reduce number of fetches for dynamic commands (#549)

Signed-off-by: David Chung <david.chung@docker.com>

Author: David Chung

pkg/cli/context.go

@@ -428,14 +428,7 @@ func (c *Context) Funcs() []template.Function {
 
 // loadBackend determines the backend to use for executing the rendered template text (e.g. run in shell).
 // During this phase, the template delimiters are changed to =% %= so put this in the comment {{/* */}}
-func (c *Context) loadBackends() error {
-	t, err := template.NewTemplate(c.src, template.Options{
-		DelimLeft:  "=%",
-		DelimRight: "%=",
-	})
-	if err != nil {
-		return err
-	}
+func (c *Context) loadBackends(t *template.Template) error {
 	t.AddFunc("print",
 		func() string {
 			c.run = func(script string) error {
@@ -567,35 +560,61 @@ func (c *Context) loadBackends() error {
 			return ""
 		})
 
-	_, err = t.Render(c)
+	_, err := t.Render(c)
+
+	// clean up after we rendered...  remove the functions
+	t.RemoveFunc("sh", "print", "instanceProvision", "managerCommit")
 	return err
 }
 
+func (c *Context) getTemplate() (*template.Template, error) {
+	if c.template == nil {
+		t, err := template.NewTemplate(c.src, template.Options{})
+		if err != nil {
+			return nil, err
+		}
+		c.template = t
+	}
+	return c.template, nil
+}
+
 // BuildFlags from parsing the body which is a template
-func (c *Context) BuildFlags() error {
-	t, err := template.NewTemplate(c.src, template.Options{})
+func (c *Context) BuildFlags() (err error) {
+	var t *template.Template
+
+	t, err = c.getTemplate()
 	if err != nil {
-		return err
+		return
 	}
-
+	t.SetOptions(template.Options{})
 	_, err = configureTemplate(t, c.plugins).Render(c)
-	return err
+	return
 }
 
 // Execute runs the command
-func (c *Context) Execute() error {
+func (c *Context) Execute() (err error) {
+	var t *template.Template
 
-	if err := c.loadBackends(); err != nil {
-		return err
+	t, err = c.getTemplate()
+	if err != nil {
+		return
 	}
 
-	t, err := template.NewTemplate(c.src, template.Options{
-		Stderr: func() io.Writer { return os.Stderr },
+	// First pass to get the backends
+	t.SetOptions(template.Options{
+		DelimLeft:  "=%",
+		DelimRight: "%=",
 	})
-	if err != nil {
+
+	if err := c.loadBackends(t); err != nil {
 		return err
 	}
 
+	// Now regular processing
+	t.SetOptions(template.Options{
+		Stderr: func() io.Writer { return os.Stderr },
+	})
+
 	c.exec = true
 	c.template = t
 	script, err := configureTemplate(t, c.plugins).Render(c)

pkg/template/template.go

@@ -143,6 +143,15 @@ func NewTemplateFromBytes(buff []byte, contextURL string, opt Options) (*Templat
 	}, nil
 }
 
+// NewFromTemplate creates a new template from existing, without any fetches (hence network calls)
+func NewFromTemplate(from *Template, opt Options) *Template {
+	return &Template{
+		options: opt,
+		url:     from.url,
+		body:    from.body,
+	}
+}
+
 // SetOptions sets the runtime flags for the engine
 func (t *Template) SetOptions(opt Options) *Template {
 	t.lock.Lock()
@@ -167,6 +176,16 @@ func (t *Template) AddFunc(name string, f interface{}) *Template {
 	return t
 }
 
+// RemoveFunc remove the functions
+func (t *Template) RemoveFunc(name ...string) *Template {
+	t.lock.Lock()
+	defer t.lock.Unlock()
+	for _, n := range name {
+		delete(t.funcs, n)
+	}
+	return t
+}
+
 // Ref returns the value keyed by name in the context of this template. See 'ref' template function.
 func (t *Template) Ref(name string) interface{} {
 	if found, has := t.globals[name]; has {
@@ -275,10 +294,6 @@ func (t *Template) build(context Context) error {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
-	if t.parsed != nil {
-		return nil
-	}
-
 	registered := []Function{}
 	fm := map[string]interface{}{}