path: root/bbot/agent.cxx

// file      : bbot/agent.cxx -*- C++ -*-
// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
// license   : TBC; see accompanying LICENSE file

#include <bbot/agent>

#include <pwd.h>    // getpwuid()
#include <limits.h> // PATH_MAX
#include <signal.h> // signal()
#include <unistd.h> // sleep(), realink(), getuid()

#include <net/if.h>     // ifreq
#include <netinet/in.h> // sockaddr_in
#include <arpa/inet.h>  // inet_ntop()
#include <sys/ioctl.h>
#include <sys/socket.h>

#include <iostream>

#include <butl/pager>
#include <butl/filesystem> // dir_iterator

#include <bbot/manifest>

#include <bbot/types>
#include <bbot/utility>
#include <bbot/diagnostics>

#include <bbot/tftp>
#include <bbot/machine>
#include <bbot/machine-manifest>
#include <bbot/bootstrap-manifest>

using namespace std;
using namespace butl;
using namespace bbot;

namespace bbot
{
  agent_options ops;

  const string bs_prot ("1");

  string tc_name;
  size_t tc_num;
  string tc_id;

  string hname;
  uid_t  uid;
  string uname;

  // Note: Linux-specific implementation.
  //
  string
  iface_addr (const string& i)
  {
    if (i.size () >= IFNAMSIZ)
      throw invalid_argument ("interface nama too long");

    auto_fd fd (socket (AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0));

    if (fd.get () == -1)
      throw_system_error (errno);

    ifreq ifr;
    ifr.ifr_addr.sa_family = AF_INET;
    strcpy (ifr.ifr_name, i.c_str ());

    if (ioctl (fd.get (), SIOCGIFADDR, &ifr) == -1)
      throw_system_error (errno);

    char buf[3 * 4 + 3 + 1]; // IPv4 address.
    if (inet_ntop (AF_INET,
                   &reinterpret_cast<sockaddr_in*> (&ifr.ifr_addr)->sin_addr,
                   buf,
                   sizeof (buf)) == nullptr)
      throw_system_error (errno);

    return buf;
  }
}

// The btrfs tool likes to print informational messages, like "Created
// snapshot such and such". Luckily, it writes them to stdout while proper
// diagnostics to stderr.
//
template <typename... A>
inline void
run_btrfs (tracer& t, A&&... a)
{
  if (verb >= 4)
    run_io (t, fdnull (), 2, 2, "btrfs", forward<A> (a)...);
  else
    run_io (t, fdnull (), fdnull (), 2, "btrfs", forward<A> (a)...);
}

template <typename... A>
inline butl::process_exit::code_type
btrfs_exit (tracer& t, A&&... a)
{
  return verb >= 4
    ? run_io_exit (t, fdnull (), 2, 2, "btrfs", forward<A> (a)...)
    : run_io_exit (t, fdnull (), fdnull (), 2, "btrfs", forward<A> (a)...);
}

// Bootstrap the machine. Return the bootstrapped machine manifest if
// successful and nullopt otherwise (in which case the machine directory
// should be cleaned and the machine ignored for now).
//
static optional<bootstrapped_machine_manifest>
bootstrap_machine (const dir_path& md,
                   const machine_manifest& mm,
                   optional<bootstrapped_machine_manifest> obmm)
{
  tracer trace ("bootstrap_machine");

  bootstrapped_machine_manifest r {
    mm,
    toolchain_manifest {tc_id.empty () ? "bogus" : tc_id},
    bootstrap_manifest {
      bootstrap_manifest::versions_type {
        {"bbot",    BBOT_VERSION},
        {"libbbot", LIBBBOT_VERSION},
        {"libbpkg", LIBBPKG_VERSION},
        {"libbutl", LIBBUTL_VERSION}
      }
    }
  };

  if (ops.fake_bootstrap ())
  {
    r.machine.mac = "de:ad:be:ef:de:ad";
  }
  else
  try
  {
    string br ("br1"); // Using private bridge for now.

