Fix the compatibility issue with loading WATM-Go in WATER-rs (#37)

* call _start when init if there is one, specifically for WATM-go

Author: erikziyunchi

crates/water/src/runtime/core.rs

@@ -9,6 +9,7 @@ use crate::runtime::*;
 #[derive(Default, Clone)]
 pub struct Host {
     pub preview1_ctx: Option<wasmtime_wasi::WasiCtx>,
+    #[cfg(feature = "multithread")]
     pub wasi_threads: Option<Arc<WasiThreadsCtx<Host>>>,
 }
 
@@ -37,10 +38,13 @@ impl H2O<Host> {
         }
 
         let engine = Engine::new(&wasm_config)?;
-        let linker: Linker<Host> = Linker::new(&engine);
 
         let module = Module::from_file(&engine, &conf.filepath)?;
 
+        let linker: Linker<Host> = Linker::new(&engine);
+
+        // linker.allow_unknown_exports(true);
+
         let host = Host::default();
         let store = Store::new(&engine, host);
 
@@ -143,8 +147,26 @@ impl H2O<Host> {
             version_common::funcs::export_config(&mut linker, conf.config_wasm.clone())?;
         }
 
+        // linker.define_unknown_imports_as_traps(&module)?;
+
         let instance = linker.instantiate(&mut store, &module)?;
 
+        // call _start function explicitly if there is one exported from the WATM module
+        let func = instance.get_func(&mut store, "_start");
+
+        if let Some(func) = func {
+            let mut res = vec![Val::null(); func.ty(&store).results().len()];
+            match func.call(&mut store, &[], &mut res) {
+                Ok(_) => {}
+                Err(e) => {
+                    return Err(anyhow::Error::msg(format!(
+                        "failed to call _start function: {}",
+                        e
+                    )))
+                }
+            }
+        }
+
         Ok(H2O {
             version: match version {
                 Some(v) => v,

crates/water/src/runtime/mod.rs

@@ -34,7 +34,6 @@ use tracing::{debug, info};
 use wasi_common::{file::FileAccessMode, WasiCtx, WasiFile};
 use wasmtime::*;
 use wasmtime_wasi::sync::{Dir, WasiCtxBuilder};
-use wasmtime_wasi_threads::WasiThreadsCtx;
 
 // =================== CURRENT CRATE IMPORTS ===================
 use crate::{