mesh-controller/examples/postgres-provisioner/main.go

// A provisioner, in the form the mesh expects one.
//
// The mesh generated a password, sealed it to the machine that must accept it, and discarded the
// plaintext — so it cannot tell PostgreSQL to start accepting it. Something on that machine reads
// what the host wrote and makes it true. This is that something.
//
// **It is an example, not part of the control plane.** The control plane decides and never
// touches a machine; this runs on the machine and touches it. A real one ships with the module
// that ships PostgreSQL (novox/hq ADR 0001 — third-party software runs *on* the mesh, not *of*
// it). What lives here is the contract, written as something that runs so it can be read rather
// than described.
//
// What it is given, both written by the host from an ordinary declaration:
//
//	$GRANTS/mesh.json          every consumer, what it asked for, and where its credential is
//	$GRANTS/<node>.secret      one consumer's password, alone in the file
//
// Two files because the mesh discarded the value and could not compose a document containing it.
package main

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"net/url"
	"os"
	"os/signal"
	"path/filepath"
	"sort"
	"strings"
	"syscall"
	"time"

	"github.com/jackc/pgx/v5"
)

// mark is what this provisioner names the roles it owns.
//
// So it never removes one a person made by hand — the mesh's own rule about origins, one level
// down (novox/hq 04-ISSUES/010). A provisioner that dropped every role it did not recognise would
// be a provisioner nobody could safely run on a database that predates it.
const mark = "mesh_"

// contribution is one consumer, as the mesh described it.
type contribution struct {
	From string `json:"from"`
	// Node is empty for a module on this machine, which is asking for something local and is not
	// this provisioner's business.
	Node   string         `json:"node"`
	Secret string         `json:"secret"`
	Values map[string]any `json:"values"`
}

type manifest struct {
	Requirement string         `json:"requirement"`
	Given       []contribution `json:"given"`
}

func main() {
	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
	defer stop()

	// Once, or whenever what it was given changes.
	//
	// **Watching is what lets this be a module.** Run once, it has to be invoked by something
	// after every declaration — a timer that runs it when nothing changed, or a unit wired to
	// restart on a file. Watching, it is an ordinary long-running service, which is a shape the
	// mesh already delivers and the host already supervises.
	//
	// The file it watches is the manifest the mesh writes. A credential changing rewrites the
	// file beside it and not the manifest, so the manifest is stamped whenever either is written
	// — which is why this compares content rather than modification time.
	if len(os.Args) > 1 && os.Args[1] == "--watch" {
		if err := watch(ctx); err != nil {
			fmt.Fprintf(os.Stderr, "mesh-provision-postgres: %v\n", err)
			os.Exit(1)
		}
		return
	}
	if err := run(ctx); err != nil {
		fmt.Fprintf(os.Stderr, "mesh-provision-postgres: %v\n", err)
		os.Exit(1)
	}
}

// watch reconciles now, and again whenever what the mesh delivered changes.
//
// By polling rather than by watching the filesystem, because the file is replaced rather than
// written in place — the host writes atomically, so an inotify watch on the path stops seeing
// anything after the first replacement, which is a watcher that silently stops working.
func watch(ctx context.Context) error {
	const every = 10 * time.Second
	var last string

	for {
		state, err := given()
		switch {
		case err != nil:
			// Said and retried. A provisioner that exits because the mesh has not written
			// anything yet is one that has to be restarted by hand after the first push.
			fmt.Fprintf(os.Stderr, "cannot read what was granted: %v\n", err)
		case state != last:
			if err := run(ctx); err != nil {
				// Reported and retried. The usual reason is that the database has not finished
				// starting, and giving up would mean a module that works only if the two
				// containers happen to come up in the right order.
				fmt.Fprintf(os.Stderr, "%v\n", err)
			} else {
				last = state
			}
		}

