admin: Implement a simple timer to pull config

[colinaaa]

Implemented mostly referenced to the #4106

• add a PullInterval filed in config
• setup a timer in a goroutine
• select with context and timer

A trick point is that

https://github.com/colinaaa/caddy/blob/169bb265104d0ade5f8bab05d82e53eef30c0129/caddy.go#L485-L494

here I did not use if-else to check whether PullInterval is non-zero, but I directly use time.After(PullInterval) so that with 0 intervals.
This will return immediately which is the same as executing without time.After

re #4106

[mholt]

Awesome, thanks for working on this! I'll take a look shortly.

[colinaaa]

Here is an example config.json for pull interval. Use another http server to serve this file at port 5000.

{
  "admin": {
    "listen": "0.0.0.0:2019",
    "config": {
      "load_interval": "3s",
      "load": { "module": "http", "url": "http://localhost:5000/config.json" }
    }
  },
  "apps": {
    "http": {
      "servers": {
        "local": {
          "listen": [":8080"]
        }
      }
    }
  }
}

[mholt]

Thanks, this is a good looking PR.

The only thing I'm not real sure about is the fact that all config loading is now asynchronous. That does make me a little nervous since the vast majority of users won't use the pull feature, and would probably expect synchronous config loading. I am not sure of the consequences if we unblock while the config is loaded...

I do appreciate the simplicity of this approach, but I'm not sure what kinds of effects it will have. What do you think?

[colinaaa]

The only thing I'm not really sure about is the fact that all config loading is now asynchronous. That does make me a little nervous since the vast majority of users won't use the pull feature, and would probably expect synchronous config loading. I am not sure of the consequences if we unblock while the config is loaded...

I do appreciate the simplicity of this approach, but I'm not sure what kinds of effects it will have. What do you think?

I agree that load config asynchronously is a breaking change. I'm doing this because it's the simplest way to implement this feature.

I will update this pr to keep loading config synchronously when no interval is specified as well as keep the code simple and clear.

[francislavoie]

There's still an issue here, CI failing. I think PullInterval needs to be wrapped with a time.Duration() to type convert it to a time.Duration.

[mholt]

On second thought... this affects how often the function specified by the load property is run. So I wonder if, for consistency, we should call this LoadInterval instead. Does that make sense?

[colinaaa]

Sounds good to me!

[mholt]

Nice, this is a lot more like what I had envisioned. Does it work well? If so, there's just the matter of deciding the name for the property. Then we can merge this.

[colinaaa]

Does it work well?

It works as expected with the config I have given at #4246 (comment)

And I tried another config without load_interval, which work as expected too!

[mholt]

Awesome, thanks for working on this and trying it out!

I had one more suggestion that I just noticed; otherwise, LGTM and let's merge this and give it a try.