mesh-host/internal/declaration/declaration.go

// Package declaration is what the host is told a machine should be.
//
// Data, never instructions. The vocabulary is finite, versioned, and anything outside it
// refuses the whole declaration rather than being skipped — a host that applied most of what
// it was sent and reported success is a node that looks configured and is not
// (novox/hq ADR 0005).
package declaration

import (
	"bytes"
	"encoding/json"
	"fmt"
	"reflect"
	"sort"
	"strings"
)

// Version is the vocabulary this host speaks. A declaration naming any other version is
// refused: an older host handed a newer vocabulary must not quietly do half of it.
const Version = 1

// Type names a kind of resource. Every addition widens what a compromised control plane can
// express, so the list is a security artefact and grows deliberately.
type Type string

const (
	TypeDirectory Type = "directory"
	TypeFile      Type = "file"
	TypeService   Type = "service"
	TypePackage   Type = "package"
	TypeContainer Type = "container"
	TypeAction    Type = "action"
)

// Resource is one thing that should be true of the machine.
//
// A struct per kind rather than one struct carrying every field, because the decoder is then
// what rejects a field the kind does not have: a `file` carrying an `image` is refused because
// File has no such field, not because a list somewhere remembered to say so. The one-struct
// form needs every kind revisited whenever a field is added, and the kind nobody revisits
// silently accepts a field the host will never read.
type Resource interface {
	// Identity is the name the control plane keeps stable across declarations. Not a position
	// and not a hash of the content: it is what lets the store say *this is the same resource
	// I applied last time*, which is what makes removal possible at all.
	Identity() string
	// Kind is the resource's type, for the store and for reporting.
	Kind() Type
	// Target is what the resource acts on, for a person reading a report.
	Target() string

	validate(where string, allowActions bool) []string
}

// The `Type` field on each kind below exists only to absorb the JSON `"type"` key, which the
// strict decoder would otherwise refuse. `Kind()` returns the constant and is what anything
// else should read.

// Directory is a directory that should exist, with a mode.
type Directory struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Path string `json:"path"`
	Mode string `json:"mode,omitempty"`
}

func (d *Directory) Identity() string { return d.ID }
func (d *Directory) Kind() Type       { return TypeDirectory }
func (d *Directory) Target() string   { return d.Path }

func (d *Directory) validate(where string, _ bool) []string {
	var problems []string
	if d.Path == "" {
		problems = append(problems, where+": a directory needs a path")
	}
	return append(problems, checkMode(where, d.Mode)...)
}

// File is a file with literal content. The host renders nothing.
type File struct {
	ID      string `json:"id"`
	Type    Type   `json:"type"`
	Path    string `json:"path"`
	Content string `json:"content"`
	Mode    string `json:"mode,omitempty"`
}

func (f *File) Identity() string { return f.ID }
func (f *File) Kind() Type       { return TypeFile }
func (f *File) Target() string   { return f.Path }

func (f *File) validate(where string, _ bool) []string {
	var problems []string
	if f.Path == "" {
		problems = append(problems, where+": a file needs a path")
	}
	return append(problems, checkMode(where, f.Mode)...)
}

// Service is a unit the host puts into a state. It does not install the unit.
//
// Two states, and they are orthogonal rather than one scale. A unit can be enabled and stopped
// (it will come back at boot), or disabled and running (started by hand, gone after a reboot).
// Folding them into one field would make the second expressible only by accident.
type Service struct {
	ID    string `json:"id"`
	Type  Type   `json:"type"`
	Unit  string `json:"unit"`
	State string `json:"state"`
	// Boot is "enabled" or "disabled" — whether the unit starts at boot. Optional: absent means
	// the host asserts nothing about it and leaves whatever is there.
	//
	// Without this the host could start a unit and not make it survive a reboot, which is a
	// declaration that reports success and stops being true at the next power cut.
	Boot string `json:"boot,omitempty"`
}

func (s *Service) Identity() string { return s.ID }
func (s *Service) Kind() Type       { return TypeService }
func (s *Service) Target() string   { return s.Unit }

func (s *Service) validate(where string, _ bool) []string {
	var problems []string
	if s.Unit == "" {
		problems = append(problems, where+": a service needs a unit")
	}
	if s.State != "running" && s.State != "stopped" {
		problems = append(problems, fmt.Sprintf(
			"%s: state %q; a service is \"running\" or \"stopped\"", where, s.State))
	}
	if s.Boot != "" && s.Boot != "enabled" && s.Boot != "disabled" {
		problems = append(problems, fmt.Sprintf(
			"%s: boot %q; a service is \"enabled\" or \"disabled\" at boot, or omits it to "+
				"leave the machine's own setting alone", where, s.Boot))
	}
	return problems
}

