text/template: allow explicit redefinition of a template

[parkr]

Hey Go team,

I am interested in redefinition of templates. Implicit redefinition is disallowed. How about explicit redefinition? Either:

• Template.Remove(name string) followed by Template.ParseFiles(names ...string)
• Template.Redefine(name, body string) / Template.RedefineFile(name, file string)

Scenario

We have a server which watches a folder for updates. When it receives a write or create for a file, it re-reads in the template. This allows for no-downtime updates to our templates, which is something we value greatly. It also allows for a more streamlined development process (developer isn't spending time restarting the server locally on changes).

With each filesystem event, we receive the event type and the file it affects. We would like to be able to redefine this template at runtime as these events are triggered.

Currently...

// Template has been changes on the filesystem.
// Re-read and parse. Store the latest.
// Always throws the 'redefinition' error.
_, err := rootTmpl.ParseFiles(event.FileName)
if err != nil {
    log.Printf("error reading template '%s': %v", event.FileName, err)
}

Unfortunately, this fails every time, with template: redefinition of template %NAME%.

Proposal

My proposal is to allow an explicit removal or redefinition. An example with Template.Remove(name string)...

// Allow explicit removal of a template to allow redefinition.
err := rootTmpl.Remove(event.FileName)
if err != nil && err != template.ErrorNoSuchTemplate {
    log.Printf("error removing template '%s': %v", event.FileName, err)
}

// This should NOT return an error.
_, err = rootTmpl.ParseFiles(event.FileName)
if err != nil {
    log.Printf("error reading template '%s': %v", event.FileName, err)
}

Current Work-Around

To get around this, we bind to SIGUSR1 and re-read all the templates (Template.ParseFilesGlob), blocking requests until it's re-defined.

[parkr]

Are there any thoughts here? Am I barking up the wrong tree?

[adg]

@robpike would an API like this allay your concerns about accidental template redefinition?

[robpike]

This is already doable without too much trouble by synchronously reparsing the full set of templates when one changes. Plus, removing a template like this will violate invariants maintained by html/template, which escapes the content based on a statically knowable set of templates.