Add /debug/liveness handler for liveness checks.

go/vt/servenv/liveness.go

@@ -0,0 +1,35 @@
+/*
+Copyright 2019 The Vitess Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package servenv
+
+import (
+	"net/http"
+)
+
+// This file registers a handler that immediately responds with HTTP 200 OK.
+// This can be used as a liveness check to make sure the process can pass a
+// minimum bar of functionality: accept a connection and reply unconditionally.
+// The handler is as simple as possible to avoid false negatives for liveness.
+// If the process is actually making progress, albeit slowly, failing the
+// liveness check and restarting the process will likely only make it fall
+// further behind on its backlog.
+
+func init() {
+	http.HandleFunc("/debug/liveness", func(rw http.ResponseWriter, r *http.Request) {
+		// Do nothing. Return success immediately.
+	})
+}

go/vt/servenv/liveness_test.go

@@ -0,0 +1,42 @@
+/*
+Copyright 2019 The Vitess Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package servenv
+
+import (
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+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)
+	}
+	defer resp.Body.Close()
+
+	// Make sure we can read the body, even though it's empty.
+	body, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("ioutil.ReadAll: %v", err)
+	}
+	t.Logf("body: %q", body)
+}