mesh-catalog/modules/zsh/shell.ts

// node-login-shell's `execute`, and the reading of the account's zsh files (novox/hq ADR 0176,
// ADR 0204).
//
// Who runs it. The node tools runtime runs as the operator account, not root (novox/hq ADR 0175
// §4), and launches this bundle as a process of its own (ADR 0188, ADR 0193). So the command runs
// as this process's own user, which is the account; a command that needs root uses sudo inside the
// shell, as a person would.
//
// The bounds are the seat's protocol (ADR 0204 §4), and each answers one way the runtime fails:
// - time: the runtime gives up on a call after thirty seconds, so the command is ended before then
//   and the answer says it timed out, rather than the caller hearing nothing;
// - the process group: the shell is started as the leader of its own group and the whole group is
//   killed on timeout, so `sleep 100 & wait` leaves no sleep behind;
// - output: a reply over about a megabyte is lost on the way back, so each stream is cut at a bound
//   and the answer says it was cut.

import { spawn } from "node:child_process";
import { existsSync } from "node:fs";
import { readFile } from "node:fs/promises";
import { homedir, userInfo } from "node:os";

/** The longest a command may run when the caller does not say, and the most it may ask for. Both
 *  below the runtime's thirty-second call limit, leaving room for the answer to travel. */
export const DEFAULT_TIMEOUT_SECONDS = 20;
export const MAX_TIMEOUT_SECONDS = 25;
/** How much of each stream is kept: two of these are far below what one reply carries. */
export const OUTPUT_CAP_BYTES = 256 * 1024;

export interface Executed {
  command: string;
  account: string;
  cwd: string;
  /** The shell's exit status; null when it was ended by a signal. */
  status: number | null;
  signal: string | null;
  stdout: string;
  stderr: string;
  /** Which streams were cut at OUTPUT_CAP_BYTES. */
  truncated: { stdout: boolean; stderr: boolean };
  timed_out: boolean;
  timeout_seconds: number;
}

export interface ExecuteOptions {
  /** The shell to run; zsh for this module, a test may use another. */
  shell?: string;
  cwd: string;
  env: NodeJS.ProcessEnv;
  timeoutSeconds: number;
  capBytes?: number;
  account?: string;
}

/** The timeout a caller asked for, held within the bounds. */
export function timeoutOf(asked: unknown): number {
  if (asked === undefined || asked === null || asked === "") return DEFAULT_TIMEOUT_SECONDS;
  const n = Number(asked);
  if (!Number.isFinite(n) || n <= 0) return DEFAULT_TIMEOUT_SECONDS;
  return Math.min(n, MAX_TIMEOUT_SECONDS);
}

/** Where the command runs: the account's home. */
export function homeOf(env: NodeJS.ProcessEnv): string {
  return env.MESH_OPERATOR_HOME?.trim() || env.HOME?.trim() || homedir();
}

/** The environment the command is given: the runtime's, without the mesh's own words, and with the
 *  account's session words when its runtime directory exists, so `systemctl --user` and anything
 *  speaking to the session bus reach the account's own manager. */
export function commandEnv(env: NodeJS.ProcessEnv, uid: number | undefined = process.getuid?.(), exists: (p: string) => boolean = existsSync): NodeJS.ProcessEnv {
  const out: NodeJS.ProcessEnv = {};
  for (const [k, v] of Object.entries(env)) {
    if (!k.startsWith("MESH_")) out[k] = v;
  }
  if (uid !== undefined) {
    const runtime = `/run/user/${uid}`;
    if (exists(runtime)) {
      out.XDG_RUNTIME_DIR ??= runtime;
      out.DBUS_SESSION_BUS_ADDRESS ??= `unix:path=${runtime}/bus`;
    }
  }
  return out;
}

/** Collects one stream up to a bound, and says whether it had to cut. */
class Capped {
  private readonly cap: number;
  private parts: Buffer[] = [];
  private size = 0;
  cut = false;
  constructor(cap: number) {
    this.cap = cap;
  }
  add(chunk: Buffer): void {
    const room = this.cap - this.size;
    if (room <= 0) {
      this.cut = true;
      return;
    }
    const kept = chunk.length > room ? chunk.subarray(0, room) : chunk;
    if (kept.length < chunk.length) this.cut = true;
    this.parts.push(kept);
    this.size += kept.length;
  }
  text(): string {
    return Buffer.concat(this.parts).toString("utf8");
  }
}

/** Run one command line as `<shell> -lc`, bounded in time and output, its process group ended on
 *  timeout. */
export function execute(command: string, o: ExecuteOptions): Promise<Executed> {
  const cap = o.capBytes ?? OUTPUT_CAP_BYTES;
  const account = o.account ?? userInfo().username;
  return new Promise((resolve) => {
    const out = new Capped(cap);
    const err = new Capped(cap);
    let timedOut = false;
    let settled = false;
    const child = spawn(o.shell ?? "zsh", ["-lc", command], {
      cwd: o.cwd,
      env: o.env,
      detached: true, // its own process group, so the whole group can be ended
      stdio: ["ignore", "pipe", "pipe"],
    });
    const killGroup = () => {
      try {
        if (child.pid) process.kill(-child.pid, "SIGKILL");
      } catch {
        // The group is already gone.
      }
    };
    child.stdout.on("data", (d: Buffer) => out.add(d));
    child.stderr.on("data", (d: Buffer) => err.add(d));
    const timer = setTimeout(() => {
      timedOut = true;
      killGroup();
    }, o.timeoutSeconds * 1000);
    const finish = (status: number | null, signal: string | null, extra = "") => {
      if (settled) return;
      settled = true;
      clearTimeout(timer);
      child.stdout.destroy();
      child.stderr.destroy();
      resolve({
        command, account, cwd: o.cwd, status, signal,
        stdout: out.text(), stderr: err.text() + extra,
        truncated: { stdout: out.cut, stderr: err.cut },
        timed_out: timedOut, timeout_seconds: o.timeoutSeconds,
      });
    };
    child.on("error", (e) => finish(null, null, e.message));
    // The shell exiting is the answer. Its streams are read until they close, but a job it left
    // running in the background may hold them open for ever, so they are given a moment and no more.
    child.on("exit", (status, signal) => {
      const grace = setTimeout(() => finish(status, signal), 200);
      child.on("close", () => {
        clearTimeout(grace);
        finish(status, signal);
      });
    });
  });
}

/** One zsh startup file as it is now, and how many of its lines are the mesh's block. */
export async function zshFile(path: string, cap: number = OUTPUT_CAP_BYTES): Promise<Record<string, unknown>> {
  const text = await readFile(path, "utf8").catch(() => null);
  if (text === null) return { path, exists: false };
  const block = /^# BEGIN mesh [^\n]*\n([\s\S]*?)^# END mesh[^\n]*$/m.exec(text);
  const lines = text === "" ? 0 : text.replace(/\n$/, "").split("\n").length;
  return {
    path,
    exists: true,
    lines,
    mesh_block_lines: block && block[1] !== "" ? block[1].replace(/\n$/, "").split("\n").length : 0,
    truncated: text.length > cap,
    content: text.length > cap ? text.slice(0, cap) : text,
  };
}