// file      : build2/cc/compile.cxx -*- C++ -*-
// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

#include <build2/cc/compile>

#include <cstdlib>  // exit()
#include <iostream> // cerr

#include <build2/depdb>
#include <build2/scope>
#include <build2/context>
#include <build2/variable>
#include <build2/algorithm>
#include <build2/diagnostics>

#include <build2/bin/target>

#include <build2/cc/target>  // h
#include <build2/cc/utility>

using namespace std;
using namespace butl;

namespace build2
{
  namespace cc
  {
    using namespace bin;

    compile::
    compile (data&& d)
        : common (move (d)),
          rule_id (string (x) += ".compile 1")
    {
    }

    struct match_data
    {
      prerequisite_member src;
      timestamp dd_mtime; // depdb mtime, timestamp_nonexistent if outdated.
    };

    static_assert (sizeof (match_data) <= target::data_size,
                   "insufficient space");

    match_result compile::
    match (action act, target& t, const string&) const
    {
      tracer trace (x, "compile::match");

      // @@ TODO:
      //
      // - check prerequisites: single source file
      // - if path already assigned, verify extension?
      //

      // Link-up to our group (this is the obj{} target group protocol which
      // means this can be done whether we match or not).
      //
      if (t.group == nullptr)
        t.group = targets.find<obj> (t.dir, t.out, t.name);

      // See if we have a source file. Iterate in reverse so that a source
      // file specified for an obj*{} member overrides the one specified for
      // the group. Also "see through" groups.
      //
      for (prerequisite_member p: reverse_group_prerequisite_members (act, t))
      {
        if (p.is_a (x_src))
        {
          // Save in the target's auxilary storage.
          //
          t.data (match_data {p, timestamp_nonexistent});
          return true;
        }
      }

      l4 ([&]{trace << "no " << x_lang << " source file for target " << t;});
      return false;
    }

    // Append or hash library options from a pair of *.export.* variables
    // (first one is cc.export.*) recursively, prerequisite libraries first.
    //
    void compile::
    append_lib_options (const scope& bs,
                        cstrings& args,
                        const target& t,
                        action act,
                        lorder lo) const
    {
      auto opt = [&args, this] (
        const file& l, const string& t, bool com, bool exp)
      {
        // Note that in our model *.export.poptions are always "interface",
        // even if set on liba{}/libs{}, unlike loptions.
        //
        assert (exp);

        const variable& var (
          com
          ? c_export_poptions
          : (t == x ? x_export_poptions : var_pool[t + ".export.poptions"]));

        append_options (args, l, var);
      };

      // In case we don't have the "small function object" optimization.
      //
      const function<void (const file&, const string&, bool, bool)> optf (opt);

      // Note that here we don't need to see group members (see apply()).
      //
      for (const prerequisite& p: group_prerequisites (t))
      {
        // Should be already searched and matched.
        //
        const target* pt (p.target.load (memory_order_consume));

        bool a;

        if (const lib* l = pt->is_a<lib> ())
          a = (pt = &link_member (*l, act, lo))->is_a<liba> ();
        else if (!(a = pt->is_a<liba> ()) && !pt->is_a<libs> ())
          continue;

        process_libraries (act, bs, lo, sys_lib_dirs,
                           pt->as<file> (), a,
                           nullptr, nullptr, optf);
      }
    }

    void compile::
    hash_lib_options (const scope& bs,
                      sha256& cs,
                      const target& t,
                      action act,
                      lorder lo) const
    {
      auto opt = [&cs, this] (
        const file& l, const string& t, bool com, bool exp)
      {
        assert (exp);

        const variable& var (
          com
          ? c_export_poptions
          : (t == x ? x_export_poptions : var_pool[t + ".export.poptions"]));

        hash_options (cs, l, var);
      };

      // In case we don't have the "small function object" optimization.
      //
      const function<void (const file&, const string&, bool, bool)> optf (opt);

      for (const prerequisite& p: group_prerequisites (t))
      {
        // Should be already searched and matched.
        //
        const target* pt (p.target.load (memory_order_consume));

        bool a;

        if (const lib* l = pt->is_a<lib> ())
          a = (pt = &link_member (*l, act, lo))->is_a<liba> ();
        else if (!(a = pt->is_a<liba> ()) && !pt->is_a<libs> ())
          continue;

        process_libraries (act, bs, lo, sys_lib_dirs,
                           pt->as<file> (), a,
                           nullptr, nullptr, optf);
      }
    }

    // Append library prefixes based on the *.export.poptions variables
    // recursively, prerequisite libraries first.
    //
    void compile::
    append_lib_prefixes (const scope& bs,
                         prefix_map& m,
                         target& t,
                         action act,
                         lorder lo) const
    {
      auto opt = [&m, this] (
        const file& l, const string& t, bool com, bool exp)
      {
        assert (exp);

        const variable& var (
          com
          ? c_export_poptions
          : (t == x ? x_export_poptions : var_pool[t + ".export.poptions"]));

        append_prefixes (m, l, var);
      };

      // In case we don't have the "small function object" optimization.
      //
      const function<void (const file&, const string&, bool, bool)> optf (opt);

      for (const prerequisite& p: group_prerequisites (t))
      {
        // Should be already searched and matched.
        //
        const target* pt (p.target.load (memory_order_consume));

        bool a;

        if (const lib* l = pt->is_a<lib> ())
          a = (pt = &link_member (*l, act, lo))->is_a<liba> ();
        else if (!(a = pt->is_a<liba> ()) && !pt->is_a<libs> ())
          continue;

        process_libraries (act, bs, lo, sys_lib_dirs,
                           pt->as<file> (), a,
                           nullptr, nullptr, optf);
      }
    }

    recipe compile::
    apply (action act, target& xt) const
    {
      tracer trace (x, "compile::apply");

      file& t (xt.as<file> ());
      match_data& md (t.data<match_data> ());

      const scope& bs (t.base_scope ());
      const scope& rs (*bs.root_scope ());

      otype ct (compile_type (t));
      lorder lo (link_order (bs, ct));

