lang: Improve graph shape with speculative execution

Most of the time, we don't need to have a dynamic call sub graph, since
the actual function call could be represented statically as it
originally was before lambda functions were implemented. Simplifying the
graph shape has important performance benefits in terms of both keep the
graph smaller (memory, etc) and in avoiding the need to run transactions
at runtime (speed) to reshape the graph.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/core/deploy/readfileabs.go

@@ -51,6 +51,8 @@ func init() {
 	funcs.ModuleRegister(ModuleName, ReadFileAbsFuncName, func() interfaces.Func { return &ReadFileAbsFunc{} }) // must register the func and name
 }
 
+var _ interfaces.DataFunc = &ReadFileAbsFunc{}
+
 // ReadFileAbsFunc is a function that reads the full contents from a file in our
 // deploy. The file contents can only change with a new deploy, so this is
 // static. In particular, this takes an absolute path relative to the root
@@ -97,6 +99,8 @@ func (obj *ReadFileAbsFunc) Info() *interfaces.Info {
 	return &interfaces.Info{
 		Pure: false, // maybe false because the file contents can change
 		Memo: false,
+		Fast: false,
+		Spec: false,
 		Sig:  types.NewType(fmt.Sprintf("func(%s str) str", readfileArgNameFilename)),
 	}
 }
@@ -168,11 +172,26 @@ func (obj *ReadFileAbsFunc) Stream(ctx context.Context) error {
 	}
 }
 
+// Copy is implemented so that the obj.built value is not lost if we copy this
+// function.
+func (obj *ReadFileAbsFunc) Copy() interfaces.Func {
+	return &ReadFileAbsFunc{
+		init: obj.init, // likely gets overwritten anyways
+		data: obj.data, // needed because we don't call SetData twice
+	}
+}
+
 // Call this function with the input args and return the value if it is possible
 // to do so at this time.
 func (obj *ReadFileAbsFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	if len(args) < 1 {
+		return nil, fmt.Errorf("not enough args")
+	}
 	filename := args[0].Str()
 
+	if obj.init == nil || obj.data == nil {
+		return nil, funcs.ErrCantSpeculate
+	}
 	fs, err := obj.init.World.Fs(obj.data.FsURI) // open the remote file system
 	if err != nil {
 		return nil, errwrap.Wrapf(err, "can't load code from file system `%s`", obj.data.FsURI)