fix(engine): Fix template rendering thread safety issue (#4828)

Remove the engine `currentTemplates` field which was shared state
across threads and thus not thread safe, and instead just pass these
reference templates as parameters down recursively.

Closes #4819

Signed-off-by: Sean Eagan <sean.eagan@att.com>

pkg/engine/engine.go

@@ -39,7 +39,6 @@ type Engine struct {
 	// If strict is enabled, template rendering will fail if a template references
 	// a value that was not passed in.
 	Strict bool
-	CurrentTemplates map[string]renderable
 	// In LintMode, some 'required' template values may be missing, so don't fail
 	LintMode bool
 }
@@ -122,7 +121,6 @@ func FuncMap() template.FuncMap {
 func (e *Engine) Render(chrt *chart.Chart, values chartutil.Values) (map[string]string, error) {
 	// Render the charts
 	tmap := allTemplates(chrt, values)
-	e.CurrentTemplates = tmap
 	return e.render(tmap)
 }
 
@@ -139,7 +137,7 @@ type renderable struct {
 // alterFuncMap takes the Engine's FuncMap and adds context-specific functions.
 //
 // The resulting FuncMap is only valid for the passed-in template.
-func (e *Engine) alterFuncMap(t *template.Template) template.FuncMap {
+func (e *Engine) alterFuncMap(t *template.Template, referenceTpls map[string]renderable) template.FuncMap {
 	// Clone the func map because we are adding context-specific functions.
 	var funcMap template.FuncMap = map[string]interface{}{}
 	for k, v := range e.FuncMap {
@@ -198,7 +196,7 @@ func (e *Engine) alterFuncMap(t *template.Template) template.FuncMap {
 
 		templates[templateName.(string)] = r
 
-		result, err := e.render(templates)
+		result, err := e.renderWithReferences(templates, referenceTpls)
 		if err != nil {
 			return "", fmt.Errorf("Error during tpl function execution for %q: %s", tpl, err.Error())
 		}
@@ -210,6 +208,12 @@ func (e *Engine) alterFuncMap(t *template.Template) template.FuncMap {
 
 // render takes a map of templates/values and renders them.
 func (e *Engine) render(tpls map[string]renderable) (rendered map[string]string, err error) {
+	return e.renderWithReferences(tpls, tpls)
+}
+
+// renderWithReferences takes a map of templates/values to render, and a map of
+// templates which can be referenced within them.
+func (e *Engine) renderWithReferences(tpls map[string]renderable, referenceTpls map[string]renderable) (rendered map[string]string, err error) {
 	// Basically, what we do here is start with an empty parent template and then
 	// build up a list of templates -- one for each file. Once all of the templates
 	// have been parsed, we loop through again and execute every template.
@@ -231,7 +235,7 @@ func (e *Engine) render(tpls map[string]renderable) (rendered map[string]string,
 		t.Option("missingkey=zero")
 	}
 
-	funcMap := e.alterFuncMap(t)
+	funcMap := e.alterFuncMap(t, referenceTpls)
 
 	// We want to parse the templates in a predictable order. The order favors
 	// higher-level (in file system) templates over deeply nested templates.
@@ -248,9 +252,9 @@ func (e *Engine) render(tpls map[string]renderable) (rendered map[string]string,
 		files = append(files, fname)
 	}
 
-	// Adding the engine's currentTemplates to the template context
+	// Adding the reference templates to the template context
 	// so they can be referenced in the tpl function
-	for fname, r := range e.CurrentTemplates {
+	for fname, r := range referenceTpls {
 		if t.Lookup(fname) == nil {
 			t = t.New(fname).Funcs(funcMap)
 			if _, err := t.Parse(r.tpl); err != nil {