      // Derive file name from target name.
      //
      const char* e (nullptr);

      if (tsys == "win32-msvc")
      {
        switch (ct)
        {
        case otype::e: e = "exe.obj"; break;
        case otype::a: e = "lib.obj"; break;
        case otype::s: e = "dll.obj"; break;
        }
      }
      else if (tsys == "mingw32")
      {
        switch (ct)
        {
        case otype::e: e = "exe.o"; break;
        case otype::a: e = "a.o";   break;
        case otype::s: e = "dll.o"; break;
        }
      }
      else if (tsys == "darwin")
      {
        switch (ct)
        {
        case otype::e: e = "o";       break;
        case otype::a: e = "a.o";     break;
        case otype::s: e = "dylib.o"; break;
        }
      }
      else
      {
        switch (ct)
        {
        case otype::e: e = "o"; break;
        case otype::a: e = "a.o"; break;
        case otype::s: e = "so.o"; break;
        }
      }

      const path& tp (t.derive_path (e));

      // Inject dependency on the output directory.
      //
      const fsdir* dir (inject_fsdir (act, t));

      // Match all the existing prerequisites. The injection code takes care
      // of the ones it is adding.
      //
      // When cleaning, ignore prerequisites that are not in the same or a
      // subdirectory of our project root.
      //
      auto& pts (t.prerequisite_targets);
      optional<dir_paths> usr_lib_dirs; // Extract lazily.

      // Start asynchronous matching of prerequisites. Wait with unlocked
      // phase to allow phase switching.
      //
      wait_guard wg (target::count_busy (), t.task_count, true);

      size_t start (pts.size ()); // Index of the first to be added.
      for (prerequisite_member p: group_prerequisite_members (act, t))
      {
        const target* pt (nullptr);

        // A dependency on a library is there so that we can get its
        // *.export.poptions. This is the "library meta-information
        // protocol". See also append_lib_options().
        //
        if (p.is_a<lib> () || p.is_a<liba> () || p.is_a<libs> ())
        {
          if (act.operation () == update_id)
          {
            // Handle imported libraries. We know that for such libraries we
            // don't need to do match() in order to get options (if any, they
            // would be set by search_library()).
            //
            if (p.proj ())
            {
              if (search_library (act,
                                  sys_lib_dirs,
                                  usr_lib_dirs,
                                  p.prerequisite) != nullptr)
                continue;
            }

            pt = &p.search (t);

            if (const lib* l = pt->is_a<lib> ())
              pt = &link_member (*l, act, lo);
          }

          continue;
        }
        else
        {
          pt = &p.search (t);

          if (act.operation () == clean_id && !pt->dir.sub (rs.out_path ()))
            continue;
        }

        match_async (act, *pt, target::count_busy (), t.task_count);
        pts.push_back (pt);
      }

      wg.wait ();

      // Finish matching all the targets that we have started.
      //
      for (size_t i (start), n (pts.size ()); i != n; ++i)
      {
        const target*& pt (pts[i]);

        // Making sure a library is updated before us will only restrict
        // parallelism. But we do need to match it in order to get its imports
        // resolved and prerequisite_targets populated. So we match it but
        // then unmatch if it is safe. And thanks to the two-pass prerequisite
        // match in link::apply() it will be safe unless someone is building
        // an obj?{} target directory.
        //
        if (build2::match (act,
                           *pt,
                           pt->is_a<liba> () || pt->is_a<libs> ()
                           ? unmatch::safe
                           : unmatch::none))
          pt = nullptr; // Ignore in execute.

      }

      // Inject additional prerequisites. We only do it when performing update
      // since chances are we will have to update some of our prerequisites in
      // the process (auto-generated source code).
      //
      if (act == perform_update_id)
      {
        // The cached prerequisite target should be the same as what is in
        // t.prerequisite_targets since we used standard search() and match()
        // above.
        //
        const file& src (*md.src.search (t).is_a<file> ());

        // Make sure the output directory exists.
        //
        // Is this the right thing to do? It does smell a bit, but then we do
        // worse things in inject_prerequisites() below. There is also no way
        // to postpone this until update since we need to extract and inject
        // header dependencies now (we don't want to be calling search() and
        // match() in update), which means we need to cache them now as well.
        // So the only alternative, it seems, is to cache the updates to the
        // database until later which will sure complicate (and slow down)
        // things.
        //
        if (dir != nullptr)
        {
          // We can do it properly by using execute_direct(). But this means
          // we will be switching to the execute phase with all the associated
          // overheads. At the same time, in case of update, creation of a
          // directory is not going to change the external state in any way
          // that would affect any parallel efforts in building the internal
          // state. So we are just going to create the directory directly.
          // Note, however, that we cannot modify the fsdir{} target since
          // this can very well be happening in parallel. But that's not a
          // problem since fsdir{}'s update is idempotent.
          //
          fsdir_rule::perform_update_direct (act, t);
        }

        depdb dd (tp + ".d");

        // First should come the rule name/version.
        //
        if (dd.expect (rule_id) != nullptr)
          l4 ([&]{trace << "rule mismatch forcing update of " << t;});

        // Then the compiler checksum. Note that here we assume it
        // incorporates the (default) target so that if the compiler changes
        // but only in what it targets, then the checksum will still change.
        //
        if (dd.expect (cast<string> (rs[x_checksum])) != nullptr)
          l4 ([&]{trace << "compiler mismatch forcing update of " << t;});

        // Then the options checksum.
        //
        // The idea is to keep them exactly as they are passed to the compiler
        // since the order may be significant.
        //
        sha256 cs;

        hash_options (cs, t, c_poptions);
        hash_options (cs, t, x_poptions);

        // Hash *.export.poptions from prerequisite libraries.
        //
        hash_lib_options (bs, cs, t, act, lo);

