spin up should check required variables

[ThorstenHans]

Currently, the spin up command does not check if required variables are provided when launching the application.

Consider a Spin application with the following manifest:

spin_manifest_version = 2

[application]
name = "test"
version = "0.1.0"
description = ""

[variables]
foo = { required = true }

[[trigger.http]]
route = "/..."
component = "test"

[component.test]
source = "target/wasm32-wasip1/release/test.wasm"
allowed_outbound_hosts = []

[component.test.variables]
foo = "{{foo}}"

[component.test.build]
command = "cargo build --target wasm32-wasip1 --release"
watch = ["src/**/*.rs", "Cargo.toml"]

use spin_sdk::http::{IntoResponse, Request, Response};
use spin_sdk::{http_component, variables};
#[http_component]
fn handle_test_var(_: Request) -> anyhow::Result<impl IntoResponse> {
    match variables::get("foo") {
        Ok(value) => println!("{}", value),
        Err(r) => println!("{}", r),
    };
    Ok(Response::new(200, ()))
}

Expected behavior

I expect spin up to bail out and prevent from launching the app - if the required variable foo is not specified (e.g., by setting the SPIN_VARIABLE_FOO environment variable)

Actual behavior

I can start the app locally using spin up. Sending a request to the app (which tries to load the foo variable will result in a runtime error:

Error::Provider("no provider resolved required variable \"foo\"")

Spin Version

I see this behavior with current spin version (tested 3.0.0, 3.1.2 and 3.2.0)

[itowlson]

Variables are lazily loaded each time they are accessed. I guess we could evaluate if the runtime config contains only the environment provider, where the set of variables and values can't change after startup, and do validation in that case.

Alternatively I think @lann has suggested switching variables to be read once at startup in which case this would happen automatically. (Sorry Lann if I am misremembering.) That's a breaking change though.