// Package is a package that should be present.
//
// Present is the whole of what it asserts, never a version: version is the package manager's
// business and the mesh does not hold a second opinion about it.
type Package struct {
	ID      string `json:"id"`
	Type    Type   `json:"type"`
	Package string `json:"package"`
}

func (p *Package) Identity() string { return p.ID }
func (p *Package) Kind() Type       { return TypePackage }
func (p *Package) Target() string   { return p.Package }

func (p *Package) validate(where string, _ bool) []string {
	if p.Package == "" {
		return []string{where + ": a package needs a package name"}
	}
	return nil
}

// Container is a container that should be running, from an image pinned by digest.
type Container struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Name string `json:"name"`
	// Image is pinned by digest (novox/hq ADR 0006) — a tag moves and a digest does not.
	Image   string            `json:"image"`
	Env     map[string]string `json:"env,omitempty"`
	Ports   []string          `json:"ports,omitempty"`
	Volumes []string          `json:"volumes,omitempty"`
	Args    []string          `json:"args,omitempty"`
	// Network is the container's network, passed to the runtime unchanged.
	//
	// Needed because the control plane must reach the store and the broker on the machine it was
	// raised on, before there is any mesh to arrange that. The alternative was publishing ports
	// and guessing an address that works from inside a container, which is the same thing with a
	// worse failure mode.
	Network string `json:"network,omitempty"`
}

func (c *Container) Identity() string { return c.ID }
func (c *Container) Kind() Type       { return TypeContainer }
func (c *Container) Target() string   { return c.Name }

func (c *Container) validate(where string, _ bool) []string {
	var problems []string
	if c.Name == "" {
		problems = append(problems, where+": a container needs a name")
	}
	return append(problems, checkImage(where, c.Image)...)
}

// Action runs something the bundle declared, and the host never learns what it means.
type Action struct {
	ID      string   `json:"id"`
	Type    Type     `json:"type"`
	Command []string `json:"command"`
	// Verify is not optional and is not a courtesy. It is the read-back AND the idempotency
	// check: the host does not know what a database is, so "is it already there" is a question
	// only the declaration can ask (novox/hq ADR 0005).
	Verify []string `json:"verify"`
	// In names a container to run inside. Empty means the machine itself.
	In string `json:"in,omitempty"`
}

func (a *Action) Identity() string { return a.ID }
func (a *Action) Kind() Type       { return TypeAction }

func (a *Action) Target() string {
	target := strings.Join(a.Command, " ")
	if a.In != "" {
		return "in " + a.In + ": " + target
	}
	return target
}

func (a *Action) validate(where string, allowActions bool) []string {
	// The bound the whole security argument rests on (novox/hq ADR 0005).
	if !allowActions {
		return []string{where +
			": an action arrived over the link, and the link may not carry one. The host " +
			"applies declarations of known shape; a command to run is not one. A bundle may " +
			"carry an action because it arrives with the binary — anyone able to put a " +
			"hostile action there could have put it in the host itself"}
	}
	var problems []string
	if len(a.Command) == 0 {
		problems = append(problems, where+": an action needs a command")
	}
	if len(a.Verify) == 0 {
		problems = append(problems, where+
			": an action needs a verify. An action that runs and reports success without "+
			"reading anything back is the fault this host exists to prevent, and verify is "+
			"also how the host knows whether the action is already done")
	}
	return problems
}

// newOf returns an empty resource of a kind, or nil if the kind is unknown.
//
// This is the whole vocabulary, in one place. A kind that is not here cannot be declared.
func newOf(t Type) Resource {
	switch t {
	case TypeDirectory:
		return &Directory{}
	case TypeFile:
		return &File{}
	case TypeService:
		return &Service{}
	case TypePackage:
		return &Package{}
	case TypeContainer:
		return &Container{}
	case TypeAction:
		return &Action{}
	}
	return nil
}