        // Extra system header dirs (last).
        //
        for (const dir_path& d: sys_inc_dirs)
          cs.append (d.string ());

        hash_options (cs, t, c_coptions);
        hash_options (cs, t, x_coptions);
        hash_std (cs);

        if (ct == otype::s)
        {
          // On Darwin, Win32 -fPIC is the default.
          //
          if (tclass == "linux" || tclass == "bsd")
            cs.append ("-fPIC");
        }

        if (dd.expect (cs.string ()) != nullptr)
          l4 ([&]{trace << "options mismatch forcing update of " << t;});

        // Finally the source file.
        //
        if (dd.expect (src.path ()) != nullptr)
          l4 ([&]{trace << "source file mismatch forcing update of " << t;});

        // If any of the above checks resulted in a mismatch (different
        // compiler, options, or source file), or if the database is newer
        // than the target (interrupted update) then force the target update.
        //
        md.dd_mtime = dd.writing () ? timestamp_nonexistent : dd.mtime ();

        inject (act, t, lo, src, dd);
        dd.close ();
      }

      switch (act)
      {
      case perform_update_id: return [this] (action a, const target& t)
        {
          return perform_update (a, t);
        };
      case perform_clean_id: return [this] (action a, const target& t)
        {
          return perform_clean (a, t);
        };
      default: return noop_recipe; // Configure update.
      }
    }

    // Reverse-lookup target type from extension.
    //
    const target_type* compile::
    map_extension (const scope& s, const string& n, const string& e) const
    {
      // We will just have to try all of the possible ones, in the "most
      // likely to match" order.
      //
      auto test = [&s, &n, &e] (const target_type& tt) -> bool
      {
        // Call the extension derivation function. Here we know that it will
        // only use the target type and name from the target key so we can
        // pass bogus values for the rest.
        //
        target_key tk {&tt, nullptr, nullptr, &n, nullopt};

        // This is like prerequisite search.
        //
        if (optional<string> de = tt.extension (tk, s, true))
          if (*de == e)
            return true;

        return false;
      };

      for (const target_type* const* p (x_inc); *p != nullptr; ++p)
        if (test (**p)) return *p;

      return nullptr;
    }

    void compile::
    append_prefixes (prefix_map& m, const target& t, const variable& var) const
    {
      tracer trace (x, "compile::append_prefixes");

      // If this target does not belong to any project (e.g, an "imported as
      // installed" library), then it can't possibly generate any headers for
      // us.
      //
      const scope* rs (t.base_scope ().root_scope ());
      if (rs == nullptr)
        return;

      const dir_path& out_base (t.dir);
      const dir_path& out_root (rs->out_path ());

      if (auto l = t[var])
      {
        const auto& v (cast<strings> (l));

        for (auto i (v.begin ()), e (v.end ()); i != e; ++i)
        {
          // -I can either be in the "-Ifoo" or "-I foo" form. For VC it can
          // also be /I.
          //
          const string& o (*i);

          if (o.size () < 2 || (o[0] != '-' && o[0] != '/') || o[1] != 'I')
            continue;

          dir_path d;
          if (o.size () == 2)
          {
            if (++i == e)
              break; // Let the compiler complain.

            d = dir_path (*i);
          }
          else
            d = dir_path (*i, 2, string::npos);

          l6 ([&]{trace << "-I '" << d << "'";});

          // If we are relative or not inside our project root, then ignore.
          //
          if (d.relative () || !d.sub (out_root))
            continue;

          // If the target directory is a sub-directory of the include
          // directory, then the prefix is the difference between the
          // two. Otherwise, leave it empty.
          //
          // The idea here is to make this "canonical" setup work auto-
          // magically:
          //
          // 1. We include all files with a prefix, e.g., <foo/bar>.
          // 2. The library target is in the foo/ sub-directory, e.g.,
          //    /tmp/foo/.
          // 3. The poptions variable contains -I/tmp.
          //
          dir_path p (out_base.sub (d) ? out_base.leaf (d) : dir_path ());

          auto j (m.find (p));

          if (j != m.end ())
          {
            if (j->second != d)
            {
              // We used to reject duplicates but it seems this can be
              // reasonably expected to work according to the order of the -I
              // options.
              //
              // Seeing that we normally have more "specific" -I paths first,
              // (so that we don't pick up installed headers, etc), we ignore
              // it.
              //
              if (verb >= 4)
                trace << "ignoring dependency prefix '" << p << "'\n"
                      << "  existing mapping to " << j->second << "\n"
                      << "  another mapping to  " << d;
            }
          }
          else
          {
            l6 ([&]{trace << "'" << p << "' = '" << d << "'";});
            m.emplace (move (p), move (d));
          }
        }
      }
    }

    auto compile::
    build_prefix_map (const scope& bs,
                      target& t,
                      action act, lorder lo) const -> prefix_map
    {
      prefix_map m;

      // First process our own.
      //
      append_prefixes (m, t, c_poptions);
      append_prefixes (m, t, x_poptions);

      // Then process the include directories from prerequisite libraries.
      //
      append_lib_prefixes (bs, m, t, act, lo);

      return m;
    }

    // Return the next make prerequisite starting from the specified
    // position and update position to point to the start of the
    // following prerequisite or l.size() if there are none left.
    //
    static string
    next_make (const string& l, size_t& p)
    {
      size_t n (l.size ());

      // Skip leading spaces.
      //
      for (; p != n && l[p] == ' '; p++) ;

      // Lines containing multiple prerequisites are 80 characters max.
      //
      string r;
      r.reserve (n);

