Save http trace to a file (#5300)

Save http trace to a file instead of displaying it onto the console.
the environment variable MINIO_HTTP_TRACE will be a filepath instead
of a boolean.

This to handle the scenario where both json and http tracing are
turned on. In that case, both http trace and json output are displayed
on the screen making the json not parsable. Loging this trace onto
a file helps us avoid that scenario.

Fixes #5263

cmd/utils_test.go

@@ -19,8 +19,10 @@ package cmd
 import (
 	"encoding/json"
 	"errors"
+	"io/ioutil"
 	"net/http"
 	"net/url"
+	"os"
 	"reflect"
 	"strings"
 	"testing"
@@ -50,6 +52,19 @@ func TestCloneHeader(t *testing.T) {
 	}
 }
 
+// Tests closing http tracing file.
+func TestStopHTTPTrace(t *testing.T) {
+	var err error
+	globalHTTPTraceFile, err = ioutil.TempFile("", "")
+	if err != nil {
+		defer os.Remove(globalHTTPTraceFile.Name())
+		stopHTTPTrace()
+		if globalHTTPTraceFile != nil {
+			t.Errorf("globalHTTPTraceFile is not nil, it is expected to be nil")
+		}
+	}
+}
+
 // Tests maximum object size.
 func TestMaxObjectSize(t *testing.T) {
 	sizes := []struct {