path: root/libbuild2/rule.cxx

// file      : libbuild2/rule.cxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#include <libbuild2/rule.hxx>

#include <libbuild2/file.hxx>
#include <libbuild2/depdb.hxx>
#include <libbuild2/scope.hxx>
#include <libbuild2/target.hxx>
#include <libbuild2/context.hxx>
#include <libbuild2/algorithm.hxx>
#include <libbuild2/filesystem.hxx>
#include <libbuild2/diagnostics.hxx>

using namespace std;
using namespace butl;

namespace build2
{
  // rule (vtable)
  //
  rule::
  ~rule ()
  {
  }

  // file_rule
  //
  // Note that this rule is special. It is the last, fallback rule. If
  // it doesn't match, then no other rule can possibly match and we have
  // an error. It also cannot be ambigious with any other rule. As a
  // result the below implementation bends or ignores quite a few rules
  // that normal implementations should follow. So you probably shouldn't
  // use it as a guide to implement your own, normal, rules.
  //
  bool file_rule::
  match (action a, target& t, const string&) const
  {
    tracer trace ("file_rule::match");

    // While strictly speaking we should check for the file's existence
    // for every action (because that's the condition for us matching),
    // for some actions this is clearly a waste. Say, perform_clean: we
    // are not doing anything for this action so not checking if the file
    // exists seems harmless.
    //
    switch (a)
    {
    case perform_clean_id:
      return true;
    default:
      {
        // While normally we shouldn't do any of this in match(), no other
        // rule should ever be ambiguous with the fallback one and path/mtime
        // access is atomic. In other words, we know what we are doing but
        // don't do this in normal rules.

        // First check the timestamp. This takes care of the special "trust
        // me, this file exists" situations (used, for example, for installed
        // stuff where we know it's there, just not exactly where).
        //
        mtime_target& mt (t.as<mtime_target> ());

        timestamp ts (mt.mtime ());

        if (ts != timestamp_unknown)
          return ts != timestamp_nonexistent;

        // Otherwise, if this is not a path_target, then we don't match.
        //
        path_target* pt (mt.is_a<path_target> ());
        if (pt == nullptr)
          return false;

        const path* p (&pt->path ());

        // Assign the path.
        //
        if (p->empty ())
        {
          // Since we cannot come up with an extension, ask the target's
          // derivation function to treat this as a prerequisite (just like in
          // search_existing_file()).
          //
          if (pt->derive_extension (true) == nullptr)
          {
            l4 ([&]{trace << "no default extension for target " << *pt;});
            return false;
          }

          p = &pt->derive_path ();
        }

        ts = mtime (*p);
        pt->mtime (ts);

        if (ts != timestamp_nonexistent)
          return true;

        l4 ([&]{trace << "no existing file for target " << *pt;});
        return false;
      }
    }
  }

  recipe file_rule::
  apply (action a, target& t) const
  {
    // Update triggers the update of this target's prerequisites so it would
    // seem natural that we should also trigger their cleanup. However, this
    // possibility is rather theoretical so until we see a real use-case for
    // this functionality, we simply ignore the clean operation.
    //
    if (a.operation () == clean_id)
      return noop_recipe;

    // If we have no prerequisites, then this means this file is up to date.
    // Return noop_recipe which will also cause the target's state to be set
    // to unchanged. This is an important optimization on which quite a few
    // places that deal with predominantly static content rely.
    //
    if (!t.has_group_prerequisites ()) // Group as in match_prerequisites().
      return noop_recipe;

    // Match all the prerequisites.
    //
    match_prerequisites (a, t);

    // Note that we used to provide perform_update() which checked that this
    // target is not older than any of its prerequisites. However, later we
    // realized this is probably wrong: consider a script with a testscript as
    // a prerequisite; chances are the testscript will be newer than the
    // script and there is nothing wrong with that.
    //
    return default_recipe;
  }

  const file_rule file_rule::instance;