      // Scan the next prerequisite while watching out for escape sequences.
      //
      for (; p != n && l[p] != ' '; p++)
      {
        char c (l[p]);

        if (p + 1 != n)
        {
          if (c == '$')
          {
            // Got to be another (escaped) '$'.
            //
            if (l[p + 1] == '$')
              ++p;
          }
          else if (c == '\\')
          {
            // This may or may not be an escape sequence depending on whether
            // what follows is "escapable".
            //
            switch (c = l[++p])
            {
            case '\\': break;
            case ' ': break;
            default: c = '\\'; --p; // Restore.
            }
          }
        }

        r += c;
      }

      // Skip trailing spaces.
      //
      for (; p != n && l[p] == ' '; p++) ;

      // Skip final '\'.
      //
      if (p == n - 1 && l[p] == '\\')
        p++;

      return r;
    }

    // Extract the include path from the VC /showIncludes output line. Return
    // empty string if the line is not an include note or include error. Set
    // the good_error flag if it is an include error (which means the process
    // will terminate with the error status that needs to be ignored).
    //
    static string
    next_show (const string& l, bool& good_error)
    {
      // The include error should be the last line that we handle.
      //
      assert (!good_error);

      // VC /showIncludes output. The first line is the file being
      // compiled. Then we have the list of headers, one per line, in this
      // form (text can presumably be translated):
      //
      // Note: including file: C:\Program Files (x86)\[...]\iostream
      //
      // Finally, if we hit a non-existent header, then we end with an error
      // line in this form:
      //
      // x.cpp(3): fatal error C1083: Cannot open include file: 'd/h.hpp':
      // No such file or directory
      //

      // Distinguishing between the include note and the include error is
      // easy: we can just check for C1083. Distinguising between the note and
      // other errors/warnings is harder: an error could very well end with
      // what looks like a path so we cannot look for the note but rather have
      // to look for an error. Here we assume that a line containing ' CNNNN:'
      // is an error. Should be robust enough in the face of language
      // translation, etc.
      //
      size_t p (l.find (':'));
      size_t n (l.size ());

      for (; p != string::npos; p = ++p != n ? l.find (':', p) : string::npos)
      {
        auto isnum = [](char c) {return c >= '0' && c <= '9';};

        if (p > 5 &&
            l[p - 6] == ' '  &&
            l[p - 5] == 'C'  &&
            isnum (l[p - 4]) &&
            isnum (l[p - 3]) &&
            isnum (l[p - 2]) &&
            isnum (l[p - 1]))
        {
          p -= 4; // Start of the error code.
          break;
        }
      }

      if (p == string::npos)
      {
        // Include note. We assume the path is always at the end but
        // need to handle both absolute Windows and POSIX ones.
        //
        size_t p (l.rfind (':'));

        if (p != string::npos)
        {
          // See if this one is part of the Windows drive letter.
          //
          if (p > 1 && p + 1 < n && // 2 chars before, 1 after.
              l[p - 2] == ' ' &&
              alpha (l[p - 1]) &&
              path::traits::is_separator (l[p + 1]))
            p = l.rfind (':', p - 2);
        }

        if (p != string::npos)
        {
          // VC uses indentation to indicate the include nesting so there
          // could be any number of spaces after ':'. Skip them.
          //
          p = l.find_first_not_of (' ', p + 1);
        }

        if (p == string::npos)
          fail << "unable to parse /showIncludes include note line";

        return string (l, p);
      }
      else if (l.compare (p, 4, "1083") == 0)
      {
        // Include error. The path is conveniently quoted with ''.
        //
        size_t p2 (l.rfind ('\''));

        if (p2 != string::npos && p2 != 0)
        {
          size_t p1 (l.rfind ('\'', p2 - 1));

          if (p1 != string::npos)
          {
            good_error = true;
            return string (l, p1 + 1 , p2 - p1 - 1);
          }
        }

        fail << "unable to parse /showIncludes include error line" << endf;
      }
      else
      {
        // Some other error.
        //
        return string ();
      }
    }

    void compile::
    inject (action act, target& t, lorder lo, const file& src, depdb& dd) const
    {
      tracer trace (x, "compile::inject");

      l6 ([&]{trace << "target: " << t;});

      // If things go wrong (and they often do in this area), give the user a
      // bit extra context.
      //
      auto df = make_diag_frame (
        [&src](const diag_record& dr)
        {
          if (verb != 0)
            dr << info << "while extracting header dependencies from " << src;
        });

      const scope& bs (t.base_scope ());
      const scope& rs (*bs.root_scope ());

      // Initialize lazily, only if required.
      //
      const process_path* xc (nullptr);
      cstrings args;

      auto init_args = [&t, act, lo, &src, &rs, &bs, &xc, &args, this] ()
      {
        xc = &cast<process_path> (rs[x_path]);
        args.push_back (xc->recall_string ());

        // Add *.export.poptions from prerequisite libraries.
        //
        append_lib_options (bs, args, t, act, lo);

        append_options (args, t, c_poptions);
        append_options (args, t, x_poptions);

        // Extra system header dirs (last).
        //
        for (const dir_path& d: sys_inc_dirs)
        {
          args.push_back ("-I");
          args.push_back (d.string ().c_str ());
        }

        // Some compile options (e.g., -std, -m) affect the preprocessor.
        //
        append_options (args, t, c_coptions);
        append_options (args, t, x_coptions);

        append_std (args);

        if (t.is_a<objs> ())
        {
          // On Darwin, Win32 -fPIC is the default.
          //
          if (tclass == "linux" || tclass == "bsd")
            args.push_back ("-fPIC");
        }

        if (cid == "msvc")
        {
          args.push_back ("/nologo");

          // See perform_update() for details on overriding the default
          // exceptions and runtime.
          //
          if (x_lang == lang::cxx && !find_option_prefix ("/EH", args))
            args.push_back ("/EHsc");

          if (!find_option_prefixes ({"/MD", "/MT"}, args))
            args.push_back ("/MD");

          args.push_back ("/EP");           // Preprocess to stdout.
          args.push_back ("/showIncludes"); // Goes to sterr becasue of /EP.
          args.push_back (x_lang == lang::c ? "/TC" : "/TP"); // Compile as.
        }
        else
        {
          args.push_back ("-M");  // Note: -MM -MG skips missing <>-included.
          args.push_back ("-MG"); // Treat missing headers as generated.

