werkdock/internal/api/quirks.go

package api

import (
	"fmt"
	"net"
	"path"
	"sort"
	"strconv"
	"strings"

	"werkdock/internal/store"
)

// A quirk is the per-image knowledge werkdock needs because two of
// docker's promises are not werkdock's to keep (RFC 0005):
//
//   - There is no network namespace, so a published port cannot be
//     mapped. The service must be told to bind the port the client is
//     to be given, which every serious image allows through an
//     environment variable.
//   - There is one uid. An image whose entrypoint starts as root and
//     drops privileges itself (the official postgres image and its
//     gosu dance) has to be started as the target user right away;
//     those entrypoints detect a non-root uid and skip the dance.
//
// This is a deliberate deviation from docker, documented in RFC 0005
// and in the CHANGELOG. An image that is not listed runs as the image
// says and keeps its port, which is correct as long as one instance of
// it runs at a time.
type quirk struct {
	// User is what to run as when neither the request nor the image
	// names a user.
	User string
	// PortEnv maps a container port to the environment variable that
	// moves the service to another port.
	PortEnv map[int]string
	// PortArg maps a container port to a command-line flag that does
	// the same, for a server that reads no environment variable for it.
	// It applies only when the command is still the image's own, so
	// werkdock never appends a flag to a command somebody else chose.
	PortArg map[int]portArg
}

// portArg is "append this flag and the port, but only to this program".
// A Flag ending in "=" is joined to the number (--port=3306); otherwise
// the two are separate words (--port 6379), which is what redis-server
// wants.
type portArg struct {
	Program string
	Flag    string
}

// words renders the flag for a chosen port.
func (a portArg) words(port int) []string {
	if strings.HasSuffix(a.Flag, "=") {
		return []string{a.Flag + strconv.Itoa(port)}
	}
	return []string{a.Flag, strconv.Itoa(port)}
}

var quirks = map[string]quirk{
	"postgres": {User: "postgres", PortEnv: map[int]string{5432: "PGPORT"}},
	// MariaDB's entrypoint reads the port out of the server's own
	// configuration (mysql_get_config 'port' "$@"), not out of the
	// environment: with MARIADB_TCP_PORT set the server stayed on 3306.
	// The flag reaches both the entrypoint's reading of it and the
	// server itself, because the entrypoint passes "$@" on.
	"mariadb": {User: "mysql", PortArg: map[int]portArg{3306: {Program: "mariadbd", Flag: "--port="}}},
	// MySQL is MariaDB's relative and behaves the same way. 33060 is the
	// X protocol, which the image also opens: it moves only when a
	// client asks for it, so two instances at once share it and the
	// second one's X plugin fails while its SQL port works.
	"mysql": {User: "mysql", PortArg: map[int]portArg{
		3306:  {Program: "mysqld", Flag: "--port="},
		33060: {Program: "mysqld", Flag: "--mysqlx-port="},
	}},
	// Redis reads no environment variable for its port — measured, not
	// assumed: with REDIS_PORT set the server stayed on 6379 and
	// werkdock reported a port nothing listened on. redis-server takes
	// --port instead. The user is needed for the same reason postgres
	// needs one: the entrypoint chowns its data directory as root, which
	// an unprivileged user namespace cannot do.
	"redis": {User: "redis", PortArg: map[int]portArg{6379: {Program: "redis-server", Flag: "--port"}}},
}

// moves reports how werkdock would move this port, if it can: by
// environment variable, by command-line flag, or not at all. A flag
// counts only when the command is the image's own program, because
// appending to a command the client chose would be a guess.
func (q quirk) moves(container int, command []string) (env string, arg *portArg) {
	if name, ok := q.PortEnv[container]; ok {
		return name, nil
	}
	a, ok := q.PortArg[container]
	if !ok || len(command) == 0 {
		return "", nil
	}
	for _, word := range command {
		if path.Base(word) == a.Program {
			return "", &a
		}
	}
	return "", nil
}