    // Start the TFTP server (server chroot is --tftp). Map:
    //
    // GET requests to .../toolchain/<name>/*
    // PUT requests to .../bootstrap/<name>/*
    //
    auto_rmdir arm ((dir_path (ops.tftp ()) /= "bootstrap") /= tc_name);
    try_mkdir_p (arm.path ());

    // Bootstrap result manifest.
    //
    path mf (arm.path () / "manifest");
    try_rmfile (mf);

    tftp_server tftpd ("Gr  ^/?(.+)$  /toolchain/" + tc_name + "/\\1\n" +
                       "Pr  ^/?(.+)$  /bootstrap/" + tc_name + "/\\1\n");

    l3 ([&]{trace << "tftp server on port " << tftpd.port ();});

    // Start the machine.
    //
    unique_ptr<machine> m (
      start_machine (md,
                     mm,
                     obmm ? obmm->machine.mac : nullopt,
                     br,
                     tftpd.port ()));

    {
      // If we are terminating with an exception then force the machine down.
      // Failed that, the machine's destructor will block waiting for its
      // completion.
      //
      auto mg (
        make_exception_guard (
          [&m, &md] ()
          {
            info << "trying to force machine " << md << " down";
            try {m->forcedown ();} catch (const failed&) {}
          }));

      // What happens if the bootstrap process hangs? The simple thing would
      // be to force the machine down after some timeout and then fail. But
      // that won't be very helpful for investigating the cause. So instead
      // the plan is to suspend it after some timeout, issue diagnostics
      // (without failing and which Build OS monitor will relay to the admin),
      // and wait for the external intervention.
      //
      auto soft_fail = [&md, &m] (const char* msg)
      {
        {
          diag_record dr (error);
          dr << msg << " for machine " << md << ", suspending";
          m->print_info (dr);
        }
        m->suspend ();
        m->wait ();
        return nullopt;
      };

      // The first request should be the toolchain download. Wait for up to 60
      // seconds for that to arrive. In a sense we use it as an indication
      // that the machine has booted and the bootstrap process has started.
      //
      size_t to;
      const size_t startup_to   (60);
      const size_t bootstrap_to (ops.bootstrap_timeout ());
      const size_t shutdown_to  (60);

      if (!tftpd.serve ((to = startup_to)))
        return soft_fail ("bootstrap startup timeout");

      l3 ([&]{trace << "completed startup in " << startup_to - to << "s";});

      // Next the bootstrap process may download additional toolchain
      // archives, build things, and then upload the result manifest. So on
      // our side we serve TFTP requests while periodically checking for the
      // manifest file.
      //
      for (to = bootstrap_to; to != 0 && !file_exists (mf); tftpd.serve (to)) ;

      if (to == 0)
        return soft_fail ("bootstrap timeout");

      l3 ([&]{trace << "completed bootstrap in " << bootstrap_to - to << "s";});

      // Shut the machine down cleanly.
      //
      if (!m->shutdown ((to = shutdown_to)))
        return soft_fail ("bootstrap shutdown timeout");

      l3 ([&]{trace << "completed shutdown in " << shutdown_to - to << "s";});
    }

    // Parse the result manifest.
    //
    r.bootstrap = parse_manifest<bootstrap_manifest> (mf, "bootstrap");

    r.machine.mac = m->mac; // Save the MAC address.
  }
  catch (const system_error& e)
  {
    fail << "bootstrap error: " << e;
  }

  serialize_manifest (r, md / "manifest", "bootstrapped machine");
  return r;
}

// Return available machines and their directories as a parallel array.
//
static pair<bootstrapped_machine_manifests, dir_paths>
enumerate_machines (const dir_path& machines)
try
{
  tracer trace ("enumerate_machines");

  bootstrapped_machine_manifests rm;
  dir_paths rd;

  if (ops.fake_machine_specified ())
  {
    auto mh (
      parse_manifest<machine_header_manifest> (
        ops.fake_machine (), "machine header"));

    rm.push_back (
      bootstrapped_machine_manifest {
        machine_manifest {
          mh.id,
          mh.name,
          mh.summary,
          machine_type::kvm,
          string ("de:ad:be:ef:de:ad"),
          nullopt},
        toolchain_manifest {tc_id},
        bootstrap_manifest {}
      });

    rd.push_back (dir_path (ops.machines ()) /= mh.name); // For diagnostics.