          // Previously we used '*' as a target name but it gets expanded to
          // the current directory file names by GCC (4.9) that comes with
          // MSYS2 (2.4). Yes, this is the (bizarre) behavior of GCC being
          // executed in the shell with -MQ '*' option and not just -MQ *.
          //
          args.push_back ("-MQ"); // Quoted target name.
          args.push_back ("^");   // Old versions can't do empty target name.
        }

        // We are using absolute source file path in order to get absolute
        // paths in the result. Any relative paths in the result are non-
        // existent, potentially auto-generated headers.
        //
        // @@ We will also have to use absolute -I paths to guarantee
        // that. Or just detect relative paths and error out?
        //
        args.push_back (src.path ().string ().c_str ());
        args.push_back (nullptr);
      };

      // Build the prefix map lazily only if we have non-existent files.
      // Also reuse it over restarts since it doesn't change.
      //
      prefix_map pm;

      // If any prerequisites that we have extracted changed, then we have to
      // redo the whole thing. The reason for this is auto-generated headers:
      // the updated header may now include a yet-non-existent header. Unless
      // we discover this and generate it (which, BTW, will trigger another
      // restart since that header, in turn, can also include auto-generated
      // headers), we will end up with an error during compilation proper.
      //
      // One complication with this restart logic is that we will see a
      // "prefix" of prerequisites that we have already processed (i.e., they
      // are already in our prerequisite_targets list) and we don't want to
      // keep redoing this over and over again. One thing to note, however, is
      // that the prefix that we have seen on the previous run must appear
      // exactly the same in the subsequent run. The reason for this is that
      // none of the files that it can possibly be based on have changed and
      // thus it should be exactly the same. To put it another way, the
      // presence or absence of a file in the dependency output can only
      // depend on the previous files (assuming the compiler outputs them as
      // it encounters them and it is hard to think of a reason why would
      // someone do otherwise). And we have already made sure that all those
      // files are up to date. And here is the way we are going to exploit
      // this: we are going to keep track of how many prerequisites we have
      // processed so far and on restart skip right to the next one.
      //
      // And one more thing: most of the time this list of headers would stay
      // unchanged and extracting them by running the compiler every time is a
      // bit wasteful. So we are going to cache them in the depdb. If the db
      // hasn't been invalidated yet (e.g., because the compiler options have
      // changed), then we start by reading from it. If anything is out of
      // date then we use the same restart and skip logic to switch to the
      // compiler run.
      //

      // Update the target "smartly". Return true if it has changed or if the
      // passed timestamp is not timestamp_unknown and is older than the
      // target.
      //
      // There would normally be a lot of headers for every source file (think
      // all the system headers) and just calling execute_direct() on all of
      // them can get expensive. At the same time, most of these headers are
      // existing files that we will never be updating (again, system headers,
      // for example) and the rule that will match them is the fallback
      // file_rule. That rule has an optimization: it returns noop_recipe
      // (which causes the target state to be automatically set to unchanged)
      // if the file is known to be up to date.
      //
      auto update = [&trace, act] (const path_target& pt, timestamp ts) -> bool
      {
        target_state os (pt.matched_state (act));

        if (os == target_state::unchanged)
        {
          if (ts == timestamp_unknown)
            return false;
          else
          {
            // We expect the timestamp to be known (i.e., existing file).
            //
            timestamp mt (pt.mtime ()); // @@ MT perf: know target state.
            assert (mt != timestamp_unknown);
            return mt > ts;
          }
        }
        else
        {
          // We only want to restart if our call to execute() actually caused
          // an update. In particular, the target could already have been in
          // target_state::changed because of a dependency extraction run for
          // some other source file.
          //
          // @@ MT perf: so we are going to switch the phase and execute for
          //    any generated header.
          //
          phase_switch ps (run_phase::execute);
          target_state ns (execute_direct (act, pt));

          if (ns != os && ns != target_state::unchanged)
          {
            l6 ([&]{trace << "updated " << pt
                          << "; old state " << os
                          << "; new state " << ns;});
            return true;
          }
          else
            return ts != timestamp_unknown ? pt.newer (ts) : false;
        }
      };

      // Update and add a header file to the list of prerequisite targets.
      // Depending on the cache flag, the file is assumed to either have come
      // from the depdb cache or from the compiler run. Return whether the
      // extraction process should be restarted.
      //
      auto add = [&trace, &update, &pm, act, &t, lo, &dd, &bs, &t, this]
        (path f, bool cache) -> bool
      {
        // Find or maybe insert the target.
        //
        auto find = [&trace, &t, this] (
          const path& f, bool insert) -> const path_target*
        {
          // Split the name into its directory part, the name part, and
          // extension. Here we can assume the name part is a valid filesystem
          // name.
          //
          // Note that if the file has no extension, we record an empty
          // extension rather than NULL (which would signify that the default
          // extension should be added).
          //
          dir_path d (f.directory ());
          string n (f.leaf ().base ().string ());
          string e (f.extension ());

          // Determine the target type.
          //
          const target_type* tt (nullptr);

          // See if this directory is part of any project out_root hierarchy.
          // Note that this will miss all the headers that come from src_root
          // (so they will be treated as generic C headers below). Generally,
          // we don't have the ability to determine that some file belongs to
          // src_root of some project. But that's not a problem for our
          // purposes: it is only important for us to accurately determine
          // target types for headers that could be auto-generated.
          //
          // While at it also try to determine if this target is from the src
          // or out tree of said project.
          //
          dir_path out;

          const scope& bs (scopes.find (d));
          if (const scope* rs = bs.root_scope ())
          {
            tt = map_extension (bs, n, e);

            if (bs.out_path () != bs.src_path () && d.sub (bs.src_path ()))
              out = out_src (d, *rs);
          }

