mesh-controller/internal/catalogue/port_into.go

package catalogue

import (
	"fmt"
	"regexp"
	"sort"
	"strconv"
	"strings"
)

// Telling a module which port it was given.
//
// **The mesh assigns the machine-side port and a module does not choose one**
// ([ADR 0038](../../02-DECISIONS/0038-the-mesh-assigns-the-port.md)); on a node given one for a
// module it is the operator's number rather than the mesh's
// ([ADR 0100](../../02-DECISIONS/0100-a-node-in-use-is-adopted-before-it-is-converged.md)). For a
// container's own listening socket that is invisible: the mesh rewrites `ports` into
// `assigned:wanted`, the software inside binds the number it has always bound, and the machine
// publishes a different one.
//
// **Two kinds of resource have no such layer.**
//
//   - **A process** runs on the machine, there is nothing to rewrite, and it binds whatever its
//     configuration says — so without this, every process binds the number written in its own
//     config, two modules declaring the same one collide, and the mesh's whole reason for
//     assigning ports is defeated by the resource kind that most needs it.
//   - **A container that DIALS the machine** — a module's own sidecar reaching the service beside
//     it over the machine's loopback — is told that address in its environment, and the mapping
//     that saves the listener does nothing for the caller: what it must dial is the machine-side
//     number, which is exactly the one the module cannot know (novox/hq 04-ISSUES/088).
//
// So a module asks. `${port:8080}` is "the machine-side port you gave me for the 8080 I said I
// listen on", and the module writes that where it would otherwise have written a literal — in a
// file's content, or in a value of a container's `env`.
//
// **The environment is filled by the control plane, exactly as a bound value is.** A port is not
// secret — the mesh holds it in the clear — so there is nothing for the host to be the only
// witness of, and the host learns no new field. That is what separates this from
// [ADR 0086](../../02-DECISIONS/0086-a-secret-reaches-a-process-as-a-file.md), which refuses a
// `${secret:…}` in an `env` outright: the objection there is to the value being in an environment
// at all, not to who fills it in.
//
// **It answers with the machine's number, wherever it is written.** A container reaching a sibling
// over the runtime's own network reaches it on the port inside that container and goes on writing
// that number literally — it is a number the module does control. This is for the machine side,
// which is the side nobody but the mesh can know.

// ofPort is where a module asks which port it was given: ${port:<the port its software uses>}.
var ofPort = regexp.MustCompile(`\$\{port:([0-9]+)\}`)

// portsUsed are the ports a written value asks about, first appearance first.
func portsUsed(content string) []int {
	var used []int
	seen := map[int]bool{}
	for _, m := range ofPort.FindAllStringSubmatch(content, -1) {
		n, err := strconv.Atoi(m[1])
		if err != nil || seen[n] {
			continue
		}
		seen[n] = true
		used = append(used, n)
	}
	return used
}

// portInto replaces a resource's ${port:…} placeholders with what this machine assigned — in a
// file's content, and in a value of a container's environment.
//
// A port the module did not say it listens on is refused, for the same reason a binding's unknown
// key is: the module is asking about something it never declared, and the answer would be a guess.
// Left alone, the literal would be written into a configuration file, or handed to a process as
// its environment, and read as a port number.
func portInto(resource map[string]any, module string, listens []Listening, with Rendering) error {
	switch fmt.Sprint(resource["type"]) {
	case "file":
		content, ok := resource["content"].(string)
		if !ok {
			return nil
		}
		filled, err := portsFilledInto(content,
			fmt.Sprintf("%s has a file that", module), module, listens, with)
		if err != nil {
			return err
		}
		resource["content"] = filled

	case "container":
		env, ok := resource["env"].(map[string]any)
		if !ok {
			return nil
		}
		// In a stated order, so a container with two bad values always refuses on the same one.
		named := make([]string, 0, len(env))
		for key := range env {
			named = append(named, key)
		}
		sort.Strings(named)

		// **A fresh map, and only when something changes.** This map came out of the module's
		// manifest and the resource around it is a shallow copy, so filling a value in place would
		// change what the catalogue holds for every other machine running the module — the trap
		// withMeshNames is written to avoid, one field along.
		var filled map[string]any
		for _, key := range named {
			written, ok := env[key].(string)
			if !ok || len(portsUsed(written)) == 0 {
				continue
			}
			value, err := portsFilledInto(written,
				fmt.Sprintf("%s's container %s sets %s to something that",
					module, resource["name"], key), module, listens, with)
			if err != nil {
				return err
			}
			if filled == nil {
				filled = map[string]any{}
				for k, v := range env {
					filled[k] = v
				}
			}
			filled[key] = value
		}
		if filled != nil {
			resource["env"] = filled
		}
	}
	return nil
}

// portsFilledInto answers every ${port:…} in one written value, or refuses. `where` names the
// place it was written, so a refusal is one edit from right whichever kind of resource it came
// out of.
func portsFilledInto(written, where, module string, listens []Listening, with Rendering) (
	string, error) {
	for _, wanted := range portsUsed(written) {
		var declared bool
		for _, l := range listens {
			if l.Port == wanted {
				declared = true
			}
		}
		if !declared {
			return "", fmt.Errorf(
				"%s says ${port:%d}, and %s does not say it listens on %d. A module is told the "+
					"port it was given for something it declared, and %s",
				where, wanted, module, wanted, orNoListens(listens))
		}
		written = strings.ReplaceAll(written, fmt.Sprintf("${port:%d}", wanted),
			strconv.Itoa(with.machinePort(module, wanted)))
	}
	return written, nil
}

// orNoListens says what would have worked, so a refusal is one edit from right.
func orNoListens(listens []Listening) string {
	if len(listens) == 0 {
		return "it declares no ports at all"
	}
	said := make([]string, 0, len(listens))
	for _, l := range listens {
		said = append(said, strconv.Itoa(l.Port))
	}
	return "it declares " + strings.Join(said, ", ")
}