    return make_pair (move (rm), move (rd));
  }

  // The first level are machine volumes.
  //
  for (const dir_entry& ve: dir_iterator (machines))
  {
    const string vn (ve.path ().string ());

    // Ignore hidden directories.
    //
    if (ve.type () != entry_type::directory || vn[0] == '.')
      continue;

    const dir_path vd (dir_path (machines) /= vn);

    // Inside we have machines.
    //
    try
    {
      for (const dir_entry& me: dir_iterator (vd))
      {
        const string mn (me.path ().string ());

        if (me.type () != entry_type::directory || mn[0] == '.')
          continue;

        const dir_path md (dir_path (vd) /= mn);

        // Our endgoal here is to obtain a bootstrapped snapshot of this
        // machine while watching out for potential race conditions (machines
        // being added/upgraded/removed; see the manual for details).
        //
        // So here is our overall plan:
        //
        // 1. Resolve current subvolume link for our bootstrap protocol.
        //
        // 2. If there is no link, cleanup and ignore this machine.
        //
        // 3. Try to create a snapshot of current subvolume (this operation is
        //    atomic). If failed (e.g., someone changed the link and removed
        //    the subvolume in the meantime), retry from #1.
        //
        // 4. Compare the snapshot to the already bootstrapped version (if
        //    any) and see if we need to re-bootstrap. If so, use the snapshot
        //    as a starting point. Rename to bootstrapped at the end (atomic).
        //
        dir_path lp (dir_path (md) /= (mn + '-' + bs_prot)); // -<P>
        dir_path tp (dir_path (md) /= (mn + '-' + tc_name)); // -<toolchain>
        bool te (dir_exists (tp));

        auto delete_t = [&tp, &trace] ()
        {
          run_btrfs (trace, "property", "set", "-ts", tp, "ro", "false");
          run_btrfs (trace, "subvolume", "delete", tp);
        };

        for (size_t retry (0);; ++retry)
        {
          if (retry != 0)
            sleep (1);

          // Resolve the link to subvolume path.
          //
          dir_path sp; // <name>-<P>.<R>
          try
          {
            char b [PATH_MAX + 1];
            ssize_t r (readlink (lp.string ().c_str (), b, sizeof (b)));

            if (r == -1)
            {
              if (errno != ENOENT)
                throw_generic_error (errno);
            }
            else if (static_cast<size_t> (r) >= sizeof (b))
              throw_generic_error (EINVAL);
            else
            {
              b[r] = '\0';
              sp = dir_path (b);
              if (sp.relative ())
                sp = md / sp;
            }
          }
          catch (const system_error& e)
          {
            fail << "unable to read subvolume link " << lp << ": " << e;
          }

          // If the resolution fails, then this means there is no current
          // machine subvolume (for this bootstrap protocol). In this case we
          // clean up our toolchain subvolume (<name>-<toolchain>) and ignore
          // this machine.
          //
          if (sp.empty ())
          {
            if (te)
              delete_t ();

            l3 ([&]{trace << "skipping " << md << ": no subvolume link";});
            break;
          }

          // <name>-<toolchain>-<xxx>
          //
          const dir_path xp (
            dir_path (md) /= path::traits::temp_name (mn + '-' + tc_name));

          if (btrfs_exit (trace, "subvolume", "snapshot", sp, xp) != 0)
          {
            if (retry >= 10)
              fail << "unable to snapshot subvolume " << sp;

            continue;
          }

          // Load the (original) machine manifest.
          //
          auto mm (
            parse_manifest<machine_manifest> (sp / "manifest", "machine"));

          // If we already have <name>-<toolchain>, see if it needs to be re-
          // bootstrapped. Things that render it obsolete:
          //
          // 1. New machine revision  (compare machine ids).
          // 2. New toolchain         (compare toolchain ids).
          // 3. New bbot/libbbot      (compare versions).
          //
          // The last case has a complication: what should we do if we have
          // bootstrapped a newer version of bbot? This would mean that we are
          // about to be stopped and upgraded (and the upgraded version will
          // probably be able to use the result). So we simply ignore this
          // machine for this run.

