

caddyhttp/proxy/policy.go

@@ -22,6 +22,7 @@ func init() {
 	RegisterPolicy("least_conn", func() Policy { return &LeastConn{} })
 	RegisterPolicy("round_robin", func() Policy { return &RoundRobin{} })
 	RegisterPolicy("ip_hash", func() Policy { return &IPHash{} })
+	RegisterPolicy("first", func() Policy { return &First{} })
 }
 
 // Random is a policy that selects up hosts from a pool at random.
@@ -131,3 +132,16 @@ func (r *IPHash) Select(pool HostPool, request *http.Request) *UpstreamHost {
 	}
 	return nil
 }
+
+// First is a policy that selects the fist available host
+type First struct{}
+
+// Select selects the first host from the pool, that is available
+func (r *First) Select(pool HostPool, request *http.Request) *UpstreamHost {
+	for _, host := range pool {
+		if host.Available() {
+			return host
+		}
+	}
+	return nil
+}

caddyhttp/proxy/policy_test.go

@@ -226,3 +226,20 @@ func TestIPHashPolicy(t *testing.T) {
 		t.Error("Expected ip hash policy host to be nil.")
 	}
 }
+
+func TestFirstPolicy(t *testing.T) {
+	pool := testPool()
+	firstPolicy := &First{}
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+
+	h := firstPolicy.Select(pool, req)
+	if h != pool[0] {
+		t.Error("Expected first policy host to be the first host.")
+	}
+
+	pool[0].Unhealthy = 1
+	h = firstPolicy.Select(pool, req)
+	if h != pool[1] {
+		t.Error("Expected first policy host to be the second host.")
+	}
+}