reverseproxy: Correct alternate port for active health checks (#3693)

* reverseproxy: construct active health-check transport from scratch (Fixes #3691)

* reverseproxy: do upstream health-check on the correct alternative port

* reverseproxy: add integration test for health-check on alternative port

* reverseproxy: put back the custom transport for health-check http client

* reverseproxy: cleanup health-check integration test

* reverseproxy: fix health-check of unix socket upstreams

* reverseproxy: skip unix socket tests on Windows

* tabs > spaces

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* make the linter (and @francislavoie) happy

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* One more lint fix

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

caddytest/integration/reverseproxy_test.go

@@ -0,0 +1,97 @@
+package integration
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"os"
+	"runtime"
+	"strings"
+	"testing"
+
+	"github.com/caddyserver/caddy/v2/caddytest"
+)
+
+func TestReverseProxyHealthCheck(t *testing.T) {
+	tester := caddytest.NewTester(t)
+	tester.InitServer(`
+	{
+		http_port     9080
+		https_port    9443
+	}
+	http://localhost:2020 {
+		respond "Hello, World!"
+	}
+	http://localhost:2021 {
+		respond "ok"
+	}
+	http://localhost:9080 {
+		reverse_proxy {
+			to localhost:2020
+	
+			health_path /health
+			health_port 2021
+			health_interval 2s
+			health_timeout 5s
+		}
+	}
+  `, "caddyfile")
+
+	tester.AssertGetResponse("http://localhost:9080/", 200, "Hello, World!")
+}
+
+func TestReverseProxyHealthCheckUnixSocket(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.SkipNow()
+	}
+	tester := caddytest.NewTester(t)
+	f, err := ioutil.TempFile("", "*.sock")
+	if err != nil {
+		t.Errorf("failed to create TempFile: %s", err)
+		return
+	}
+	// a hack to get a file name within a valid path to use as socket
+	socketName := f.Name()
+	os.Remove(f.Name())
+
+	server := http.Server{
+		Handler: http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			if strings.HasPrefix(req.URL.Path, "/health") {
+				w.Write([]byte("ok"))
+				return
+			}
+			w.Write([]byte("Hello, World!"))
+		}),
+	}
+
+	unixListener, err := net.Listen("unix", socketName)
+	if err != nil {
+		t.Errorf("failed to listen on the socket: %s", err)
+		return
+	}
+	go server.Serve(unixListener)
+	t.Cleanup(func() {
+		server.Close()
+	})
+	runtime.Gosched() // Allow other goroutines to run
+
+	tester.InitServer(fmt.Sprintf(`
+	{
+		http_port     9080
+		https_port    9443
+	}
+	http://localhost:9080 {
+		reverse_proxy {
+			to unix/%s
+	
+			health_path /health
+			health_port 2021
+			health_interval 2s
+			health_timeout 5s
+		}
+	}
+	`, socketName), "caddyfile")
+
+	tester.AssertGetResponse("http://localhost:9080/", 200, "Hello, World!")
+}