          // Return -1 if older, 0 if the same, and +1 if newer.
          //
          auto compare_bbot = [] (const bootstrap_manifest& m) -> int
          {
            auto cmp = [&m] (const string& n, uint64_t v) -> int
            {
              auto i = m.versions.find (n);
              return
                i == m.versions.end () || i->second < v
                ? -1
                : i->second > v ? 1 : 0;
            };

            // Start from the top assuming a new dependency cannot be added
            // without changing the dependent's version.
            //
            int r;
            return
              (r = cmp ("bbot",       BBOT_VERSION)) != 0 ? r :
              (r = cmp ("libbbot", LIBBBOT_VERSION)) != 0 ? r :
              (r = cmp ("libbpkg", LIBBPKG_VERSION)) != 0 ? r :
              (r = cmp ("libbutl", LIBBUTL_VERSION)) != 0 ? r : 0;
          };

          optional<bootstrapped_machine_manifest> bmm;
          if (te)
          {
            bmm = parse_manifest<bootstrapped_machine_manifest> (
              tp / "manifest", "bootstrapped machine");

            if (bmm->machine.id != mm.id)
            {
              l3 ([&]{trace << "re-bootstrapping " << tp << ": new machine";});
              te = false;
            }

            if (!tc_id.empty () && bmm->toolchain.id != tc_id)
            {
              l3 ([&]{trace << "re-bootstrapping " << tp << ": new toolchain";});
              te = false;
            }

            if (int i = compare_bbot (bmm->bootstrap))
            {
              if (i < 0)
              {
                l3 ([&]{trace << "re-bootstrapping " << tp << ": new bbot";});
                te = false;
              }
              else
              {
                l3 ([&]{trace << "ignoring " << tp << ": old bbot";});
                run_btrfs (trace, "subvolume", "delete", xp);
                break;
              }
            }

            if (!te)
              delete_t ();
          }
          else
            l3 ([&]{trace << "bootstrapping " << tp;});

          if (!te)
          {
            // Use the <name>-<toolchain>-<xxx> snapshot that we have made to
            // bootstrap the new machine. Then atomically rename it to
            // <name>-<toolchain>.
            //
            bmm = bootstrap_machine (xp, mm, move (bmm));

            if (!bmm)
            {
              l3 ([&]{trace << "ignoring " << tp << ": failed to bootstrap";});
              run_btrfs (trace, "subvolume", "delete", xp);
              break;
            }

            try
            {
              mvdir (xp, tp);
            }
            catch (const system_error& e)
            {
              fail << "unable to rename " << xp << " to " << tp;
            }

            l2 ([&]{trace << "bootstrapped " << bmm->machine.name;});

            // Check the bootstrapped bbot version as above and ignore this
            // machine if it's newer than us.
            //
            if (int i = compare_bbot (bmm->bootstrap))
            {
              assert (i > 0);
              l3 ([&]{trace << "ignoring " << tp << ": old bbot";});
              break;
            }
          }
          else
            run_btrfs (trace, "subvolume", "delete", xp);

          // Add the machine to the lists.
          //
          rm.push_back (move (*bmm));
          rd.push_back (move (tp));

          break;
        }
      }
    }
    catch (const system_error& e)
    {
      fail << "unable to iterate over " << vd << ": " << e << endf;
    }
  }

  return make_pair (move (rm), move (rd));
}
catch (const system_error& e)
{
  fail << "unable to iterate over " << machines << ": " << e << endf;
}

static result_manifest
perform_task (const dir_path& md,
              const bootstrapped_machine_manifest& mm,
              const task_manifest& tm)
try
{
  tracer trace ("perform_task");

  result_manifest r {
    tm.name,
    tm.version,
    result_status::abort,
    operation_results {}};

  if (ops.fake_build ())
    return r;

