fix: Use fresh instance on each new plugin invocation (#413)

* fix: Use fresh instance on each new plugin invocation

* Skip test that depends on 1password page html, see #414

* Pass in instance we want to close to deferred function

---------

Co-authored-by: Gabor Javorszky <gabor@suborbital.dev>

sat/engine2/enginetest/engine_test.go

@@ -126,6 +126,8 @@ func TestEngineSetRespHeader(t *testing.T) {
 }
 
 func TestEngineFetch(t *testing.T) {
+	t.Skip("This test gets skipped until https://github.com/suborbital/e2core/issues/414 is resolved")
+
 	ref, err := engine2.WasmRefFromFile("./testdata/fetch/fetch.wasm")
 	if err != nil {
 		t.Error(err)

sat/engine2/runtime/pool.go

@@ -66,14 +66,21 @@ func (ip *InstancePool) RemoveInstance() error {
 
 // UseInstance provides an instance from the environment's pool to be used by a callback function
 func (ip *InstancePool) UseInstance(ctx *scheduler.Ctx, instFunc func(*instance.Instance, int32)) error {
-	// grab an instance from the available queue and then
-	// return it to the environment when finished
+	// grab an instance from the available queue
 	inst := <-ip.availableInstances
 
-	defer func() {
-		ip.availableInstances <- inst
+	go func() {
+		// prepare a new instance
+		if err := ip.AddInstance(); err != nil {
+			panic(err)
+		}
 	}()
 
+	defer func(it *instance.Instance) {
+		it.Close()
+		it = nil
+	}(inst)
+
 	// generate a random identifier as a reference to the instance in use to
 	// easily allow the Wasm module to reference itself when calling back over the FFI
 	ident, err := instance.Store(inst)