  // alias_rule
  //
  bool alias_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  recipe alias_rule::
  apply (action a, target& t) const
  {
    // Inject dependency on our directory (note: not parent) so that it is
    // automatically created on update and removed on clean.
    //
    inject_fsdir (a, t, false);

    match_prerequisites (a, t);
    return default_recipe;
  }

  const alias_rule alias_rule::instance;

  // fsdir_rule
  //
  bool fsdir_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  recipe fsdir_rule::
  apply (action a, target& t) const
  {
    // Inject dependency on the parent directory. Note that it must be first
    // (see perform_update_direct()).
    //
    inject_fsdir (a, t);

    match_prerequisites (a, t);

    switch (a)
    {
    case perform_update_id: return &perform_update;
    case perform_clean_id: return &perform_clean;
    default: assert (false); return default_recipe;
    }
  }

  static bool
  fsdir_mkdir (const target& t, const dir_path& d)
  {
    // Even with the exists() check below this can still be racy so only print
    // things if we actually did create it (similar to build2::mkdir()).
    //
    auto print = [&t, &d] ()
    {
      if (verb >= 2)
        text << "mkdir " << d;
      else if (verb && t.ctx.current_diag_noise)
        text << "mkdir " << t;
    };

    // Note: ignoring the dry_run flag.
    //
    mkdir_status ms;

    try
    {
      ms = try_mkdir (d);
    }
    catch (const system_error& e)
    {
      print ();
      fail << "unable to create directory " << d << ": " << e << endf;
    }

    if (ms == mkdir_status::success)
    {
      print ();
      return true;
    }

    return false;
  }

  target_state fsdir_rule::
  perform_update (action a, const target& t)
  {
    target_state ts (target_state::unchanged);

    // First update prerequisites (e.g. create parent directories) then create
    // this directory.
    //
    // @@ outer: should we assume for simplicity its only prereqs are fsdir{}?
    //
    if (!t.prerequisite_targets[a].empty ())
      ts = straight_execute_prerequisites (a, t);

    // The same code as in perform_update_direct() below.
    //
    const dir_path& d (t.dir); // Everything is in t.dir.

    // Generally, it is probably correct to assume that in the majority of
    // cases the directory will already exist. If so, then we are going to get
    // better performance by first checking if it indeed exists. See
    // butl::try_mkdir() for details.
    //
    // @@ Also skip prerequisites? Can't we return noop in apply?
    //
    if (!exists (d) && fsdir_mkdir (t, d))
      ts |= target_state::changed;

    return ts;
  }

  void fsdir_rule::
  perform_update_direct (action a, const target& t)
  {
    // First create the parent directory. If present, it is always first.
    //
    const target* p (t.prerequisite_targets[a].empty ()
                     ? nullptr
                     : t.prerequisite_targets[a][0]);

    if (p != nullptr && p->is_a<fsdir> ())
      perform_update_direct (a, *p);

    // The same code as in perform_update() above.
    //
    const dir_path& d (t.dir);

    if (!exists (d))
      fsdir_mkdir (t, d);
  }

  target_state fsdir_rule::
  perform_clean (action a, const target& t)
  {
    // The reverse order of update: first delete this directory, then clean
    // prerequisites (e.g., delete parent directories).
    //
    // Don't fail if we couldn't remove the directory because it is not empty
    // (or is current working directory). In this case rmdir() will issue a
    // warning when appropriate.
    //
    target_state ts (rmdir (t.dir, t, t.ctx.current_diag_noise ? 1 : 2)
                     ? target_state::changed
                     : target_state::unchanged);

    if (!t.prerequisite_targets[a].empty ())
      ts |= reverse_execute_prerequisites (a, t);

    return ts;
  }

  const fsdir_rule fsdir_rule::instance;

  // noop_rule
  //
  bool noop_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  recipe noop_rule::
  apply (action, target&) const
  {
    return noop_recipe;
  }

  const noop_rule noop_rule::instance;

