mesh-controller/internal/catalogue/roster.go

package catalogue

import (
	"bytes"
	"fmt"
	"sort"
	"strings"
	"text/template"
)

// What only the mesh knows, written where a module asks for it — in the module's own format.
//
// **The graph is the control plane's; the format is the module's.** The mesh knows which machines
// exist, what they are called and where they are. Turning that into a hosts file, a resolver's
// zones, an ssh known_hosts is somebody's configuration language, and the mesh has no business
// knowing it. So the mesh hands the roster to a template the module wrote and renders it; it never
// learns what the file means.
//
// This used to be a closed list of fact names, each with its format written in Go here — a hosts
// file, a resolver's zones. Every new consumer meant a new formatter in the control plane, in the
// consumer's configuration language. Now the data is the mesh's and the format is a template the
// module ships: the two built-in cases (the network module's `/etc/hosts`, dnsmasq's zones) render
// the same way any module's would, and the control plane holds no format at all.
//
// It replaced three modules that existed only because computed output needed somewhere to live —
// they ran no software, could not be swapped for anything, and appeared in the graph as modules
// while being a data channel wearing a costume (novox/hq ADR 0040).

// A RosterFile is a file the mesh renders from the roster of machines, in the format the module
// gives as a Go text/template. The template sees a rosterView: `.Node` (this machine's bare name),
// `.Suffix` (what its mesh name ends in), and two sets of `{Name, FQDN, Address}` — `.Names`, every
// name the mesh serves, and `.Machines`, only the nodes of the mesh. Which set a template ranges is
// how the hq issue 111 distinction is drawn: a container's hosts wants every name; a resolver told
// the suffix is its own wants only the machines.
type RosterFile struct {
	// Path is where on the machine the rendered file goes. Absolute, or it is refused here rather
	// than discovered as a daemon that reads nothing.
	Path string `json:"path"`
	// Template is the module's format, a Go text/template over the rosterView. It is the module's,
	// not the mesh's: the mesh renders it and does not read it.
	Template string `json:"template"`
}

// rosterView is what a RosterFile's template sees. A closed shape — a template referencing a field
// the mesh does not compute fails to render here, not on a machine.
type rosterView struct {
	Node     string
	Suffix   string
	Names    []rosterEntry
	Machines []rosterEntry
}

// rosterEntry is one machine as a template sees it: its bare name, its full mesh name, its address.
type rosterEntry struct {
	Name    string
	FQDN    string
	Address string
}

// FactsInto renders the roster files a module asked for, as files it will be given.
//
// The module owns everything after the file exists: loading it, restarting on it, what a resolver
// or a client does with it. This only puts it there. `every` is every name the mesh serves;
// `machines` is only the machines — the two must not be confused (novox/hq 04-ISSUES/111), so both
// are given and the template chooses.
func FactsInto(m Manifest, r Resolution, every, machines map[string]string, suffix string) ([]map[string]any, error) {
	if len(m.Facts) == 0 {
		return nil, nil
	}
	names := make([]string, 0, len(m.Facts))
	for name := range m.Facts {
		names = append(names, name)
	}
	sort.Strings(names)

	view := rosterView{
		Node:     r.Node,
		Suffix:   strings.TrimPrefix(suffixOr(suffix), "."),
		Names:    entriesFrom(every, suffix),
		Machines: entriesFrom(machines, suffix),
	}

	out := make([]map[string]any, 0, len(names))
	for _, name := range names {
		fact := m.Facts[name]
		if !strings.HasPrefix(fact.Path, "/") {
			return nil, fmt.Errorf(
				"%s asks for %q at %q, which is not an absolute path", m.Module, name, fact.Path)
		}
		content, err := renderRoster(fact.Template, view)
		if err != nil {
			return nil, fmt.Errorf("%s cannot render %q: %w", m.Module, name, err)
		}
		out = append(out, map[string]any{
			"id": "fact-" + name, "type": "file", "path": fact.Path, "mode": "0644",
			"content": content,
		})
	}
	return out, nil
}

// renderRoster runs a module's template over the roster. A template that will not parse, or reads
// a field the mesh does not have, is an error here — where the manifest is — rather than an empty
// file on a machine.
func renderRoster(tmpl string, view rosterView) (string, error) {
	t, err := template.New("roster").Option("missingkey=error").Parse(tmpl)
	if err != nil {
		return "", err
	}
	var b bytes.Buffer
	if err := t.Execute(&b, view); err != nil {
		return "", err
	}
	return b.String(), nil
}

// entriesFrom is a name→address map as sorted roster entries.
//
// **A machine with no address is left out.** The mesh has a record for it — somebody added it —
// and does not yet know where it is, which is the ordinary state between adding a machine and it
// joining. Writing the name anyway would give a name that resolves to nothing, and a connection to
// that hangs; leaving it out fails at once and says the name is unknown.
func entriesFrom(addresses map[string]string, suffix string) []rosterEntry {
	out := make([]rosterEntry, 0, len(addresses))
	for _, name := range sortedNames(addresses) {
		internal, bare := meshName(name, suffix)
		out = append(out, rosterEntry{Name: bare, FQDN: internal, Address: addresses[name]})
	}
	return out
}

// meshName is a machine's internal name and its bare one, from either. The control plane keys
// the names it hands a resolution by the internal name (`homer.internal`), the same map a
// container gets as its hosts; a caller that keys by the bare name gets the same answer. The
// suffix is the one the control plane composed those names with, handed down rather than written
// here a second time — the alternative was `homer.internal.internal` on every machine.
func meshName(name, suffix string) (internal, bare string) {
	dotted := "." + strings.TrimPrefix(suffixOr(suffix), ".")
	if strings.HasSuffix(name, dotted) {
		return name, strings.TrimSuffix(name, dotted)
	}
	return name + dotted, name
}

// suffixOr is the suffix given, or the one the mesh composes names with when none was handed down.
// The one place the default is written, so a fact and a name cannot disagree about it.
func suffixOr(suffix string) string {
	if suffix == "" {
		return "internal"
	}
	return suffix
}

func sortedNames(addresses map[string]string) []string {
	out := make([]string, 0, len(addresses))
	for name, at := range addresses {
		// A machine the mesh cannot place is left out rather than named at nothing.
		if at == "" {
			continue
		}
		out = append(out, name)
	}
	sort.Strings(out)
	return out
}