mgmt/lang/core/struct_lookup.go

// Mgmt
// Copyright (C) James Shubin and the project contributors
// Written by James Shubin <james@shubin.ca> and the project contributors
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.
//
// Additional permission under GNU GPL version 3 section 7
//
// If you modify this program, or any covered work, by linking or combining it
// with embedded mcl code and modules (and that the embedded mcl code and
// modules which link with this program, contain a copy of their source code in
// the authoritative form) containing parts covered by the terms of any other
// license, the licensors of this program grant you additional permission to
// convey the resulting work. Furthermore, the licensors of this program grant
// the original author, James Shubin, additional permission to update this
// additional permission if he deems it necessary to achieve the goals of this
// additional permission.

package core

import (
	"context"
	"fmt"

	"github.com/purpleidea/mgmt/lang/funcs"
	"github.com/purpleidea/mgmt/lang/interfaces"
	"github.com/purpleidea/mgmt/lang/types"
	"github.com/purpleidea/mgmt/util/errwrap"
)

const (
	// StructLookupFuncName is the name this function is registered as. This
	// starts with an underscore so that it cannot be used from the lexer.
	StructLookupFuncName = funcs.StructLookupFuncName

	// arg names...
	structLookupArgNameStruct = "struct"
	structLookupArgNameField  = "field"
)

func init() {
	funcs.Register(StructLookupFuncName, func() interfaces.Func { return &StructLookupFunc{} }) // must register the func and name
}

var _ interfaces.BuildableFunc = &StructLookupFunc{} // ensure it meets this expectation

// StructLookupFunc is a struct field lookup function.
type StructLookupFunc struct {
	Type *types.Type // Kind == Struct, that is used as the struct we lookup
	Out  *types.Type // type of field we're extracting

	built bool // was this function built yet?

	init  *interfaces.Init
	last  types.Value // last value received to use for diff
	field string

	result types.Value // last calculated output
}

// String returns a simple name for this function. This is needed so this struct
// can satisfy the pgraph.Vertex interface.
func (obj *StructLookupFunc) String() string {
	return StructLookupFuncName
}

// ArgGen returns the Nth arg name for this function.
func (obj *StructLookupFunc) ArgGen(index int) (string, error) {
	seq := []string{structLookupArgNameStruct, structLookupArgNameField}
	if l := len(seq); index >= l {
		return "", fmt.Errorf("index %d exceeds arg length of %d", index, l)
	}
	return seq[index], nil
}

// helper
func (obj *StructLookupFunc) sig() *types.Type {
	st := "?1"
	out := "?2"
	if obj.Type != nil {
		st = obj.Type.String()
	}
	if obj.Out != nil {
		out = obj.Out.String()
	}

	return types.NewType(fmt.Sprintf(
		"func(%s %s, %s str) %s",
		structLookupArgNameStruct, st,
		structLookupArgNameField,
		out,
	))
}

// FuncInfer takes partial type and value information from the call site of this
// function so that it can build an appropriate type signature for it. The type
// signature may include unification variables.
func (obj *StructLookupFunc) FuncInfer(partialType *types.Type, partialValues []types.Value) (*types.Type, []*interfaces.UnificationInvariant, error) {
	// func(struct ?1, field str) ?2

	// This particular function should always get called with a known string
	// for the second argument. Without it being known statically, we refuse
	// to build this function.

	if l := 2; len(partialValues) != l {
		return nil, nil, fmt.Errorf("function must have %d args", l)
	}
	if err := partialValues[1].Type().Cmp(types.TypeStr); err != nil {
		return nil, nil, errwrap.Wrapf(err, "function field name must be a str")
	}
	s := partialValues[1].Str() // must not panic
	if s == "" {
		return nil, nil, fmt.Errorf("function must not have an empty field name")
	}
	// This can happen at runtime too, but we save it here for Build()!
	obj.field = s // store for later

	// Figure out more about the sig if any information is known statically.
	if len(partialType.Ord) > 0 && partialType.Map[partialType.Ord[0]] != nil {
		obj.Type = partialType.Map[partialType.Ord[0]] // assume this
		if obj.Type.Kind == types.KindStruct && obj.Type.Map != nil {
			if typ, exists := obj.Type.Map[s]; exists {
				obj.Out = typ
			}
		}
	}

	// This isn't precise enough because we must guarantee that the field is
	// in the struct and that ?1 is actually a struct, but that's okay it is
	// something that we'll verify at build time!