  // adhoc_rule
  //
  const dir_path adhoc_rule::recipes_build_dir ("recipes.out");

  bool adhoc_rule::
  match (action a, target& t, const string& h, optional<action> fallback) const
  {
    return !fallback && match (a, t, h);
  }

  bool adhoc_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  // Scope operation callback that cleans up recipe builds.
  //
  target_state adhoc_rule::
  clean_recipes_build (action, const scope& rs, const dir&)
  {
    context& ctx (rs.ctx);

    const dir_path& out_root (rs.out_path ());

    dir_path d (out_root / rs.root_extra->build_dir / recipes_build_dir);

    if (exists (d))
    {
      if (rmdir_r (ctx, d))
      {
        // Clean up build/ if it also became empty (e.g., in case of a build
        // with a transient configuration).
        //
        d = out_root / rs.root_extra->build_dir;
        if (empty (d))
          rmdir (ctx, d);

        return target_state::changed;
      }
    }

    return target_state::unchanged;
  }

  // adhoc_script_rule
  //
  void adhoc_script_rule::
  dump (ostream& os, const string& ind) const
  {
    // @@ TODO: indentation is multi-line recipes is off (would need to insert
    //          indentation after every newline). Maybe if we pre-parse them?
    //

    // Do we need the header?
    //
    if (diag)
    {
      os << ind << '%';

      if (diag)
      {
        os << " [";
        os << "diag="; to_stream (os, name (*diag), true /* quote */, '@');
        os << ']';
      }

      os << endl;
    }

    os << ind << string (braces, '{') << endl
       << ind << code
       << ind << string (braces, '}');
  }

  bool adhoc_script_rule::
  match (action a, target& t, const string&, optional<action> fb) const
  {
    if (!fb)
      ;
    // If this is clean for a file target and we are supplying the update,
    // then we will also supply the standard clean.
    //
    else if (a   == perform_clean_id  &&
             *fb == perform_update_id &&
             t.is_a<file> ())
      ;
    else
      return false;

    // It's unfortunate we have to resort to this but we need to remember this
    // in apply().
    //
    t.data (fb.has_value ());

    return true;
  }

  recipe adhoc_script_rule::
  apply (action a, target& t) const
  {
    // Derive file names for the target and its ad hoc group members, if any.
    //
    for (target* m (&t); m != nullptr; m = m->adhoc_member)
    {
      if (auto* p = m->is_a<path_target> ())
        p->derive_path ();
    }

    // Inject dependency on the output directory.
    //
    // We do it always instead of only if one of the targets is path-based in
    // case the recipe creates temporary files or some such.
    //
    inject_fsdir (a, t);

    // Match prerequisites.
    //
    match_prerequisite_members (a, t);

    // See if we are providing the standard clean as a fallback.
    //
    if (t.data<bool> ())
      return &perform_clean_depdb;

    // For update inject dependency on the tool target(s).
    //
    // @@ We could see that it's a target and do it but not sure if we should
    //    bother. We dropped this idea of implicit targets in tests. Maybe we
    //    should verify path assigned, like we do there? I think we will have
    //    to.
    //
    // if (a == perform_update_id)
    //  inject (a, t, tgt);

    if (a == perform_update_id && t.is_a<file> ())
    {
      return [this] (action a, const target& t)
      {
        return perform_update_file (a, t);
      };
    }
    else
    {
      return [this] (action a, const target& t)
      {
        return default_action (a, t);
      };
    }
  }

  target_state adhoc_script_rule::
  perform_update_file (action a, const target& xt) const
  {
    tracer trace ("adhoc_rule::perform_update_file");

    const file& t (xt.as<file> ());
    const path& tp (t.path ());

    // Update prerequisites and determine if any of them render this target
    // out-of-date.
    //
    timestamp mt (t.load_mtime ());
    optional<target_state> ps (execute_prerequisites (a, t, mt));

    bool update (!ps);

