Spin Build Targets Check SIP

docs/content/sips/0XX-spin-build-targets-check.md

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+title = "SIP 0XX - Spin Build Targets Check"
+template = "main"
+date = "2024-06-13T12:00:00Z"
+---
+Summary: Extend `spin build` to validate component targets against trigger types.
+
+Owner(s): 
+    [[email protected]](mailto:[email protected])
+    [[email protected]](mailto:[email protected])
+
+Created: June 13, 2024
+
+# Background
+When associating a Spin component to a specific trigger in the Spin manifest, there is an implied world that the Spin component is targeting (e.g. the http trigger type implies `fermyon:spin/http-trigger`). However, we don't know if the Spin component actually implements that world until instantiation time (i.e. when a request / event comes in).
+
+As a visual example, if a user mistankenly associates an http trigger with a component that targeted the redis world:
+
+`examples/hello-world/spin.toml:`
+```examples/hello-world/spin.toml
+[[trigger.http]]
+route = "/hello"
+component = "hello"
+
+[component.hello]
+source = "redis-component.wasm" # this is a redis component which is wrong
+```
+
+When issuing a curl to this endpoint:
+```
+❯ curl -i http://127.0.0.1:3000/hello
+HTTP/1.1 500 Internal Server Error
+content-length: 0
+date: Tue, 04 Jun 2024 21:53:41 GMT
+```
+
+And in the output of `spin up` we see:
+```
+❯ spin up -f examples/hello-world 
+Logging component stdio to "examples/hello-world/.spin/logs/"
+
+Serving http://127.0.0.1:3000
+Available Routes:
+  hello: http://127.0.0.1:3000/hello
+    A simple component that returns hello.
+2024-06-04T21:53:16.160773Z ERROR spin_trigger_http: Error processing request: Expected component to either export `wasi:http/[email protected]` or `fermyon:spin/inbound-http` but it exported neither    
+```
+
+# Proposal
+
+Validate that each Spin component structurally conforms to (or targets) the expected world implied by the associated trigger type at build time rather than runtime.
+
+Prior art for performing a structural target of components against a world is [`wasm-tools component targets ...`](https://github.com/bytecodealliance/wasm-tools/blob/9340ed2466a50b4dbc580b13ba18a417dee91433/src/bin/wasm-tools/component.rs#L683) subcommand. We could potentially use [wac target predicate implemtation](https://github.com/bytecodealliance/wac/blob/4c96def294e6e779c52cfc5a93e05ed4c73ee60f/crates/wac-parser/src/resolution.rs#L2708) for a cleaner more actionable user diagnostic. Effectively, spin build could invoke the targets check using the trigger implied world and the source component.
+
+An "approximate" visual for the spin build experience we could enable:
+
+```
+❯ spin build -f examples/hello-world 
+Component "hello" is not a valid http component.
+
+Error: 
+    Expected component to either export `wasi:http/[email protected]` or `fermyon:spin/inbound-http` but it exported neither    
+```
+
+# Open question: How does this work with custom triggers?
+One possible solution is to allow trigger extensions the abilitiy to hook into build time validation via the `TriggerExecutor` trait. For each `spin build`, the `spin-cli` could construct a `build` command to invoke each trigger with allowing the custom trigger an opportunity to enforce validation on the target of each component before `up`.