	return obj.sig(), []*interfaces.UnificationInvariant{}, nil
}

// Build is run to turn the polymorphic, undetermined function, into the
// specific statically typed version. It is usually run after Unify completes,
// and must be run before Info() and any of the other Func interface methods are
// used. This function is idempotent, as long as the arg isn't changed between
// runs.
func (obj *StructLookupFunc) Build(typ *types.Type) (*types.Type, error) {
	// typ is the KindFunc signature we're trying to build...
	if typ.Kind != types.KindFunc {
		return nil, fmt.Errorf("input type must be of kind func")
	}

	if len(typ.Ord) != 2 {
		return nil, fmt.Errorf("the structlookup function needs exactly two args")
	}
	if typ.Out == nil {
		return nil, fmt.Errorf("return type of function must be specified")
	}
	if typ.Map == nil {
		return nil, fmt.Errorf("invalid input type")
	}

	tStruct, exists := typ.Map[typ.Ord[0]]
	if !exists || tStruct == nil {
		return nil, fmt.Errorf("first arg must be specified")
	}

	tField, exists := typ.Map[typ.Ord[1]]
	if !exists || tField == nil {
		return nil, fmt.Errorf("second arg must be specified")
	}
	if err := tField.Cmp(types.TypeStr); err != nil {
		return nil, errwrap.Wrapf(err, "field must be an str")
	}

	// NOTE: We actually don't know which field this is yet, only its type!
	// We cached the discovered field during Infer(), but it turns out it's
	// not actually necessary for us to know it to build the struct. It is
	// needed to make sure the lossy Infer unification variables are right.
	if tStruct.Kind != types.KindStruct {
		return nil, fmt.Errorf("first arg must be of kind struct, got: %s", tStruct.Kind)
	}
	if obj.field == "" {
		// programming error
		return nil, fmt.Errorf("did not infer correctly")
	}
	ix := -1 // not found
	for i, x := range tStruct.Ord {
		if x != obj.field {
			continue
		}
		// found
		if ix != -1 {
			// programming error
			return nil, fmt.Errorf("duplicate field found")
		}
		ix = i // found it here!
		//break // keep checking for extra safety
	}
	if ix == -1 {
		return nil, fmt.Errorf("field %s was not found in struct", obj.field)
	}
	tF, exists := tStruct.Map[tStruct.Ord[ix]]
	if !exists {
		return nil, fmt.Errorf("field %s was not found in struct", obj.field)
	}
	// The return value must match the type of the field we're pulling out!
	if err := typ.Out.Cmp(tF); err != nil {
		return nil, fmt.Errorf("field %s type error: %+v", obj.field, err)
	}

	obj.Type = tStruct // struct type
	obj.Out = typ.Out  // type of return value
	obj.built = true

	return obj.sig(), nil
}

// Copy is implemented so that the obj.field value is not lost if we copy this
// function. That value is learned during FuncInfer, and previously would have
// been lost by the time we used it in Build.
func (obj *StructLookupFunc) Copy() interfaces.Func {
	return &StructLookupFunc{
		Type: obj.Type, // don't copy because we use this after unification
		Out:  obj.Out,

		built: obj.built,
		init:  obj.init,  // likely gets overwritten anyways
		field: obj.field, // this we really need!
	}
}

// Validate tells us if the input struct takes a valid form.
func (obj *StructLookupFunc) Validate() error {
	if !obj.built {
		return fmt.Errorf("function wasn't built yet")
	}
	if obj.Type == nil { // build must be run first
		return fmt.Errorf("type is still unspecified")
	}
	if obj.Type.Kind != types.KindStruct {
		return fmt.Errorf("type must be a kind of struct")
	}
	if obj.Out == nil {
		return fmt.Errorf("return type must be specified")
	}

	for _, t := range obj.Type.Map {
		if obj.Out.Cmp(t) == nil {
			return nil // found at least one match
		}
	}
	return fmt.Errorf("return type is not in the list of available struct fields")
}

// Info returns some static info about itself. Build must be called before this
// will return correct data.
func (obj *StructLookupFunc) Info() *interfaces.Info {
	// Since this function implements FuncInfer we want sig to return nil to
	// avoid an accidental return of unification variables when we should be
	// getting them from FuncInfer, and not from here. (During unification!)
	var sig *types.Type
	if obj.built {
		sig = obj.sig() // helper
	}
	return &interfaces.Info{
		Pure: true,
		Memo: true,
		Fast: true,
		Spec: true,
		Sig:  sig,
		Err:  obj.Validate(),
	}
}

// Init runs some startup code for this function.
func (obj *StructLookupFunc) Init(init *interfaces.Init) error {
	obj.init = init
	return nil
}

// Call returns the result of this function.
func (obj *StructLookupFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
	if len(args) < 2 {
		return nil, fmt.Errorf("not enough args")
	}
	st := args[0].(*types.StructValue)
	field := args[1].Str()

	if field == "" {
		return nil, fmt.Errorf("received empty field")
	}
	// TODO: Is it a hack to grab this first value?
	if obj.field == "" {
		// This can happen at compile time too. Bonus!
		obj.field = field // store first field
	}
	if field != obj.field {
		return nil, fmt.Errorf("input field changed from: `%s`, to: `%s`", obj.field, field)
	}
	result, exists := st.Lookup(obj.field)
	if !exists {
		return nil, fmt.Errorf("could not lookup field: `%s` in struct", field)
	}

	return result, nil
}