    // We use depdb to track changes to the script itself, input file names,
    // tools, etc.
    //
    depdb dd (tp + ".d");
    {
      // First should come the rule name/version.
      //
      if (dd.expect ("adhoc 1") != nullptr)
        l4 ([&]{trace << "rule mismatch forcing update of " << t;});

      // Then the tool checksums.
      //
      // @@ TODO: obtain checksums of all the targets used as commands in
      //          the script.
      //
      //if (dd.expect (csum) != nullptr)
      //  l4 ([&]{trace << "compiler mismatch forcing update of " << t;});

      // Then the script checksum.
      //
      // @@ TODO: for now we hash the unexpanded text but it should be
      //          expanded. This will take care of all the relevant input
      //          file name changes as well as any other variables the
      //          script may reference.
      //
      //          It feels like we need a special execute mode that instead
      //          of executing hashes the commands.
      //
      if (dd.expect (sha256 (code).string ()) != nullptr)
        l4 ([&]{trace << "recipe change forcing update of " << t;});
    }

    // Update if depdb mismatch.
    //
    if (dd.writing () || dd.mtime > mt)
      update = true;

    dd.close ();

    // If nothing changed, then we are done.
    //
    if (!update)
      return *ps;

    if (verb >= 2)
    {
      //@@ TODO

      //print_process (args);

      text << trim (string (code));
    }
    else if (verb)
    {
      // @@ TODO:
      //
      // - derive diag if absent (should probably do in match?)
      //
      // - we are printing target, not source (like in most other places)
      //
      // - printing of ad hoc target group (the {hxx cxx}{foo} idea)
      //
      // - if we are printing prerequisites, should we print all of them
      //   (including tools)?
      //

      text << (diag ? diag->c_str () : "adhoc") << ' ' << t;
    }

    if (!t.ctx.dry_run)
    {
      // @@ TODO
      //
      touch (t.ctx, tp, true, verb_never);
      dd.check_mtime (tp);
    }

    t.mtime (system_clock::now ());
    return target_state::changed;
  }

  target_state adhoc_script_rule::
  default_action (action a, const target& t) const
  {
    tracer trace ("adhoc_rule::default_action");

    execute_prerequisites (a, t);

    if (verb >= 2)
    {
      //@@ TODO

      //print_process (args);

      text << trim (string (code));
    }
    else if (verb)
    {
      // @@ TODO: as above

      text << (diag ? diag->c_str () : "adhoc") << ' ' << t;
    }

    if (!t.ctx.dry_run)
    {
      // @@ TODO
      //
    }

    return target_state::changed;
  }

  // cxx_rule
  //
  bool cxx_rule::
  match (action, target&, const string&) const
  {
    return true;
  }

  // adhoc_cxx_rule
  //
  adhoc_cxx_rule::
  ~adhoc_cxx_rule ()
  {
    delete impl.load (memory_order_relaxed); // Serial execution.
  }

  void adhoc_cxx_rule::
  dump (ostream& os, const string& ind) const
  {
    // @@ TODO: indentation is multi-line recipes is off (would need to insert
    //          indentation after every newline).
    //
    os << ind << string (braces, '{') << " c++" << endl
       << ind << code
       << ind << string (braces, '}');
  }

  // From module.cxx.
  //
  void
  create_module_context (context&, const location&);

  const target&
  update_in_module_context (context&, const scope&, names tgt,
                            const location&, const path& bf);

  pair<void*, void*>
  load_module_library (const path& lib, const string& sym, string& err);

  bool adhoc_cxx_rule::
  match (action a, target& t, const string& hint) const
  {
    tracer trace ("adhoc_cxx_rule::match");

    context& ctx (t.ctx);
    const scope& rs (t.root_scope ());

    // The plan is to reduce this to the build system module case as much as
    // possible. Specifically, we switch to the load phase, create a module-
    // like library with the recipe text as a rule implementation, then build
    // and load it.
    //
    // Since the recipe can be shared among multiple targets, several threads
    // can all be trying to do this in parallel.

