[TT-1681] Add default XML template (TykTechnologies#3510)

## Description
https://tyktech.atlassian.net/browse/TT-1681


## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
- [ ] Refactoring or add test (improvements in base code or adds test coverage to functionality)

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes that apply -->
<!-- If you're unsure about any of these, don't hesitate to ask; we're here to help! -->
- [ ] Make sure you are requesting to **pull a topic/feature/bugfix branch** (right side). If pulling from your own
      fork, don't request your `master`!
- [ ] Make sure you are making a pull request against the **`master` branch** (left side). Also, you should start
      *your branch* off *our latest `master`*.
- [ ] My change requires a change to the documentation.
  - [ ] If you've changed APIs, describe what needs to be updated in the documentation.
  - [ ] If new config option added, ensure that it can be set via ENV variable
- [ ] I have updated the documentation accordingly.
- [ ] Modules and vendor dependencies have been updated; run `go mod tidy && go mod vendor`
- [ ] When updating library version must provide reason/explanation for this update.
- [ ] I have added tests to cover my changes.
- [ ] All new and existing tests passed.
- [ ] Check your code additions will not fail linting checks:
  - [ ] `go fmt -s`
  - [ ] `go vet`

gateway/handler_error.go

@@ -95,6 +95,12 @@ type ErrorHandler struct {
 	BaseMiddleware
 }
 
+// TemplateExecutor is an interface used to switch between text/templates and html/template.
+// It only switch to text/template (templatesRaw) when contentType is XML related
+type TemplateExecutor interface {
+	Execute(wr io.Writer, data interface{}) error
+}
+
 // HandleError is the actual error handler and will store the error details in analytics if analytics processing is enabled.
 func (e *ErrorHandler) HandleError(w http.ResponseWriter, r *http.Request, errMsg string, errCode int, writeResponse bool) {
 	defer e.Base().UpdateRequestSession(r)
@@ -120,7 +126,6 @@ func (e *ErrorHandler) HandleError(w http.ResponseWriter, r *http.Request, errMs
 		w.Header().Set(headers.ContentType, contentType)
 		response.Header = http.Header{}
 		response.Header.Set(headers.ContentType, contentType)
-
 		templateName := "error_" + strconv.Itoa(errCode) + "." + templateExtension
 
 		// Try to use an error template that matches the HTTP error code and the content type: 500.json, 400.xml, etc.
@@ -158,12 +163,22 @@ func (e *ErrorHandler) HandleError(w http.ResponseWriter, r *http.Request, errMs
 		if errMsg != errCustomBodyResponse.Error() {
 			w.WriteHeader(errCode)
 			response.StatusCode = errCode
+			var tmplExecutor TemplateExecutor
+			tmplExecutor = tmpl
 
 			apiError := APIError{template.HTML(template.JSEscapeString(errMsg))}
+			if contentType == headers.ApplicationXML || contentType == headers.TextXML {
+				apiError.Message = template.HTML(errMsg)
+
+				//we look up in the last defined templateName to obtain the template.
+				rawTmpl := templatesRaw.Lookup(templateName)
+				tmplExecutor = rawTmpl
+			}
+
 			var log bytes.Buffer
 
 			rsp := io.MultiWriter(w, &log)
-			tmpl.Execute(rsp, &apiError)
+			tmplExecutor.Execute(rsp, &apiError)
 			response.Body = ioutil.NopCloser(&log)
 		}
 	}

gateway/handler_error_test.go

@@ -16,7 +16,8 @@ import (
 
 func TestHandleError_text_xml(t *testing.T) {
 	file := filepath.Join(config.Global().TemplatePath, "error_500.xml")
-	xml := `<error>
+	xml := `<?xml version = "1.0" encoding = "UTF-8"?>
+<error>
 	<code>500</code>
 	<message>{{.Message}}</message>
 </error>`
@@ -25,7 +26,7 @@ func TestHandleError_text_xml(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.Remove(file)
-	expect := `
+	expect := `<?xml version = "1.0" encoding = "UTF-8"?>
 <error>
 	<code>500</code>
 	<message>There was a problem proxying the request</message>
@@ -59,3 +60,65 @@ func TestHandleError_text_xml(t *testing.T) {
 		},
 	})
 }
+
+func TestHandleDefaultErrorXml(t *testing.T) {
+
+	expect := `<?xml version = "1.0" encoding = "UTF-8"?>
+<error>There was a problem proxying the request</error>`
+	ts := StartTest()
+	defer ts.Close()
+
+	BuildAndLoadAPI(func(spec *APISpec) {
+		spec.Proxy.ListenPath = "/"
+		spec.Proxy.TargetURL = "http://localhost:66666"
+	})
+	ts.Run(t, test.TestCase{
+		Path: "/",
+		Code: http.StatusInternalServerError,
+		Headers: map[string]string{
+			headers.ContentType: headers.TextXML,
+		},
+		BodyMatchFunc: func(b []byte) bool {
+			return strings.TrimSpace(expect) == string(bytes.TrimSpace(b))
+		},
+	})
+
+	ts.Run(t, test.TestCase{
+		Path: "/",
+		Code: http.StatusInternalServerError,
+		Headers: map[string]string{
+			headers.ContentType: headers.TextXML + "; charset=UTF-8",
+		},
+		BodyMatchFunc: func(b []byte) bool {
+			return strings.TrimSpace(expect) == string(bytes.TrimSpace(b))
+		},
+	})
+}
+
+func TestHandleDefaultErrorJSON(t *testing.T) {
+
+	expect := `
+{
+    "error": "There was a problem proxying the request"
+}
+`
+
+	ts := StartTest()
+	defer ts.Close()
+
+	BuildAndLoadAPI(func(spec *APISpec) {
+		spec.Proxy.ListenPath = "/"
+		spec.Proxy.TargetURL = "http://localhost:66666"
+	})
+	ts.Run(t, test.TestCase{
+		Path: "/",
+		Code: http.StatusInternalServerError,
+		Headers: map[string]string{
+			headers.ContentType: headers.ApplicationJSON,
+		},
+		BodyMatchFunc: func(b []byte) bool {
+			return strings.TrimSpace(expect) == string(bytes.TrimSpace(b))
+		},
+	})
+
+}

gateway/server.go

@@ -18,6 +18,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	textTemplate "text/template"
 	"time"
 
 	"github.com/TykTechnologies/again"
@@ -56,6 +57,7 @@ var (
 	pubSubLog            = log.WithField("prefix", "pub-sub")
 	rawLog               = logger.GetRaw()
 	templates            *template.Template
+	templatesRaw         *textTemplate.Template
 	analytics            RedisAnalyticsHandler
 	GlobalEventsJSVM     JSVM
 	memProfFile          *os.File
@@ -224,6 +226,7 @@ func setupGlobals(ctx context.Context) {
 	// Load all the files that have the "error" prefix.
 	templatesDir := filepath.Join(config.Global().TemplatePath, "error*")
 	templates = template.Must(template.ParseGlob(templatesDir))
+	templatesRaw = textTemplate.Must(textTemplate.ParseGlob(templatesDir))
 
 	CoProcessInit()
 

templates/error.xml

@@ -0,0 +1,2 @@
+<?xml version = "1.0" encoding = "UTF-8"?>
+<error>{{.Message}}</error>