          // If it is outside any project, or the project doesn't have such an
          // extension, assume it is a plain old C header.
          //
          if (tt == nullptr)
          {
            // If the project doesn't "know" this extension then we won't
            // possibly find an explicit target of this type.
            //
            if (!insert)
              return nullptr;

            tt = &h::static_type;
          }

          // Find or insert target.
          //
          // @@ OPT: move d, out, n
          //
          const target* r;
          if (insert)
            r = &search (t, *tt, d, out, n, &e, nullptr);
          else
          {
            // Note that we skip any target type-specific searches (like for
            // an existing file) and go straight for the target object since
            // we need to find the target explicitly spelled out.
            //
            r = targets.find (*tt, d, out, n, e, trace);
          }

          return static_cast<const path_target*> (r);
        };

        const path_target* pt (nullptr);

        // If it's not absolute then it does not exist.
        //
        if (!f.absolute ())
        {
          f.normalize ();

          // This is probably as often an error as an auto-generated file, so
          // trace at level 4.
          //
          l4 ([&]{trace << "non-existent header '" << f << "'";});

          // If we already did this and build_prefix_map() returned empty,
          // then we would have failed below.
          //
          if (pm.empty ())
            pm = build_prefix_map (bs, t, act, lo);

          // First try the whole file. Then just the directory.
          //
          // @@ Has to be a separate map since the prefix can be
          //    the same as the file name.
          //
          // auto i (pm.find (f));

          // Find the most qualified prefix of which we are a sub-path.
          //
          auto i (pm.end ());

          if (!pm.empty ())
          {
            const dir_path& d (f.directory ());
            i = pm.upper_bound (d);

            // Get the greatest less than, if any. We might still not be a
            // sub. Note also that we still have to check the last element if
            // upper_bound() returned end().
            //
            if (i == pm.begin () || !d.sub ((--i)->first))
              i = pm.end ();
          }

          if (i != pm.end ())
          {
            // If this is a prefixless mapping, then only use it if we can
            // resolve it to an existing target (i.e., it is explicitly
            // spelled out in a buildfile).
            //
            // Note that at some point we will probably have a list of
            // directories.
            //
            if (i->first.empty ())
            {
              path p (i->second / f);
              l4 ([&]{trace << "trying as auto-generated " << p;});
              pt = find (p, false);
              if (pt != nullptr)
                f = move (p);
            }
            else
            {
              f = i->second / f;
              l4 ([&]{trace << "mapped as auto-generated " << f;});
              pt = find (f, true);
            }
          }

          if (pt == nullptr)
            fail << "header '" << f << "' not found and cannot be generated";
        }
        else
        {
          // We used to just normalize the path but that could result in an
          // invalid path (e.g., on CentOS 7 with Clang 3.4) because of the
          // symlinks. So now we realize (i.e., realpath(3)) it instead. If
          // it comes from the depdb, in which case we've already done that.
          //
          if (!cache)
            f.realize ();

          l6 ([&]{trace << "injecting " << f;});
          pt = find (f, true);
        }

        // Cache the path.
        //
        const path& pp (pt->path (move (f)));

        // Match to a rule.
        //
        build2::match (act, *pt);

        // Update.
        //
        // If this header came from the depdb, make sure it is no older than
        // the db itself (if it has changed since the db was written, then
        // chances are the cached data is stale).
        //
        bool restart (update (*pt, cache ? dd.mtime () : timestamp_unknown));

        // Verify/add it to the dependency database. We do it after update in
        // order not to add bogus files (non-existent and without a way to
        // update).
        //
        if (!cache)
          dd.expect (pp);

        // Add to our prerequisite target list.
        //
        t.prerequisite_targets.push_back (pt);

        return restart;
      };

      // If nothing so far has invalidated the dependency database, then
      // try the cached data before running the compiler.
      //
      bool cache (dd.reading ());

      // But, before we do all that, make sure the source file itself if up to
      // date.
      //
      if (update (src, dd.mtime ()))
      {
        // If the file got updated or is newer than the database, then we
        // cannot rely on the cache any further. However, the cached data
        // could actually still be valid so the compiler run will validate it.
        //
        // We do need to update the database timestamp, however. Failed that,
        // we will keep re-validating the cached data over and over again.
        //
        if (cache)
        {
          cache = false;
          dd.touch ();
        }
      }

      size_t skip_count (0);
      for (bool restart (true); restart; cache = false)
      {
        restart = false;

        if (cache)
        {
          // If any, this is always the first run.
          //
          assert (skip_count == 0);

          while (dd.more ())
          {
            string* l (dd.read ());

            // If the line is invalid, run the compiler.
            //
            if (l == nullptr)
            {
              restart = true;
              break;
            }

            restart = add (path (move (*l)), true);
            skip_count++;

            // The same idea as in the source file update above.
            //
            if (restart)
            {
              l6 ([&]{trace << "restarting";});
              dd.touch ();
              break;
            }
          }
        }
        else
        {
          try
          {
            if (args.empty ())
              init_args ();

            if (verb >= 3)
              print_process (args);

            // For VC with /EP we need a pipe to stderr and stdout should go
            // to /dev/null.
            //
            process pr (*xc,
                        args.data (),
                        0,
                        cid == "msvc" ? -2 : -1,
                        cid == "msvc" ? -1 : 2);

            try
            {
              // We may not read all the output (e.g., due to a restart).
              // Before we used to just close the file descriptor to signal to
              // the other end that we are not interested in the rest. This
              // works fine with GCC but Clang (3.7.0) finds this impolite and
              // complains, loudly (broken pipe). So now we are going to skip
              // until the end.
              //
              ifdstream is (move (cid == "msvc" ? pr.in_efd : pr.in_ofd),
                            fdstream_mode::text | fdstream_mode::skip,
                            ifdstream::badbit);