    // The only way to guarantee that the name of our module matches its
    // implementation is to based the name on the implementation hash.
    //
    // Unfortunately, this means we will be creating a new project (and
    // leaving behind the old one as garbage) for every change to the
    // recipe. On the other hand, if the recipe is moved around unchanged, we
    // will reuse the same project. In fact, two different recipes (e.g., in
    // different buildfiles) with the same text will share the project.
    //
    // @@ Shouldn't we also include buildfile path and line seeing that we
    //    add them as #line? Or can we do something clever for this case
    //    (i.e., if update is successful, then this information is no longer
    //    necessary, unless update is caused by something external, like
    //    change of compiler). Also location in comment. Why not just
    //    overwrite the source file every time we compile it, to KISS?
    //
    string id (sha256 (code).abbreviated_string (12));

    // We use the relaxed memory order because any change must go through the
    // serial load phase. In other words, all we need here is atomicity with
    // ordering/visibility provided by the phase mutex.
    //
    cxx_rule* impl (this->impl.load (memory_order_relaxed));

    while (impl == nullptr) // Breakout loop.
    {
      // Switch the phase to (serial) load and re-check.
      //
      phase_switch ps (ctx, run_phase::load);

      if ((impl = this->impl.load (memory_order_relaxed)) != nullptr)
        break;

      using create_function = cxx_rule* (const location&);
      using load_function = create_function* ();

      dir_path pd (rs.out_path () /
                   rs.root_extra->build_dir /
                   recipes_build_dir /= id);

      string sym ("load_" + id);

      // Update the project is necessary.
      //
      path of;
      try
      {
        const uint16_t verbosity (3);

        ofdstream ofs;

        optional<bool> altn (false); // Standard naming scheme.
        if (!is_src_root (pd, altn))
        {

          // Write ad hoc config.build that loads the ~build2 configuration.
          // This way the configuration will be always in sync with ~build2
          // and we can update the recipe manually (e.g., for debugging).
          //
          create_project (
            pd,
            dir_path (),                             /* amalgamation */
            {},                                      /* boot_modules */
            "cxx.std = latest",                      /* root_pre */
            {"cxx."},                                /* root_modules */
            "",                                      /* root_post */
            string ("config"),                       /* config_module */
            string ("config.config.load = ~build2"), /* config_file */
            false,                                   /* buildfile */
            "build2 core",                           /* who */
            verbosity);                              /* verbosity */


          // Write the rule source file.
          //
          of = path (pd / "rule.cxx");

          if (verb >= verbosity)
            text << (verb >= 2 ? "cat >" : "save ") << of;

          ofs.open (of);

          ofs << "#include \"location.hxx\""                            << '\n'
              << '\n';

          // Include every header that can plausibly be needed by a rule.
          //
          ofs << "#include <libbuild2/types.hxx>"                       << '\n'
              << "#include <libbuild2/forward.hxx>"                     << '\n'
              << "#include <libbuild2/utility.hxx>"                     << '\n'
              << '\n'
              << "#include <libbuild2/file.hxx>"                        << '\n'
              << "#include <libbuild2/rule.hxx>"                        << '\n'
              << "#include <libbuild2/depdb.hxx>"                       << '\n'
              << "#include <libbuild2/scope.hxx>"                       << '\n'
              << "#include <libbuild2/target.hxx>"                      << '\n'
              << "#include <libbuild2/context.hxx>"                     << '\n'
              << "#include <libbuild2/variable.hxx>"                    << '\n'
              << "#include <libbuild2/algorithm.hxx>"                   << '\n'
              << "#include <libbuild2/filesystem.hxx>"                  << '\n'
              << "#include <libbuild2/diagnostics.hxx>"                 << '\n'
              << '\n';

          // Normally the recipe code will have one level of indentation so
          // let's not indent the namespace level to match.
          //
          ofs << "namespace build2"                                     << '\n'
              << "{"                                                    << '\n'
              << '\n';