  // The overall plan is as follows:
  //
  // 1. Snapshot the (bootstrapped) machine.
  //
  // 2. Save the task manifest to the TFTP directory (to be accessed by the
  //    worker).
  //
  // 3. Start the TFTP server and the machine.
  //
  // 4. Serve TFTP requests while watching out for the result manifest.
  //
  // 5. Clean up (force the machine down and delete the snapshot).
  //

  // TFTP server mapping (server chroot is --tftp):
  //
  // GET requests to .../build/<name>/get/*
  // PUT requests to .../build/<name>/put/*
  //
  auto_rmdir arm ((dir_path (ops.tftp ()) /= "build") /= tc_name);

  dir_path gd (dir_path (arm.path ()) /= "get");
  dir_path pd (dir_path (arm.path ()) /= "put");

  try_mkdir_p (gd);
  try_mkdir_p (pd);

  path tf (gd / "manifest"); // Task manifest file.
  path rf (pd / "manifest"); // Result manifest file.

  serialize_manifest (tm, tf, "task");

  if (ops.fake_machine_specified ())
  {
    // Simply wait for the file to appear.
    //
    for (size_t i (0); !file_exists (rf); sleep (1))
      if (i++ % 10 == 0)
        l3 ([&]{trace << "waiting for result manifest";});

    r = parse_manifest<result_manifest> (rf, "result");
  }
  else
  {
    // <name>-<toolchain>-<xxx>
    //
    const dir_path xp (
      md.directory () /= path::traits::temp_name (md.leaf ().string ()));

    run_btrfs (trace, "subvolume", "snapshot", md, xp);

    string br ("br1"); // Using private bridge for now.

    // Start the TFTP server.
    //
    tftp_server tftpd ("Gr  ^/?(.+)$  /build/" + tc_name + "/get/\\1\n" +
                       "Pr  ^/?(.+)$  /build/" + tc_name + "/put/\\1\n");

    l3 ([&]{trace << "tftp server on port " << tftpd.port ();});

    // Start the machine.
    //
    unique_ptr<machine> m (
      start_machine (xp,
                     mm.machine,
                     mm.machine.mac,
                     br,
                     tftpd.port ()));

    // Note: the machine handling logic is similar to bootstrap.
    //
    {
      auto mg (
        make_exception_guard (
          [&m, &xp] ()
          {
            info << "trying to force machine " << xp << " down";
            try {m->forcedown ();} catch (const failed&) {}
          }));

      auto soft_fail = [&xp, &m, &r] (const char* msg, bool wait = true)
      {
        {
          diag_record dr (error);
          dr << msg << " for machine " << xp << ", suspending";
          m->print_info (dr);
        }
        m->suspend ();

        if (wait)
          m->wait ();

        return r;
      };

      // The first request should be the task manifest download. Wait for up
      // to 60 seconds for that to arrive. In a sense we use it as an
      // indication that the machine has booted and the worker process has
      // started.
      //
      size_t to;
      const size_t startup_to (60);
      const size_t build_to   (ops.build_timeout ());

      if (!tftpd.serve ((to = startup_to)))
        return soft_fail ("build startup timeout");

      l3 ([&]{trace << "completed startup in " << startup_to - to << "s";});

      // Next the worker builds things and then uploads the result manifest.
      // So on our side we serve TFTP requests while checking for the manifest
      // file.
      //
      for (to = build_to; to != 0 && !file_exists (rf); tftpd.serve (to)) ;

      if (to == 0)
        return soft_fail ("build timeout");

      l3 ([&]{trace << "completed build in " << build_to - to << "s";});

      // Parse the result manifest.
      //
      r = parse_manifest<result_manifest> (rf, "result");

      // If the build terminated abnormally, suspent the machine for
      // investigation (note that here we don't wait or return).
      //
      if (r.status == result_status::abnormal)
        soft_fail ("build terminated abnormally", false);

