mesh-controller/internal/catalogue/secrets_into_files.go

package catalogue

import (
	"fmt"
	"regexp"
	"sort"
)

// A credential and a configuration file meeting.
//
// **The gap this closes.** A granted credential arrives as a file whose entire content is the
// password. That is what a program reading a password file wants — and most programs do not read
// one. They read `KEY=value`, or a JSON document with the password at some path inside it, or a
// YAML file in a home directory. Before this, a module in that position could be handed the bare
// value or nothing, and both are useless.
//
// The mesh cannot compose the document, because it discarded the value (novox/hq ADR 0024). So
// the module supplies the document with a hole in it, the mesh delivers the value sealed beside
// it, and the host — the only thing that ever sees both — puts one into the other on the machine.
//
// The host has always been able to do this. Nothing filled the values in, so the hole could be
// written and never closed, and the host refused the file. That refusal was correct and the
// feature was unreachable.

// placeholder is what a module's file content says where a sealed value belongs: ${secret:name}.
//
// The same expression the host matches, written out again rather than shared. The two
// repositories agree on a wire format, and a format read on both sides is exactly the thing that
// must not be quietly changed on one of them; a test asserts they still agree.
var placeholder = regexp.MustCompile(`\$\{secret:([a-z0-9][a-z0-9-]*)\}`)

// secretsUsed are the names a file's content asks for, in the order they first appear.
func secretsUsed(content string) []string {
	var used []string
	seen := map[string]bool{}
	for _, m := range placeholder.FindAllStringSubmatch(content, -1) {
		if !seen[m[1]] {
			seen[m[1]] = true
			used = append(used, m[1])
		}
	}
	return used
}

// sealedFor is every credential a module may name from inside one of its own files.
//
// **Exactly what it already declared, and nothing else.** A module reaches its own secrets and the
// credentials it was granted for what it requires — both written down in its own manifest. It
// cannot name another module's, which is not an oversight: two modules on one machine are as
// separate as two on different machines, and letting one read the other's credential by guessing a
// name would end that, to save writing a file.
func sealedFor(m Manifest, needs []Needed, with Rendering) (map[string]string, error) {
	sealed := map[string]string{}
	for name := range m.OwnSecrets {
		if value := with.Needed[m.Module][name]; value != "" {
			sealed[name] = value
		}
	}
	for _, to := range sortedKeys(m.Secrets) {
		if _, taken := sealed[to]; taken {
			// A module whose own secret and whose requirement share a name. Refused rather than
			// settled by precedence: whichever won, the manifest would read as though the other
			// had, and the file would hold the credential for the wrong thing while every check
			// passed.
			return nil, fmt.Errorf(
				"%s has a secret of its own called %q and also requires %q, so a file saying "+
					"${secret:%s} could mean either — rename one of them", m.Module, to, to, to)
		}
		for i := range needs {
			if needs[i].Name == to && needs[i].Sealed != "" {
				sealed[to] = needs[i].Sealed
			}
		}
	}
	return sealed, nil
}

// intoFile gives a file the sealed values its content asks for.
//
// A name the module never declared is refused here rather than on the machine. The host would
// refuse it too — but it would do so having already been handed a declaration, which reads as the
// mesh sending something broken, and the name it could not find is a manifest's typo.
func intoFile(resource map[string]any, sealed map[string]string, module string) error {
	if fmt.Sprint(resource["type"]) != "file" {
		return nil
	}
	content, ok := resource["content"].(string)
	if !ok {
		return nil
	}
	used := secretsUsed(content)
	if len(used) == 0 {
		return nil
	}
	into := map[string]any{}
	for _, name := range used {
		value := sealed[name]
		if value == "" {
			return fmt.Errorf(
				"%s has a file that says ${secret:%s}, and %s has no secret of its own by that "+
					"name and requires nothing called that either. A file may name %s",
				module, name, module, namesOr(sealed))
		}
		into[name] = value
	}
	resource["secrets"] = into
	return nil
}

// namesOr says what a module could have written, because the answer to "that name is wrong" is
// almost always one of two or three right ones.
func namesOr(sealed map[string]string) string {
	if len(sealed) == 0 {
		return "nothing — it has no secrets of its own and requires nothing that grants one"
	}
	var names []string
	for name := range sealed {
		names = append(names, fmt.Sprintf("%q", name))
	}
	sort.Strings(names)
	return join(names)
}

func join(names []string) string {
	switch len(names) {
	case 1:
		return names[0]
	case 2:
		return names[0] + " or " + names[1]
	}
	out := ""
	for i, n := range names[:len(names)-1] {
		if i > 0 {
			out += ", "
		}
		out += n
	}
	return out + " or " + names[len(names)-1]
}