          // If we want the user to be able to supply a custom constuctor,
          // then we have to give the class a predictable name (i.e., we
          // cannot use id as part of its name) and put it into an unnamed
          // namespace. One clever idea is to call the class `constructor` but
          // the name could also be used for a custom destructor (still could
          // work) or for name qualification (would definitely look bizarre).
          //
          // In this light the most natural name is probable `rule`. The issue
          // is we already have this name in the build2 namespace (and its our
          // indirect base). In fact, any name that we choose could in the
          // future conflict with something in that namespace so maybe it
          // makes sense to bite the bullet and pick a name that is least
          // likely to be used by the user directly (can always use cxx_rule
          // instead).
          //
          ofs << "namespace"                                            << '\n'
              << "{"                                                    << '\n'
              << "class rule: public cxx_rule"                          << '\n'
              << "{"                                                    << '\n'
              << "public:"                                              << '\n'
              << '\n';

          // Inherit base constructor. This way the user may provide their own
          // but don't have to.
          //
          ofs << "  using cxx_rule::cxx_rule;"                          << '\n'
              << '\n';

          // An extern "C" function cannot throw which can happen in case of a
          // user-defined constructor. So we need an extra level of
          // indirection. We incorporate id to make sure it doesn't conflict
          // with anything user-defined.
          //
          ofs << "  static cxx_rule*"                                   << '\n'
              << "  create_" << id << " (const location& l)"            << '\n'
              << "  {"                                                  << '\n'
              << "    return new rule (l);"                             << '\n'
              << "  }"                                                  << '\n'
              << '\n';

          // Use the #line directive to point diagnostics to the code in the
          // buildfile. Note that there is no easy way to restore things to
          // point back to the source file (other than another #line with a
          // line and a file). Seeing that we don't have much after, let's not
          // bother for now.
          //
          ofs << "#line RECIPE_LINE RECIPE_FILE"                        << '\n';

          // Note that the code always includes trailing newline.
          //
          ofs << code
              << "};"                                                   << '\n'
              << '\n';

          // Add an alias that we can use unambiguously in the load function.
          //
          ofs << "using rule_" << id << " = rule;"                      << '\n'
              << "}"                                                    << '\n'
              << '\n';

          // Entry point.
          //
          ofs << "extern \"C\""                                         << '\n'
              << "#ifdef _WIN32"                                        << '\n'
              << "__declspec(dllexport)"                                << '\n'
              << "#endif"                                               << '\n'
              << "cxx_rule* (*" << sym << " ()) (const location&)"      << '\n'
              << "{"                                                    << '\n'
              << "  return &rule_" << id << "::create_" << id << ";"    << '\n'
              << "}"                                                    << '\n'
              << '\n';

          ofs << "}"                                                    << '\n';

          ofs.close ();


          // Write buildfile.
          //
          of = path (pd / std_buildfile_file);

          if (verb >= verbosity)
            text << (verb >= 2 ? "cat >" : "save ") << of;

          ofs.open (of);

          ofs << "import imp_libs += build2%lib{build2}"                << '\n'
              << "libs{" << id << "}: cxx{rule} hxx{location} $imp_libs"<< '\n';

          ofs.close ();
        }

        // Create/update the recipe location header.
        //
        // For update, preserve the file timestamp in order not to render the
        // recipe out of date.
        //
        of = path (pd / "location.hxx");

        entry_time et (file_time (of));

        if (verb >= verbosity)
          text << (verb >= 2 ? "cat >" : "save ") << of;

        ofs.open (of);

        // Recipe file and line for the #line directive above. Note that the
        // code starts from the next line thus +1. We also need to escape
        // backslashes (Windows paths).
        //
        {
          const string& f (!loc.file.path.empty ()
                           ? loc.file.path.string ()
                           : loc.file.name ? *loc.file.name : string ());

          ofs << "#define RECIPE_FILE \"" << sanitize_strlit (f) << '"' << '\n'
              << "#define RECIPE_LINE "   << loc.line + 1               << '\n';
        }

        ofs.close ();

        if (et.modification != timestamp_nonexistent)
          file_time (of, et);
      }
      catch (const io_error& e)
      {
        fail << "unable to write to " << of << ": " << e;
      }
      catch (const system_error& e)
      {
        fail << "unable to get/set timestamp for " << of << ": " << e;
      }

