chore: improve /healthz endpoint performance (#2014)

* Add benchmark for /healthz endpoint

This benchmark will measure not only speed but also how much memory
each request to /healthz allocate. I'm adding the benchmark to have a
baseline to compare.

Signed-off-by: Leandro López (inkel) <inkel.ar@gmail.com>

* Hardcode /healthz body into a variable

We know it will always be this value, there's no need to import and
call json.MarshalIndent.

Signed-off-by: Leandro López (inkel) <inkel.ar@gmail.com>

* Initialize /healthz body only once

Signed-off-by: Leandro López (inkel) <inkel.ar@gmail.com>

server/server.go

@@ -17,7 +17,6 @@ package server
 
 import (
 	"context"
-	"encoding/json"
 	"flag"
 	"fmt"
 	"log"
@@ -899,20 +898,14 @@ func mkSubDir(parentDir string, subDir string) (string, error) {
 
 // Healthz returns the health check response. It always returns a 200 currently.
 func (s *Server) Healthz(w http.ResponseWriter, _ *http.Request) {
-	data, err := json.MarshalIndent(&struct {
-		Status string `json:"status"`
-	}{
-		Status: "ok",
-	}, "", "  ")
-	if err != nil {
-		w.WriteHeader(http.StatusInternalServerError)
-		fmt.Fprintf(w, "Error creating status json response: %s", err)
-		return
-	}
 	w.Header().Set("Content-Type", "application/json")
-	w.Write(data) // nolint: errcheck
+	w.Write(healthzData) // nolint: errcheck
 }
 
+var healthzData = []byte(`{
+  "status": "ok"
+}`)
+
 // ParseAtlantisURL parses the user-passed atlantis URL to ensure it is valid
 // and we can use it in our templates.
 // It removes any trailing slashes from the path so we can concatenate it

server/server_test.go

@@ -147,6 +147,25 @@ func TestHealthz(t *testing.T) {
 }`, string(body))
 }
 
+type mockRW struct{}
+
+var _ http.ResponseWriter = mockRW{}
+var mh = http.Header{}
+
+func (w mockRW) WriteHeader(int)           {}
+func (w mockRW) Write([]byte) (int, error) { return 0, nil }
+func (w mockRW) Header() http.Header       { return mh }
+
+var w = mockRW{}
+var s = &server.Server{}
+
+func BenchmarkHealthz(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		s.Healthz(w, nil)
+	}
+}
+
 func TestParseAtlantisURL(t *testing.T) {
 	cases := []struct {
 		In     string