// quirkFor looks the table up by the image's bare repository name, so
// postgres, library/postgres and docker.io/library/postgres:16-alpine
// are the same image.
func quirkFor(reference string) quirk {
	ref, err := store.ParseReference(reference)
	if err != nil {
		return quirk{}
	}
	repo := ref.Repository
	if i := strings.LastIndex(repo, "/"); i >= 0 {
		repo = repo[i+1:]
	}
	return quirks[repo]
}

// port is one published port and where it actually listens.
type port struct {
	Container int
	Host      int
	Protocol  string
}

// parsePortKey reads docker's "5432/tcp".
func parsePortKey(key string) (int, string, error) {
	spec, proto, found := strings.Cut(key, "/")
	if !found {
		proto = "tcp"
	}
	n, err := strconv.Atoi(spec)
	if err != nil || n <= 0 || n > 65535 {
		return 0, "", fmt.Errorf("invalid port %q", key)
	}
	return n, proto, nil
}

// freePort asks the kernel for an unused port. There is a window
// between this and the service binding it; werkdock accepts it, as
// every port picker without a network namespace must.
func freePort() (int, error) {
	ln, err := net.Listen("tcp", "127.0.0.1:0")
	if err != nil {
		return 0, err
	}
	defer ln.Close()
	return ln.Addr().(*net.TCPAddr).Port, nil
}

// publishPorts decides where a container's ports really listen.
// For a port the image can be moved (the quirk table), a requested
// host port is honoured and an empty one is chosen free; the service
// is then told through its environment variable. For every other port
// the answer is the identity: the sandbox shares the host's network,
// so the port the service binds is the port the client reaches.
func publishPorts(exposed map[string]struct{}, bindings map[string][]portBinding, q quirk, command []string) ([]port, map[string]string, []string, error) {
	keys := map[string]bool{}
	for k := range exposed {
		keys[k] = true
	}
	for k := range bindings {
		keys[k] = true
	}
	env := map[string]string{}
	var args []string
	var ports []port
	for _, key := range sortedPortKeys(keys) {
		container, proto, err := parsePortKey(key)
		if err != nil {
			return nil, nil, nil, err
		}
		requested := 0
		if list := bindings[key]; len(list) > 0 && list[0].HostPort != "" {
			if requested, err = strconv.Atoi(list[0].HostPort); err != nil {
				return nil, nil, nil, fmt.Errorf("invalid host port %q for %s", list[0].HostPort, key)
			}
		}
		name, arg := q.moves(container, command)
		movable := name != "" || arg != nil
		host := requested
		switch {
		case movable && requested == 0:
			if host, err = freePort(); err != nil {
				return nil, nil, nil, err
			}
		case !movable && requested != 0 && requested != container:
			return nil, nil, nil, fmt.Errorf(
				"cannot publish %s on host port %d: werkdock shares the host's network and cannot map ports; "+
					"the service would have to bind %d itself, and werkdock does not know how to tell this image to",
				key, requested, requested)
		case !movable:
			host = container
		}
		ports = append(ports, port{Container: container, Host: host, Protocol: proto})
		switch {
		case name != "":
			env[name] = strconv.Itoa(host)
		case arg != nil:
			args = append(args, arg.words(host)...)
		}
	}
	sortPorts(ports)
	return ports, env, args, nil
}

func sortSlice(keys []string) []string { sort.Strings(keys); return keys }

// sortedPortKeys makes the chosen ports reproducible when more than one
// is published: map iteration order must not decide which gets which.
func sortedPortKeys(keys map[string]bool) []string {
	out := make([]string, 0, len(keys))
	for k := range keys {
		out = append(out, k)
	}
	return sortSlice(out)
}

func sortPorts(ports []port) {
	for i := 1; i < len(ports); i++ {
		for j := i; j > 0 && ports[j].Container < ports[j-1].Container; j-- {
			ports[j], ports[j-1] = ports[j-1], ports[j]
		}
	}
}