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/funcs/funcs.go

@@ -38,6 +38,7 @@ import (
 
 	docsUtil "github.com/purpleidea/mgmt/docs/util"
 	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/util"
 )
 
 const (
@@ -113,6 +114,15 @@ const (
 	// CollectFuncOutType is the expected return type, the data field is an
 	// encoded resource blob.
 	CollectFuncOutType = "[]" + CollectFuncOutStruct
+
+	// ErrCantSpeculate is an error that explains that we can't speculate
+	// when trying to Call a function. This often gets called by the Value()
+	// method of the Expr. This can be useful if we want to distinguish
+	// between "something is broken" and "I just can't produce a value at
+	// this time", which can be identified and skipped over. If it's the
+	// former, then it's okay to error early and shut everything down since
+	// we know this function is never going to work the way it's called.
+	ErrCantSpeculate = util.Error("can't speculate")
 )
 
 // registeredFuncs is a global map of all possible funcs which can be used. You