		select {
		case <-ctx.Done():
			return nil
		case <-time.After(every):
		}
	}
}

// given is everything the mesh has delivered, as one string, so a change of any of it is one
// comparison.
//
// The credentials are included by their **digest**, never their content: this is compared, logged
// on nothing, and held in memory for as long as the process runs, and a secret does not belong in
// any of that when a hash answers the same question.
func given() (string, error) {
	grants := os.Getenv("GRANTS")
	if grants == "" {
		grants = "/var/lib/postgres/grants"
	}
	entries, err := os.ReadDir(grants)
	if err != nil {
		return "", err
	}
	var names []string
	for _, e := range entries {
		names = append(names, e.Name())
	}
	sort.Strings(names)

	sum := sha256.New()
	for _, name := range names {
		body, err := os.ReadFile(filepath.Join(grants, name))
		if err != nil {
			return "", err
		}
		fmt.Fprintf(sum, "%s:%x\n", name, sha256.Sum256(body))
	}
	return hex.EncodeToString(sum.Sum(nil)), nil
}

func run(ctx context.Context) error {
	grants := os.Getenv("GRANTS")
	if grants == "" {
		grants = "/var/lib/postgres/grants"
	}
	raw, err := os.ReadFile(filepath.Join(grants, "mesh.json"))
	if err != nil {
		if os.IsNotExist(err) {
			// Nothing has been granted here. Not a failure: a provider with no consumers is an
			// ordinary state, and one this must be able to reach from any other.
			fmt.Printf("nothing has been granted to this machine\n")
			return nil
		}
		return err
	}
	var m manifest
	if err := json.Unmarshal(raw, &m); err != nil {
		return fmt.Errorf("the manifest at %s is not readable: %w", grants, err)
	}

	where, err := connectionString()
	if err != nil {
		return err
	}
	db, err := pgx.Connect(ctx, where)
	if err != nil {
		return err
	}
	defer db.Close(ctx)

	// **Reconciling, not applying a change.** It runs after every declaration and is never told
	// what changed, so it must reach the same state from wherever it starts.
	wanted := map[string]bool{}
	for _, c := range sorted(m.Given) {
		if c.Node == "" {
			continue
		}
		name, _ := c.Values["name"].(string)
		if name == "" {
			return fmt.Errorf("%s asked for a database and did not name it", c.Node)
		}
		password, err := os.ReadFile(c.Secret)
		if err != nil {
			// The manifest says there is a credential and the host has not written it. Refused
			// rather than creating a role with no password — a login nothing can use, which
			// nothing would report until something tried to connect.
			return fmt.Errorf("%s's credential should be at %s and is not there", c.Node, c.Secret)
		}

		role := mark + c.Node
		wanted[role] = true
		if err := ensureRole(ctx, db, role, strings.TrimSpace(string(password))); err != nil {
			return err
		}
		if err := ensureDatabase(ctx, db, name, role); err != nil {
			return err
		}
	}

	// And everything this provisioner made that nobody asks for any more. **The half usually
	// missing**: a consumer that goes away otherwise keeps a working login for ever and nothing
	// says so.
	return revokeOrphans(ctx, db, wanted)
}

func ensureRole(ctx context.Context, db *pgx.Conn, role, password string) error {
	var exists bool
	if err := db.QueryRow(ctx,
		`select true from pg_roles where rolname = $1`, role).Scan(&exists); err != nil && err != pgx.ErrNoRows {
		return err
	}
	// Set every time rather than only on creation. The mesh replaces the file when it rotates,
	// and a provisioner that only ever created would leave the old password working — a rotation
	// that reports success and changes nothing.
	verb := "create"
	if exists {
		verb = "alter"
	}
	_, err := db.Exec(ctx, fmt.Sprintf("%s role %s with login password %s",
		verb, quoteName(role), quoteString(password)))
	if err != nil {
		return err
	}
	if !exists {
		fmt.Printf("created %s\n", role)
	}
	return nil
}

func ensureDatabase(ctx context.Context, db *pgx.Conn, name, owner string) error {
	var exists bool
	if err := db.QueryRow(ctx,
		`select true from pg_database where datname = $1`, name).Scan(&exists); err != nil && err != pgx.ErrNoRows {
		return err
	}
	if exists {
		return nil
	}
	if _, err := db.Exec(ctx, fmt.Sprintf("create database %s owner %s",
		quoteName(name), quoteName(owner))); err != nil {
		return err
	}
	fmt.Printf("created database %s owned by %s\n", name, owner)
	return nil
}