// Vocabulary is every kind this host speaks.
func Vocabulary() []Type {
	return []Type{
		TypeAction, TypeContainer, TypeDirectory, TypeFile, TypePackage, TypeService,
	}
}

// Declaration is what a machine should be, in the order it should be made so.
type Declaration struct {
	Version int
	// For names the node this is meant for. A host with an identity refuses one addressed
	// elsewhere; a host without one — the first node, applying the bundle it carries — has
	// nothing to check against.
	For string
	// Resources, in the order they are applied. The host does not sort them: ordering is a
	// decision, and deciding is not what the host does (novox/hq ADR 0005).
	Resources []Resource
}

// RefusalError refuses a whole declaration, naming every problem at once.
//
// Every problem rather than the first: a caller fixing one at a time learns the next only by
// running again, and a declaration is generated, so a person reading this is debugging the
// generator.
type RefusalError struct {
	Problems []string
}

func (e *RefusalError) Error() string {
	return fmt.Sprintf(
		"this declaration is refused, and none of it was applied:\n  - %s\n\n"+
			"A host that applied the parts it understood would leave a machine that looks "+
			"configured and is not.",
		strings.Join(e.Problems, "\n  - "))
}

// Parse reads a declaration that arrived over the link, and refuses anything it does not fully
// understand — including any action, which the link may not carry (novox/hq ADR 0005).
func Parse(raw []byte) (*Declaration, error) { return parse(raw, false) }

// ParseTrusted reads a declaration from a source already as privileged as the host itself: the
// bundle it carries, or a file handed to it by someone who is running it as root.
//
// Actions are permitted here and nowhere else. The asymmetry is deliberate and is the entire
// content of ADR 0005: refusing actions from the bundle buys nothing, because whoever built the
// bundle built the binary; refusing them from the link buys the bound on what a compromised
// control plane can express.
func ParseTrusted(raw []byte) (*Declaration, error) { return parse(raw, true) }

// envelope is the declaration with its resources still unread.
//
// Two passes, because which fields are legal depends on the "type" inside each resource. The
// first pass takes the envelope and each resource's bytes; the second decodes each one into
// the struct for its kind, strictly.
type envelope struct {
	Version   int               `json:"declaration"`
	For       string            `json:"for,omitempty"`
	Resources []json.RawMessage `json:"resources"`
}

func parse(raw []byte, allowActions bool) (*Declaration, error) {
	var env envelope
	if err := strictDecode(raw, &env); err != nil {
		return nil, &RefusalError{Problems: []string{"not a declaration: " + err.Error()}}
	}

	if env.Version != Version {
		// Everything below assumes the vocabulary, so there is nothing further to say.
		return nil, &RefusalError{Problems: []string{fmt.Sprintf(
			"declaration version %d; this host speaks version %d. Refused whole rather than "+
				"partly, so a newer vocabulary is never half-applied by an older host",
			env.Version, Version)}}
	}

	d := &Declaration{Version: env.Version, For: env.For}
	var problems []string

	if len(env.Resources) == 0 {
		problems = append(problems, "no resources. An empty declaration is a mistake, not a "+
			"machine with nothing on it — say so with an explicit empty list if that is meant")
	}

	seen := map[string]int{}
	for i, rawResource := range env.Resources {
		// Peek, leniently. This pass only needs to know which struct to decode into; reading
		// strictly here would report an unknown field before knowing which fields are known.
		var head struct {
			ID   string `json:"id"`
			Type Type   `json:"type"`
		}
		_ = json.Unmarshal(rawResource, &head)

		where := fmt.Sprintf("resource %d", i)
		if head.ID != "" {
			where = fmt.Sprintf("resource %q", head.ID)
		}

		if head.ID == "" {
			problems = append(problems, where+": no id. Identity is what lets the host know "+
				"this is the same resource it applied last time")
		} else if first, ok := seen[head.ID]; ok {
			problems = append(problems, fmt.Sprintf(
				"%s: id already used by resource %d. Two resources with one identity cannot "+
					"both be tracked", where, first))
		} else {
			seen[head.ID] = i
		}

		resource := newOf(head.Type)
		if resource == nil {
			problems = append(problems, fmt.Sprintf(
				"%s: unknown type %q. This host understands %s", where, head.Type, vocabulary()))
			continue
		}