      // Force the machine down (there is no need wasting time on clean
      // shutdown since the next step is to drop the snapshot).
      //
      m->forcedown ();
    }

    run_btrfs (trace, "subvolume", "delete", xp);
  }

  // Update package name/version if the returned value as "unknown".
  //
  if (r.version == bpkg::version ("0"))
  {
    assert (r.status == result_status::abnormal);

    r.name = tm.name;
    r.version = tm.version;
  }

  return r;
}
catch (const system_error& e)
{
  fail << "build error: " << e << endf;
}

extern "C" void
handle_signal (int sig)
{
  switch (sig)
  {
  case SIGHUP:  exit (3); // Unimplemented feature.
  case SIGTERM: exit (0);
  default:      assert (false);
  }
}

int
main (int argc, char* argv[])
try
{
  cli::argv_scanner scan (argc, argv, true);
  ops.parse (scan);

  verb = ops.verbose ();

  if (ops.systemd_daemon ())
    systemd_diagnostics (true); // With critical errors.

  tracer trace ("main");

  uid = getuid ();
  uname = getpwuid (uid)->pw_name;

  {
    char buf[HOST_NAME_MAX + 1];

    if (gethostname (buf, sizeof (buf)) == -1)
      fail << "unable to obtain hostname: "
           << system_error (errno, generic_category ()); // Sanitize.

    hname = buf;
  }

  // On POSIX ignore SIGPIPE which is signaled to a pipe-writing process if
  // the pipe reading end is closed. Note that by default this signal
  // terminates a process. Also note that there is no way to disable this
  // behavior on a file descriptor basis or for the write() function call.
  //
  if (signal (SIGPIPE, SIG_IGN) == SIG_ERR)
    fail << "unable to ignore broken pipe (SIGPIPE) signal: "
         << system_error (errno, generic_category ()); // Sanitize.

  // Version.
  //
  if (ops.version ())
  {
    cout << "bbot-agent " << BBOT_VERSION_STR << endl
         << "libbbot " << LIBBBOT_VERSION_STR << endl
         << "libbutl " << LIBBUTL_VERSION_STR << endl
         << "Copyright (c) 2014-2017 Code Synthesis Ltd" << endl
         << "TBC; All rights reserved" << endl;

    return 0;
  }

  // Help.
  //
  if (ops.help ())
  {
    pager p ("bbot-agent help", false);
    print_bbot_agent_usage (p.stream ());

    // If the pager failed, assume it has issued some diagnostics.
    //
    return p.wait () ? 0 : 1;
  }

  tc_name = ops.toolchain_name ();
  tc_num  = ops.toolchain_num ();
  tc_id   = ops.toolchain_id ();


  // Controller URLs.
  //
  if (argc < 2 &&
      !ops.dump_machines () &&
      !ops.fake_request_specified ())
  {
    fail << "controller url expected" <<
      info << "run " << argv[0] << " --help for details";
  }

  strings controllers;

  for (int i (1); i != argc; ++i)
    controllers.push_back (argv[i]);

  // Handle SIGHUP and SIGTERM.
  //
  if (signal (SIGHUP,  &handle_signal) == SIG_ERR ||
      signal (SIGTERM, &handle_signal) == SIG_ERR)
    fail << "unable to set signal handler: "
         << system_error (errno, generic_category ()); // Sanitize.

  if (ops.systemd_daemon ())
  {
    diag_record dr;

    dr << info << "bbot agent " << BBOT_VERSION_STR <<
      info << "toolchain name " << tc_name <<
      info << "toolchain num  " << tc_num <<
      info << "toolchain id   " << tc_id <<
      info << "CPU(s)         " << ops.cpu () <<
      info << "RAM(kB)        " << ops.ram ();

    for (const string& u: controllers)
      dr << info << "controller url " << u;
  }

  // The work loop. The steps we go through are:
  //
  // 1. Enumerate the available machines, (re-)bootstrapping any if necessary.
  //
  // 2. Poll controller(s) for build tasks.
  //
  // 3. If no build tasks are available, go to #1 (after sleeping a bit).
  //
  // 4. If a build task is returned, do it, upload the result, and go to #1
  //    (immediately).
  //
  for (bool sleep (false);; ::sleep (sleep ? 60 : 0), sleep = false)
  {
    // Enumerate the machines.
    //
    auto mp (enumerate_machines (ops.machines ()));
    bootstrapped_machine_manifests& ms (mp.first);
    dir_paths& ds (mp.second);

