text: // Copyright 2011 Google Inc. All Rights Reserved.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #include <string>
  #include <vector>
  #include <queue>
  using namespace std;
  
  // Subprocess wraps a single async subprocess.  It is entirely
  // passive: it expects the caller to notify it when its fds are ready
  // for reading, as well as call Finish() to reap the child once done()
  // is true.
  struct Subprocess {
    Subprocess();
    ~Subprocess();
    bool Start(const string& command);
    void OnFDReady(int fd);
    // Returns true on successful process exit.
    bool Finish();
  
    bool done() const {
      return stdout_.fd_ == -1 && stderr_.fd_ == -1;
    }
  
    struct Stream {
      Stream();
      ~Stream();
      int fd_;
      string buf_;
    };
    Stream stdout_, stderr_;
    pid_t pid_;
  };
  
  // SubprocessSet runs a poll() loop around a set of Subprocesses.
  // DoWork() waits for any state change in subprocesses; finished_
  // is a queue of subprocesses as they finish.
  struct SubprocessSet {
    void Add(Subprocess* subprocess);
    void DoWork();
    Subprocess* NextFinished();
  
    vector<Subprocess*> running_;
    queue<Subprocess*> finished_;
  };
  