[PATCH] CORE-6348 add ability to depool vtgates via liveness check en…

…dpoint (vitessio#3)

`/debug/liveness` endpoint now returns a 503 when `/etc/etsy/depool` is present on the filesystem

note: included "unit" test currently relies on `/etc/etsy` existing and being writable. run with `go test go/vt/servenv/*.go`

Signed-off-by: Mackenzie Starr <mstarr@etsy.com>

go/vt/servenv/liveness.go

@@ -18,6 +18,7 @@ package servenv
 
 import (
 	"net/http"
+	"os"
 )
 
 // This file registers a handler that immediately responds with HTTP 200 OK.
@@ -30,6 +31,9 @@ import (
 
 func init() {
 	http.HandleFunc("/debug/liveness", func(rw http.ResponseWriter, r *http.Request) {
-		// Do nothing. Return success immediately.
+		if _, err := os.Stat("/etc/etsy/depool"); !os.IsNotExist(err) {
+			rw.WriteHeader(http.StatusServiceUnavailable)
+			return
+		}
 	})
 }

go/vt/servenv/liveness_test.go

@@ -20,13 +20,15 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestLivenessHandler(t *testing.T) {
 	server := httptest.NewServer(nil)
 	defer server.Close()
-
 	resp, err := http.Get(server.URL + "/debug/liveness")
 	if err != nil {
 		t.Fatalf("http.Get: %v", err)
@@ -40,3 +42,42 @@ func TestLivenessHandler(t *testing.T) {
 	}
 	t.Logf("body: %q", body)
 }
+
+func TestLivenessHandlerDepool(t *testing.T) {
+	assert := assert.New(t)
+	server := httptest.NewServer(nil)
+	defer server.Close()
+
+	var resp, err = http.Get(server.URL + "/debug/liveness")
+	if err != nil {
+		t.Fatalf("http.Get: %v", err)
+	}
+	assert.Equal(
+		http.StatusOK,
+		resp.StatusCode,
+		"should return 200 response code when depool file is not present",
+	)
+
+	// requires
+	// ```
+	// sudo mkdir -p /etc/etsy
+	// sudo chmod 777 /etc/etsy
+	// ```
+	var path = "/etc/etsy/depool"
+	_, err = os.Create(path)
+	if err != nil {
+		t.Error(err)
+	}
+	defer os.Remove(path)
+
+	resp, err = http.Get(server.URL + "/debug/liveness")
+	if err != nil {
+		t.Fatalf("http.Get: %v", err)
+	}
+	assert.Equal(
+		http.StatusServiceUnavailable,
+		resp.StatusCode,
+		"should return 503 response code when depool file is present",
+	)
+	defer resp.Body.Close()
+}