fix: changing libwaku's error handling format

[gabrielmer]

Description

For some reason, when immediately chaining sendRequestToWakuThread's response with the handleRes template, there's cases in which the errors are not handled properly. See this message for a more detailed description of the issue.

It's probably due to some misusage of Nim templates, but until we further understand what's wrong with out chaining approach, modifying to a working version.

Update

The template usage was wrong - changed it to a proc and now works great :)

Changes

Issue

#3039

[github-actions]

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[gabrielmer]

@arnetheduck whenever you're available could you please take a quick look? maybe you have a better understanding on what's going on 😶 we're not sure why does

waku_thread
  .sendRequestToWakuThread(
    ctx,
    RequestType.LIFECYCLE,
    NodeLifecycleRequest.createShared(NodeLifecycleMsgType.START_NODE),
  )
  .handleRes(callback, userData)

behave differently than

let res = waku_thread
  .sendRequestToWakuThread(
    ctx,
    RequestType.LIFECYCLE,
    NodeLifecycleRequest.createShared(NodeLifecycleMsgType.START_NODE),
  )

res.handleRes(callback, userData)

handleRes is defined here

nwaku/library/libwaku.nim

Lines 55 to 72 in 0f8e874

	template handleRes[T: string | void](
	res: Result[T, string], callback: WakuCallBack, userData: pointer
	) =
	## Handles the Result responses, which can either be Result[string, string] or
	## Result[void, string]. Notice that in case of Result[void, string], it is enough to
	## just return RET_OK and not provide any additional feedback through the callback.
	if res.isErr():
	foreignThreadGc:
	let msg = "libwaku error: " & $res.error
	callback(RET_ERR, unsafeAddr msg[0], cast[csize_t](len(msg)), userData)
	return RET_ERR
	when T is string:
	let msg = $res.get()
	if msg.len > 0:
	foreignThreadGc:
	callback(RET_OK, unsafeAddr msg[0], cast[csize_t](len(msg)), userData)
	return RET_OK

[arnetheduck]

[12:22]arnetheduck: the way the code is written, the handleRes template is calling sendRequestToWakuThread twice when there's an error, I'm pretty sure that's not what you intended
[12:24]arnetheduck: parameters in templates get their full ast expanded at every point of use - if you use res twice, you get two expansions of sendRequestToWakuThread...
[12:25]arnetheduck: that's why moving to let "fixes" it - instead of two calls, it accesses the variable twice