      // Update the library target in the module context.
      //
      const target* l (nullptr);
      do // Breakout loop.
      {
        bool nested (ctx.module_context == &ctx);

        // Create the build context if necessary.
        //
        if (ctx.module_context == nullptr)
        {
          if (!ctx.module_context_storage)
            fail (loc) << "unable to update ad hoc recipe for target " << t <<
              info << "building of ad hoc recipes is disabled";

          create_module_context (ctx, loc);
        }

        // "Switch" to the module context.
        //
        context& ctx (*t.ctx.module_context);

        // Load the project in the module context.
        //
        // Note that it's possible it has already been loaded (see above about
        // the id calculation).
        //
        path bf (pd / std_buildfile_file);
        scope& rs (load_project (ctx, pd, pd, false /* forwarded */));

        auto find_target = [&ctx, &rs, &pd, &id] ()
        {
          const target_type* tt (rs.find_target_type ("libs"));
          assert (tt != nullptr);

          const target* t (
            ctx.targets.find (*tt, pd, dir_path () /* out */, id));
          assert (t != nullptr);

          return t;
        };

        // If the project has already been loaded then, as an optimization,
        // check if the target has already been updated (this will make a
        // difference we if we have identical recipes in several buildfiles).
        //
        if (!source_once (rs, rs, bf))
        {
          l = find_target ();

          if (l->executed_state (perform_update_id) != target_state::unknown)
            break;
        }

        if (nested)
        {
          // This means there is a perform update action already in progress
          // in this context. So we are going to switch the phase and
          // perform direct match and update (similar how we do this for
          // generated headers).
          //
          // Note that since neither match nor execute are serial phases, it
          // means other targets in this context can be matched and executed
          // in paralellel with us.
          //
          if (l == nullptr)
            l = find_target ();

          phase_switch mp (ctx, run_phase::match);
          if (build2::match (perform_update_id, *l) != target_state::unchanged)
          {
            phase_switch ep (ctx, run_phase::execute);
            execute (a, *l);
          }
        }
        else
        {
          // Cutoff the existing diagnostics stack and push our own entry.
          //
          diag_frame::stack_guard diag_cutoff (nullptr);

          auto df = make_diag_frame (
            [this, &t] (const diag_record& dr)
            {
              dr << info (loc) << "while updating ad hoc recipe for target "
                 << t;
            });

          l = &update_in_module_context (
            ctx, rs, names {name (pd, "libs", id)},
            loc, bf);
        }
      } while (false);

      // Load the library.
      //
      const path& lib (l->as<file> ().path ());

      // Note again that it's possible the library has already been loaded
      // (see above about the id calculation).
      //
      string err;
      pair<void*, void*> hs (load_module_library (lib, sym, err));

      // These normally shouldn't happen unless something is seriously broken.
      //
      if (hs.first == nullptr)
        fail (loc) << "unable to load recipe library " << lib << ": " << err;

      if (hs.second == nullptr)
        fail (loc) << "unable to lookup " << sym << " in recipe library "
                   << lib << ": " << err;

      {
        auto df = make_diag_frame (
          [this](const diag_record& dr)
          {
            if (verb != 0)
              dr << info (loc) << "while initializing ad hoc recipe";
          });

        load_function* lf (function_cast<load_function*> (hs.second));
        create_function* cf (lf ());

        impl = cf (loc);
        this->impl.store (impl, memory_order_relaxed); // Still in load phase.
      }
    }

    return impl->match (a, t, hint);
  }

  recipe adhoc_cxx_rule::
  apply (action a, target& t) const
  {
    return impl.load (memory_order_relaxed)->apply (a, t);
  }
}