Add a start callback for HTTP updater

[baruch]

The callback is called when the upgrade actually starts rather than just
the initial query so that the user can know that it will not take longer
and can also prepare for the upgrade by shutting down other works.

[igrr]

This almost looks good to me, but I would propose to replace plain C function pointer with an std::function<void(void)>, similar to what we do in ESP8266WebServer library.
Obvious benefit is that you can bind a member function to a pointer/reference using std::bind, and pass the result into std::function. With plain C function pointer there is no way to do that.

[mangelajo]

If you had time to add progress and end callbacks like in ArduinoOTA, that'd be great.

Cheers!,

[baruch]

The problem with progress and end is that at that time there is no TCP/UDP active as they are cancelled by the upgrade process.

As for the method calls, I can change to that, they are not that common in Arduino code though and all other callbacks throughout the system is normal function calls too.

[mangelajo]

@baruch , but you don't need any other open TCP/UDP connection to show progress, the use case is device itself showing on a display/leds. I use it in all my projects so the end user can have some indication of the upgrade progress.

I'd be glad to see an standard interface for those callbacks for consistency within esp8266-arduino at least.

[ghost]

Can one of the admins verify this patch?

[krzychb]

@esp8266-arduino-bot,
You are not a human, are you?
Great avatar 😄 👍

[igrr]

Sorry for the noise. My regression testing setup is misbehaving.

[igrr]

@baruch just to clarify regarding callback functions: if an argument is an std::function<void(void)>, you can pass plain C function pointer and it will be implicitly converted. This will behave just like other Arduino functions, attachInterrupt for example. However, this also allows the user to pass in any other function object (lambda function, anything that std::bind returns, or even something custom). So in general, replacing a C function pointer argument with a corresponding std::function won't hurt.

[karlp]

After just running into this lack in the http update server, I've started work on this based on the ArduinoOTA style.

[earlephilhower]

The core and libraries have changed enough that this PR now has a merge conflict.

Could you merge it manually with the latest core, so we can consider it for future releases?

[earlephilhower]

Closing as abandoned for #6796