              // In some cases we may need to ignore the error return
              // status. The good_error flag keeps track of that. Similarly
              // we sometimes expect the error return status based on the
              // output we see. The bad_error flag is for that.
              //
              bool good_error (false), bad_error (false);

              size_t skip (skip_count);
              for (bool first (true), second (false);
                   !(restart || is.eof ()); )
              {
                string l;
                getline (is, l);

                if (is.fail ())
                {
                  if (is.eof ()) // Trailing newline.
                    break;

                  throw io_error ("");
                }

                l6 ([&]{trace << "header dependency line '" << l << "'";});

                // Parse different dependency output formats.
                //
                if (cid == "msvc")
                {
                  if (first)
                  {
                    // The first line should be the file we are compiling. If
                    // it is not, then something went wrong even before we
                    // could compile anything (e.g., file does not exist). In
                    // this case the first line (and everything after it) is
                    // presumably diagnostics.
                    //
                    if (l != src.path ().leaf ().string ())
                    {
                      text << l;
                      bad_error = true;
                      break;
                    }

                    first = false;
                    continue;
                  }

                  string f (next_show (l, good_error));

                  if (f.empty ()) // Some other diagnostics.
                  {
                    text << l;
                    bad_error = true;
                    break;
                  }

                  // Skip until where we left off.
                  //
                  if (skip != 0)
                  {
                    // We can't be skipping over a non-existent header.
                    //
                    assert (!good_error);
                    skip--;
                  }
                  else
                  {
                    restart = add (path (move (f)), false);
                    skip_count++;

                    // If the header does not exist, we better restart.
                    //
                    assert (!good_error || restart);

                    if (restart)
                      l6 ([&]{trace << "restarting";});
                  }
                }
                else
                {
                  // Make dependency declaration.
                  //
                  size_t pos (0);

                  if (first)
                  {
                    // Empty output should mean the wait() call below will
                    // return false.
                    //
                    if (l.empty ())
                    {
                      bad_error = true;
                      break;
                    }

                    assert (l[0] == '^' && l[1] == ':' && l[2] == ' ');

                    first = false;
                    second = true;

                    // While normally we would have the source file on the
                    // first line, if too long, it will be moved to the next
                    // line and all we will have on this line is "^: \".
                    //
                    if (l.size () == 4 && l[3] == '\\')
                      continue;
                    else
                      pos = 3; // Skip "^: ".

                    // Fall through to the 'second' block.
                  }

                  if (second)
                  {
                    second = false;
                    next_make (l, pos); // Skip the source file.
                  }

                  while (pos != l.size ())
                  {
                    string f (next_make (l, pos));

                    // Skip until where we left off.
                    //
                    if (skip != 0)
                    {
                      skip--;
                      continue;
                    }

                    restart = add (path (move (f)), false);
                    skip_count++;

                    if (restart)
                    {
                      l6 ([&]{trace << "restarting";});
                      break;
                    }
                  }
                }
              }

              // In case of VC, we are parsing stderr and if things go south,
              // we need to copy the diagnostics for the user to see.
              //
              // Note that the eof check is important: if the stream is at
              // eof, this and all subsequent writes to cerr will fail (and
              // you won't see a thing).
              //
              if (is.peek () != ifdstream::traits_type::eof () &&
                  cid == "msvc" &&
                  bad_error)
                cerr << is.rdbuf ();

              is.close ();

              // We assume the child process issued some diagnostics.
              //
              if (!pr.wait ())
              {
                if (!good_error) // Ignore expected errors (restart).
                  throw failed ();
              }
              else if (bad_error)
                fail << "expected error exist status from " << x_lang
                     << " compiler";
            }
            catch (const io_error&)
            {
              pr.wait ();
              fail << "unable to read " << x_lang << " compiler header "
                   << "dependency output";
            }
          }
          catch (const process_error& e)
          {
            error << "unable to execute " << args[0] << ": " << e;

            // In a multi-threaded program that fork()'ed but did not exec(),
            // it is unwise to try to do any kind of cleanup (like unwinding
            // the stack and running destructors).
            //
            if (e.child)
              exit (1);

            throw failed ();
          }
        }
      }
    }

    // Filter cl.exe noise (msvc.cxx).
    //
    void
    msvc_filter_cl (ifdstream&, const path& src);

    target_state compile::
    perform_update (action act, const target& xt) const
    {
      const file& t (xt.as<file> ());
      const path& tp (t.path ());
      const match_data& md (t.data<match_data> ());

      // Update prerequisites and determine if any relevant ones render us
      // out-of-date. Note that currently we treat all the prerequisites
      // as potentially affecting the result (for simplicity/performance).
      //
      const file* s;
      {
        timestamp mt;

        // If the depdb was overwritten or it's newer than the target, then
        // do unconditional update.
        //
        if (md.dd_mtime == timestamp_nonexistent ||
            md.dd_mtime > (mt = t.load_mtime ()))
          mt = timestamp_nonexistent;

        auto p (execute_prerequisites<file> (x_src, act, t, mt));

        if ((s = p.first) == nullptr)
          return p.second;
      }

      const scope& bs (t.base_scope ());
      const scope& rs (*bs.root_scope ());

      otype ct (compile_type (t));
      lorder lo (link_order (bs, ct));

      const process_path& xc (cast<process_path> (rs[x_path]));
      cstrings args {xc.recall_string ()};

      // Translate paths to relative (to working directory) ones. This
      // results in easier to read diagnostics.
      //
      path relo (relative (tp));
      path rels (relative (s->path ()));

      append_options (args, t, c_poptions);
      append_options (args, t, x_poptions);

      // Add *.export.poptions from prerequisite libraries.
      //
      append_lib_options (bs, args, t, act, lo);

