coroutine

The standard explicitly says this is not possible. As per Note 1 in stmt.return.coroutine#1

... A coroutine shall not enclose a return statement ([stmt.return]).

[Note 1: For this determination, it is irrelevant whether the return statement is enclosed by a discarded statement ([stmt.if]). — end note]

So you won't be able to return from a coroutine even if it's in a discarded statement. You can specialize the function template instead of using if constexpr.

If a coroutine was launched and no dispatcher was specified (eg. GlobalScope.launch {}), what dispatcher is used?

If no dispatcher is specified in the coroutine builder, you get the dispatcher from whichever CoroutineScope you're starting your coroutine in. If there is no dispatcher in that scope's context, the coroutine will use Dispatchers.Default (see the doc of launch for instance).

Note that the scope is the receiver of the coroutine builder call:

• if you see GlobalScope.launch { ... } then GlobalScope is the scope
• if you see scope.launch { ... }, look at that scope
• if you see launch { .. } in the wild, some instance of CoroutineScope must be available as this in that piece of code, so that's the parent scope (see below for an example on where it could be coming from)

Here is some info about the dispatchers used in the most common coroutine scopes:

If the scope is GlobalScope, then it doesn't have any dispatcher, so as mentioned before the coroutines will use Dispatchers.Default.

If the scope is lifecycleScope or viewModelScope provided by Android, then coroutines will use Dispatchers.Main.immediate by default.

If the scope is created with the CoroutineScope() factory function without particular dispatcher, Dispatchers.Default will be used (see the "Custom usage" section in the documentation).

If the scope is created using MainScope() and without particular dispatcher, it will use Dispatchers.Main as per the same documentation.

If the scope is provided by runBlocking, then it will use a special dispatcher that works like an event loop and executes your coroutines in the thread that called runBlocking (which is nice because that thread would be blocked anyway).

If the scope is provided by another (outer) coroutine builder like launch or async (which means your coroutine is a child of that coroutine), then the dispatcher will be taken from the scope that was used to launch the parent coroutine, unless they override it. So you can go all the way up until you reach one of the options mentioned above:

If it's not the bandwidth that limits you (but I cannot check this), there is a solution less complicated than the celery and rabbitmq but it is not as scalable as the celery and rabbitmq, it will be limited by your number of CPU.

Instead of splitting calls on celery workers, you split them on multiple processes.

I modified the fetch function like this:

A cold stream does not start producing values until one starts to collect them. A hot stream on the other hand starts producing values immediately.

I would recommend to read below to understand hot and cold steams with usage:

https://balwindersinghrajput.medium.com/complete-guide-to-livedata-and-flow-answering-why-where-when-and-which-6b31496ba7f3

https://developer.android.com/kotlin/flow/stateflow-and-sharedflow

The issue is that your compile SDK is 31, you are targetting API 31 (Android 12) and not setting the exported attribute.

You need to specify android:exported="true" in the manifest.

If your app targets Android 12 and contains activities, services, or broadcast receivers that use intent filters, you must explicitly declare the android: exported attribute for these app components.

I managed to fix this by upgrading compileSdk to 31 and kotlin gradle plugin to 1.5.10

I don't know if it is critical for your problem but modifying this

In some sense, the mess you experience is a consequence of Kotlin coroutines having been an early success, before they became stable. In their experimental days, one thing they lacked was structured concurrency, and a ton of web material got written about them in that state (such as your link 1 from 2017). Some of the then-valid preconceptions remained with people even after their maturation, and got perpetuated in even more recent posts.

The actual situation is quite clear — all you have to understand is coroutine hierarchy, which is mediated through the Job objects. It doesn't matter whether it's a launch or an async, or any further coroutine builder — they all behave uniformly.

With this in mind, let's go through your examples:

This seems to be a legacy implementation for MSVSC. MSVSC implemented coroutines before the standard was formally complete, so there are two implementations of async (/async and /async:strict). I seem to have the old, non–standard-compliant version turned on.

The standard is clear that you cannot use plain return statements in coroutines (emphasis added):

Coroutines cannot use variadic arguments, plain return statements, or placeholder return types (auto or Concept). Constexpr functions, constructors, destructors, and the main function cannot be coroutines.

— https://en.cppreference.com/w/cpp/language/coroutines

You can verify that this is a legacy behavior with a simple example (view in Godbolt):