feat: add wasmtime host with epoch-bounded WasmProvider

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/server/src/lib.rs

@@ -3,3 +3,4 @@ pub mod fetch;
 pub mod http;
 pub mod model;
 pub mod service;
+pub mod wasm;

crates/server/src/wasm.rs

@@ -0,0 +1,182 @@
+use std::path::Path;
+
+use wasmtime::component::{Component, Linker};
+use wasmtime::{Config, Engine, Store};
+use wasmtime_wasi::{ResourceTable, WasiCtx, WasiCtxBuilder, WasiCtxView, WasiView};
+
+use crate::error::HostError;
+use crate::model::{Comment, Entry, FetchedResponse, ParseOutcome};
+use crate::service::ProviderHandle;
+
+wasmtime::component::bindgen!({
+    world: "provider",
+    path: "../../wit",
+});
+
+use exports::whoareyou::provider::lookup::{
+    LookupError as WitLookupError, Response as WitResponse,
+};
+
+/// How many epoch ticks a guest call may run. The epoch thread ticks every
+/// 100 ms → 50 ticks ≈ 5 s budget per call.
+const EPOCH_DEADLINE_TICKS: u64 = 50;
+pub const EPOCH_TICK: std::time::Duration = std::time::Duration::from_millis(100);
+
+pub struct HostState {
+    ctx: WasiCtx,
+    table: ResourceTable,
+}
+
+impl WasiView for HostState {
+    fn ctx(&mut self) -> WasiCtxView<'_> {
+        WasiCtxView {
+            ctx: &mut self.ctx,
+            table: &mut self.table,
+        }
+    }
+}
+
+pub fn engine() -> Result<Engine, HostError> {
+    let mut config = Config::new();
+
+    config.epoch_interruption(true);
+
+    Ok(Engine::new(&config)?)
+}
+
+pub fn linker(engine: &Engine) -> Result<Linker<HostState>, HostError> {
+    let mut linker = Linker::new(engine);
+
+    wasmtime_wasi::p2::add_to_linker_sync(&mut linker)?;
+
+    Ok(linker)
+}
+
+/// Spawn the thread that advances the engine epoch so runaway guest calls
+/// trap instead of hanging the service. Call once at startup.
+pub fn spawn_epoch_thread(engine: &Engine) {
+    let engine = engine.clone();
+
+    std::thread::spawn(move || {
+        loop {
+            std::thread::sleep(EPOCH_TICK);
+            engine.increment_epoch();
+        }
+    });
+}
+
+pub struct WasmProvider {
+    name: String,
+    version: String,
+    engine: Engine,
+    pre: ProviderPre<HostState>,
+}
+
+impl WasmProvider {
+    /// Compile a component from disk and read its metadata once.
+    /// Fails fast if the component does not satisfy the provider world.
+    pub fn load(
+        engine: &Engine,
+        linker: &Linker<HostState>,
+        path: &Path,
+    ) -> Result<Self, HostError> {
+        let component = Component::from_file(engine, path)?;
+        let pre = ProviderPre::new(linker.instantiate_pre(&component)?)?;
+
+        let mut provider = Self {
+            name: String::new(),
+            version: String::new(),
+            engine: engine.clone(),
+            pre,
+        };
+
+        let mut store = provider.new_store();
+        let instance = provider.pre.instantiate(&mut store)?;
+        let info = instance
+            .whoareyou_provider_lookup()
+            .call_metadata(&mut store)?;
+
+        provider.name = info.name;
+        provider.version = info.version;
+
+        Ok(provider)
+    }
+
+    pub fn version(&self) -> &str {
+        &self.version
+    }
+
+    fn new_store(&self) -> Store<HostState> {
+        // No preopens, no env, no inherited stdio — fully sandboxed guest.
+        let ctx = WasiCtxBuilder::new().build();
+
+        let mut store = Store::new(
+            &self.engine,
+            HostState {
+                ctx,
+                table: ResourceTable::new(),
+            },
+        );
+
+        store.set_epoch_deadline(EPOCH_DEADLINE_TICKS);
+
+        store
+    }
+}
+
+impl ProviderHandle for WasmProvider {
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn requests(&self, number: &str) -> Result<Vec<String>, HostError> {
+        let mut store = self.new_store();
+        let instance = self.pre.instantiate(&mut store)?;
+
+        let requests = instance
+            .whoareyou_provider_lookup()
+            .call_requests(&mut store, number)?;
+
+        Ok(requests.into_iter().map(|r| r.url).collect())
+    }
+
+    fn parse(&self, number: &str, responses: &[FetchedResponse]) -> ParseOutcome {
+        let wit_responses: Vec<WitResponse> = responses
+            .iter()
+            .map(|r| WitResponse {
+                status: r.status,
+                body: r.body.clone(),
+            })
+            .collect();
+
+        let mut store = self.new_store();
+
+        let result: Result<Result<_, WitLookupError>, wasmtime::Error> = (|| {
+            let instance = self.pre.instantiate(&mut store)?;
+
+            instance
+                .whoareyou_provider_lookup()
+                .call_parse(&mut store, number, &wit_responses)
+        })();
+
+        match result {
+            Ok(Ok(entry)) => ParseOutcome::Ok(Entry {
+                messages: entry.messages,
+                history: entry.history,
+                comments: entry
+                    .comments
+                    .into_iter()
+                    .map(|c| Comment {
+                        timestamp: c.timestamp,
+                        title: c.title,
+                        message: c.message,
+                    })
+                    .collect(),
+            }),
+            Ok(Err(WitLookupError::NoData)) => ParseOutcome::NoData,
+            Ok(Err(WitLookupError::ParseFailed(message))) => ParseOutcome::Failed(message),
+            // Trap (incl. epoch deadline exceeded) or instantiation failure.
+            Err(error) => ParseOutcome::Failed(format!("component error: {error}")),
+        }
+    }
+}