      // Extra system header dirs (last).
      //
      for (const dir_path& d: sys_inc_dirs)
      {
        args.push_back ("-I");
        args.push_back (d.string ().c_str ());
      }

      append_options (args, t, c_coptions);
      append_options (args, t, x_coptions);

      string out, out1; // Storage.

      append_std (args);

      if (cid == "msvc")
      {
        // The /F*: option variants with separate names only became available
        // in VS2013/12.0. Why do we bother? Because the command line suddenly
        // becomes readable.
        //
        uint64_t ver (cast<uint64_t> (rs[x_version_major]));

        args.push_back ("/nologo");

        // While we want to keep the low-level build as "pure" as possible,
        // the two misguided defaults, exceptions and runtime, just have to be
        // fixed. Otherwise the default build is pretty much unusable. But we
        // also make sure that the user can easily disable our defaults: if we
        // see any relevant options explicitly specified, we take our hands
        // off.
        //
        // For C looks like no /EH* (exceptions supported but no C++ objects
        // destroyed) is a reasonable default.
        //
        if (x_lang == lang::cxx && !find_option_prefix ("/EH", args))
          args.push_back ("/EHsc");

        // The runtime is a bit more interesting. At first it may seem like a
        // good idea to be a bit clever and use the static runtime if we are
        // building obja{}. And for obje{} we could decide which runtime to
        // use based on the library link order: if it is static-only, then we
        // could assume the static runtime. But it is indeed too clever: when
        // building liba{} we have no idea who is going to use it. It could be
        // an exe{} that links both static and shared libraries (and is
        // therefore built with the shared runtime). And to safely use the
        // static runtime, everything must be built with /MT and there should
        // be no DLLs in the picture. So we are going to play it safe and
        // always default to the shared runtime.
        //
        // In a similar vein, it would seem reasonable to use the debug runtime
        // if we are compiling with debug. But, again, there will be fireworks
        // if we have some projects built with debug and some without and then
        // we try to link them together (which is not an unreasonable thing to
        // do). So by default we will always use the release runtime.
        //
        if (!find_option_prefixes ({"/MD", "/MT"}, args))
          args.push_back ("/MD");

        // The presence of /Zi or /ZI causes the compiler to write debug info
        // to the .pdb file. By default it is a shared file called vcNN.pdb
        // (where NN is the VC version) created (wait for it) in the current
        // working directory (and not the directory of the .obj file). Also,
        // because it is shared, there is a special Windows service that
        // serializes access. We, of course, want none of that so we will
        // create a .pdb per object file.
        //
        // Note that this also changes the name of the .idb file (used for
        // minimal rebuild and incremental compilation): cl.exe take the /Fd
        // value and replaces the .pdb extension with .idb.
        //
        // Note also that what we are doing here appears to be incompatible
        // with PCH (/Y* options) and /Gm (minimal rebuild).
        //
        if (find_options ({"/Zi", "/ZI"}, args))
        {
          if (ver >= 18)
            args.push_back ("/Fd:");
          else
            out1 = "/Fd";

          out1 += relo.string ();
          out1 += ".pdb";

          args.push_back (out1.c_str ());
        }

        if (ver >= 18)
        {
          args.push_back ("/Fo:");
          args.push_back (relo.string ().c_str ());
        }
        else
        {
          out = "/Fo" + relo.string ();
          args.push_back (out.c_str ());
        }

        args.push_back ("/c");                              // Compile only.
        args.push_back (x_lang == lang::c ? "/TC" : "/TP"); // Compile as.
        args.push_back (rels.string ().c_str ());
      }
      else
      {
        if (ct == otype::s)
        {
          // On Darwin, Win32 -fPIC is the default.
          //
          if (tclass == "linux" || tclass == "bsd")
            args.push_back ("-fPIC");
        }

        args.push_back ("-o");
        args.push_back (relo.string ().c_str ());

        args.push_back ("-c");
        args.push_back (rels.string ().c_str ());
      }

      args.push_back (nullptr);

      if (verb >= 2)
        print_process (args);
      else if (verb)
        text << x_name << ' ' << *s;

      try
      {
        // VC cl.exe sends diagnostics to stdout. It also prints the file name
        // being compiled as the first line. So for cl.exe we redirect stdout
        // to a pipe, filter that noise out, and send the rest to stderr.
        //
        // For other compilers redirect stdout to stderr, in case any of them
        // tries to pull off something similar. For sane compilers this should
        // be harmless.
        //
        bool filter (cid == "msvc");

        process pr (xc, args.data (), 0, (filter ? -1 : 2));

        if (filter)
        {
          try
          {
            ifdstream is (
              move (pr.in_ofd), fdstream_mode::text, ifdstream::badbit);

            msvc_filter_cl (is, rels);

            // If anything remains in the stream, send it all to stderr. Note
            // that the eof check is important: if the stream is at eof, this
            // and all subsequent writes to cerr will fail (and you won't see
            // a thing).
            //
            if (is.peek () != ifdstream::traits_type::eof ())
              cerr << is.rdbuf ();

            is.close ();
          }
          catch (const io_error&) {} // Assume exits with error.
        }

        if (!pr.wait ())
          throw failed ();

        // Should we go to the filesystem and get the new mtime? We
        // know the file has been modified, so instead just use the
        // current clock time. It has the advantage of having the
        // subseconds precision.
        //
        t.mtime (system_clock::now ());
        return target_state::changed;
      }
      catch (const process_error& e)
      {
        error << "unable to execute " << args[0] << ": " << e;

        // In a multi-threaded program that fork()'ed but did not exec(),
        // it is unwise to try to do any kind of cleanup (like unwinding
        // the stack and running destructors).
        //
        if (e.child)
          exit (1);

        throw failed ();
      }
    }

    target_state compile::
    perform_clean (action act, const target& xt) const
    {
      const file& t (xt.as<file> ());

      if (cid == "msvc")
        return clean_extra (act, t, {".d", ".idb", ".pdb"});
      else
        return clean_extra (act, t, {".d"});
    }
  }
}