    // Prepare task request.
    //
    // @@ TODO: key fingerprint.
    //
    task_request_manifest tq {hname, nullopt, machine_header_manifests {}};

    for (const bootstrapped_machine_manifest& m: ms)
      tq.machines.emplace_back (m.machine.id,
                                m.machine.name,
                                m.machine.summary);

    if (ops.dump_machines ())
    {
      for (const machine_header_manifest& m: tq.machines)
        serialize_manifest (m, cout, "stdout", "machine");

      return 0;
    }

    if (tq.machines.empty ())
    {
      warn << "no build machines for toolchain " << tc_name;
      sleep = true;
      continue;
    }

    // Send task requests.
    //
    //
    string url;
    task_response_manifest tr;

    if (ops.fake_request_specified ())
    {
      auto t (parse_manifest<task_manifest> (ops.fake_request (), "task"));

      tr = task_response_manifest {
        "fake-session", // Dummy session.
        string (),      // Empty challange.
        url,            // Empty result URL.
        move (t)};

      url = "http://example.org";
    }
    else
    {
      for (const string& u: controllers)
      {
        try
        {
          http_curl c (trace,
                       path ("-"),
                       path ("-"),
                       curl::post,
                       u,
                       "--header", "Content-Type: text/manifest",
                       "--max-time", ops.request_timeout ());

          serialize_manifest (tq, c.out, u, "task request");
          c.out.close ();

          tr = parse_manifest<task_response_manifest> (
            c.in, u, "task response");
          c.in.close ();

          if (!c.wait ())
            throw_generic_error (EIO);
        }
        catch (const system_error& e)
        {
          error << "unable to request task from " << u << ": " << e;
          continue;
        }

        if (!tr.session.empty ()) // Got a task.
        {
          url = u;
          break;
        }
      }
    }

    if (tr.session.empty ()) // No task from any of the controllers.
    {
      sleep = true;
      continue;
    }

    // We have a build task.
    //
    // First find the index of the machine we were asked to use (and also
    // verify it is one of those we sent).
    //
    size_t i (0);
    for (const machine_header_manifest& m: tq.machines)
    {
      if (m.name == tr.task->machine)
        break;

      ++i;
    }

    if (i == ms.size ())
    {
      error << "task from " << url << " for unknown machine "
            << tr.task->machine;

      if (ops.dump_task ())
        return 0;

      continue;
    }

    task_manifest& t (*tr.task);

    if (ops.dump_task ())
    {
      serialize_manifest (t, cout, "stdout", "task");
      return 0;
    }

    // If we have our own repository certificate fingerprints, then use them
    // to replace what we have received from the controller.
    //
    if (!ops.trust ().empty ())
      t.trust = ops.trust ();

    const dir_path& d (ds[i]); // The -<toolchain> directory.
    const bootstrapped_machine_manifest& m (ms[i]);

    result_manifest r (perform_task (d, m, t));

    if (ops.dump_result ())
    {
      serialize_manifest (r, cout, "stdout", "result");
      return 0;
    }

    // Upload the result.
    //
    // @@ TODO challange
    //
    result_request_manifest rq {tr.session, nullopt, move (r)};
    {
      const string& u (*tr.result_url);

      try
      {
        http_curl c (trace,
                     path ("-"),
                     nullfd,     // Not expecting any data in response.
                     curl::post,
                     u,
                     "--header", "Content-Type: text/manifest",
                     "--max-time", ops.request_timeout ());

        serialize_manifest (rq, c.out, u, "task request");
        c.out.close ();

        if (!c.wait ())
          throw_generic_error (EIO);
      }
      catch (const system_error& e)
      {
        error << "unable to upload result to " << u << ": " << e;
        continue;
      }
    }

    l2 ([&]{trace << "built " << t.name << '/' << t.version << " "
                  << "on " << t.machine << " "
                  << "for " << url;});
  }
}
catch (const failed&)
{
  return 1; // Diagnostics has already been issued.
}
catch (const cli::exception& e)
{
  error << e;
  return 1;
}