		// A field the kind does not have is refused, and the struct is what says so — there
		// is no list of exclusions for anyone to keep current.
		//
		// Asked separately rather than taken from the decoder's error, because the decoder
		// stops at the first unknown field and this record promises every problem at once. A
		// caller fixing one field at a time learns the next only by running again.
		if unknown := unknownFields(rawResource, resource); len(unknown) > 0 {
			for _, field := range unknown {
				problems = append(problems, fmt.Sprintf(
					"%s: a %s does not use %q, and it is set. Refused rather than ignored",
					where, head.Type, field))
			}
			continue
		}
		if err := json.Unmarshal(rawResource, resource); err != nil {
			problems = append(problems, fmt.Sprintf("%s: %s", where, err))
			continue
		}

		problems = append(problems, resource.validate(where, allowActions)...)
		d.Resources = append(d.Resources, resource)
	}

	if len(problems) > 0 {
		return nil, &RefusalError{Problems: problems}
	}
	return d, nil
}

func strictDecode(raw []byte, into any) error {
	// DisallowUnknownFields is the whole point rather than strictness for its own sake: a
	// field the host does not know is a thing the control plane believes it asked for.
	dec := json.NewDecoder(bytes.NewReader(raw))
	dec.DisallowUnknownFields()
	return dec.Decode(into)
}

// unknownFields names every JSON key the kind's struct has no field for.
//
// The struct's own tags are the list of what is legal, so adding a field to a kind is the
// whole of adding it — there is nowhere else that has to agree.
func unknownFields(raw []byte, into Resource) []string {
	var got map[string]json.RawMessage
	if err := json.Unmarshal(raw, &got); err != nil {
		return nil // not an object; the decode below will say so properly
	}

	known := map[string]bool{}
	t := reflect.TypeOf(into).Elem()
	for i := 0; i < t.NumField(); i++ {
		name, _, _ := strings.Cut(t.Field(i).Tag.Get("json"), ",")
		if name != "" && name != "-" {
			known[name] = true
		}
	}

	var unknown []string
	for field := range got {
		if !known[field] {
			unknown = append(unknown, field)
		}
	}
	sort.Strings(unknown)
	return unknown
}

func checkMode(where, mode string) []string {
	if mode == "" {
		return nil
	}
	if len(mode) != 4 || mode[0] != '0' {
		return []string{fmt.Sprintf(
			"%s: mode %q; write it as four octal digits such as \"0644\", so it means the "+
				"same thing here as it does in the manifest it came from", where, mode)}
	}
	for _, c := range mode[1:] {
		if c < '0' || c > '7' {
			return []string{fmt.Sprintf("%s: mode %q is not octal", where, mode)}
		}
	}
	return nil
}

// checkImage insists on a digest.
//
// A tag moves and a digest does not. The bundle's whole claim is that what it names is exact
// (novox/hq ADR 0006), and a bundle pinning `postgres:17` pins nothing — it names whatever
// that tag points at on the day the host happens to run.
func checkImage(where, image string) []string {
	if image == "" {
		return []string{where + ": a container needs an image"}
	}
	name, digest, found := strings.Cut(image, "@")
	if !found || name == "" {
		return []string{fmt.Sprintf(
			"%s: image %q is not pinned. Write it as name@sha256:... — a tag moves, and a "+
				"bundle that pinned a tag would not be pinned", where, image)}
	}
	if !strings.HasPrefix(digest, "sha256:") || len(digest) != len("sha256:")+64 {
		return []string{fmt.Sprintf(
			"%s: image digest %q is not a sha256 digest", where, digest)}
	}
	return nil
}

func vocabulary() string {
	kinds := Vocabulary()
	names := make([]string, 0, len(kinds))
	for _, t := range kinds {
		names = append(names, string(t))
	}
	sort.Strings(names)
	return strings.Join(names, ", ")
}