func revokeOrphans(ctx context.Context, db *pgx.Conn, wanted map[string]bool) error {
	rows, err := db.Query(ctx,
		`select rolname from pg_roles where rolname like $1 and rolcanlogin order by rolname`,
		mark+"%")
	if err != nil {
		return err
	}
	var found []string
	for rows.Next() {
		var role string
		if err := rows.Scan(&role); err != nil {
			rows.Close()
			return err
		}
		found = append(found, role)
	}
	rows.Close()
	if err := rows.Err(); err != nil {
		return err
	}

	for _, role := range found {
		if wanted[role] {
			continue
		}
		// Login removed rather than the role dropped. Dropping fails while the role owns
		// anything, and a provisioner that failed there would stop reconciling everything else —
		// so the credential stops working immediately and what it owns is somebody's to decide
		// about.
		if _, err := db.Exec(ctx, fmt.Sprintf("alter role %s with nologin",
			quoteName(role))); err != nil {
			return err
		}
		fmt.Printf("revoked %s — nothing in the mesh asks for it\n", role)
	}
	return nil
}

// sorted puts consumers in a stable order, so two runs do the same work in the same sequence and
// the output of one can be compared with another.
func sorted(given []contribution) []contribution {
	out := append([]contribution{}, given...)
	sort.Slice(out, func(i, j int) bool { return out[i].Node < out[j].Node })
	return out
}

// quoteName and quoteString exist because PostgreSQL takes no parameters in DDL.
//
// Both double the quote character, which is the whole of the escaping rule. Worth doing properly
// even here: a password is chosen by the mesh and a node name by a person, and "the value happens
// to be safe today" is not a property anything should rest on.
func quoteName(s string) string   { return `"` + strings.ReplaceAll(s, `"`, `""`) + `"` }
func quoteString(s string) string { return `'` + strings.ReplaceAll(s, `'`, `''`) + `'` }

// connectionString is where this provisioner reaches the database it owns.
//
// **The password comes from a file**, because that is how the mesh delivers one. A module's own
// secret — a superuser password here — is sealed to the machine and written by the host; a
// provisioner told to take it from an environment variable would need somebody to read the file
// and pass it in, which is a person in the middle of the one path that exists so there is not
// one.
//
// It is also the difference between a credential that lives in a file and one that lives in a
// process listing: `docker inspect` prints environment, and a superuser password printed by an
// ordinary diagnostic is a superuser password in whatever collected that diagnostic.
//
// MESH_PROVISION_POSTGRES alone still works, for a provisioner somebody runs by hand.
func connectionString() (string, error) {
	where := strings.TrimSpace(os.Getenv("MESH_PROVISION_POSTGRES"))
	if where == "" {
		return "", fmt.Errorf(
			"MESH_PROVISION_POSTGRES is not set, so this provisioner does not know which " +
				"database it owns")
	}
	path := strings.TrimSpace(os.Getenv("MESH_PROVISION_PASSWORD_FILE"))
	if path == "" {
		return where, nil
	}
	raw, err := os.ReadFile(path)
	if err != nil {
		return "", fmt.Errorf(
			"cannot read the password this provisioner was given at %s: %w", path, err)
	}
	password := strings.TrimSpace(string(raw))
	if password == "" {
		// An empty file connects as nobody and is refused by the database, three layers from
		// here, as an authentication problem with no cause anybody changed.
		return "", fmt.Errorf("%s is empty, so this provisioner has no password", path)
	}

	parsed, err := url.Parse(where)
	if err != nil {
		return "", fmt.Errorf("MESH_PROVISION_POSTGRES is not a URL: %w", err)
	}
	user := parsed.User.Username()
	if user == "" {
		user = "postgres"
	}
	parsed.User = url.UserPassword(user, password)
	return parsed.String(), nil
}