lib/Basic/Unix/TaskQueue.inc - third_party/swift - Git at Google

 //===--- TaskQueue.inc - Unix-specific TaskQueue ----------------*- C++ -*-===//
 //
 // This source file is part of the Swift.org open source project
 //
 // Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 //===----------------------------------------------------------------------===//

 #include "swift/Basic/TaskQueue.h"

 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/Support/ErrorHandling.h"

 #include <string>
 #include <cerrno>

 #if HAVE_POSIX_SPAWN
 #include <spawn.h>
 #endif

 #if HAVE_UNISTD_H
 #include <unistd.h>
 #endif

 #include <poll.h>
 #include <sys/types.h>
 #include <sys/wait.h>

 #if !defined(__APPLE__)
 extern char **environ;
 #else
 extern "C" {
   // _NSGetEnviron is from crt_externs.h which is missing in the iOS SDK.
   extern char ***_NSGetEnviron(void);
 }
 #endif

 namespace swift {
 namespace sys {

 class Task {
   /// The path to the executable which this Task will execute.
   const char *ExecPath;

   /// Any arguments which should be passed during execution.
   ArrayRef<const char *> Args;

   /// The environment which will be used during execution. If empty, uses
   /// this process's environment.
   ArrayRef<const char *> Env;

   /// Context which should be associated with this task.
   void *Context;

   /// True if the errors of the Task should be stored in Errors instead of Output.
   bool SeparateErrors;

   /// The pid of this Task when executing.
   pid_t Pid;

   /// A pipe for reading output from the child process.
   int Pipe;

   /// A pipe for reading errors from the child prcess, if SeparateErrors is true.
   int ErrorPipe;

   /// The current state of the Task.
   enum {
     Preparing,
     Executing,
     Finished
   } State;

   /// Once the Task has finished, this contains the buffered output of the Task.
   std::string Output;

   /// Once the Task has finished, if SeparateErrors is true, this contains the errors
   /// from the Task.
   std::string Errors;

 public:
   Task(const char *ExecPath, ArrayRef<const char *> Args,
        ArrayRef<const char *> Env, void *Context, bool SeparateErrors)
       : ExecPath(ExecPath), Args(Args), Env(Env), Context(Context),
         SeparateErrors(SeparateErrors), Pid(-1), Pipe(-1), ErrorPipe(-1),
         State(Preparing) {
     assert((Env.empty() || Env.back() == nullptr) &&
            "Env must either be empty or null-terminated!");
   }

   const char *getExecPath() const { return ExecPath; }
   ArrayRef<const char *> getArgs() const { return Args; }
   StringRef getOutput() const { return Output; }
   StringRef getErrors() const { return Errors; }
   void *getContext() const { return Context; }
   pid_t getPid() const { return Pid; }
   int getPipe() const { return Pipe; }
   int getErrorPipe() const { return ErrorPipe; }

   /// \brief Begins execution of this Task.
   /// \returns true on error, false on success
   bool execute();

   /// \brief Reads data from the pipes, if any is available.
   /// \returns true on error, false on success
   bool readFromPipes();

   /// \brief Performs any post-execution work for this Task, such as reading
   /// piped output and closing the pipe.
   void finishExecution();
 };

 } // end namespace sys
 } // end namespace swift

 bool Task::execute() {
   assert(State < Executing && "This Task cannot be executed twice!");
   State = Executing;

   // Construct argv.
   SmallVector<const char *, 128> Argv;
   Argv.push_back(ExecPath);
   Argv.append(Args.begin(), Args.end());
   Argv.push_back(0); // argv is expected to be null-terminated.

   // Set up the pipe.
   int FullPipe[2];
   pipe(FullPipe);
   Pipe = FullPipe[0];

   int FullErrorPipe[2];
   if (SeparateErrors) {
     pipe(FullErrorPipe);
     ErrorPipe = FullErrorPipe[0];
   }

   // Get the environment to pass down to the subtask.
   const char *const *envp = Env.empty() ? nullptr : Env.data();
   if (!envp) {
 #if __APPLE__
     envp = *_NSGetEnviron();
 #else
     envp = environ;
 #endif
   }

   const char **argvp = Argv.data();

 #if HAVE_POSIX_SPAWN
   posix_spawn_file_actions_t FileActions;
   posix_spawn_file_actions_init(&FileActions);

   posix_spawn_file_actions_adddup2(&FileActions, FullPipe[1], STDOUT_FILENO);

   if (SeparateErrors) {
     posix_spawn_file_actions_adddup2(&FileActions, FullErrorPipe[1],
                                      STDERR_FILENO);
   } else {
     posix_spawn_file_actions_adddup2(&FileActions, STDOUT_FILENO,
                                      STDERR_FILENO);
   }

   posix_spawn_file_actions_addclose(&FileActions, FullPipe[0]);
   if (SeparateErrors) {
     posix_spawn_file_actions_addclose(&FileActions, FullErrorPipe[0]);
   }

   // Spawn the subtask.
   int spawnErr = posix_spawn(&Pid, ExecPath, &FileActions, nullptr,
                              const_cast<char **>(argvp),
                              const_cast<char **>(envp));

   posix_spawn_file_actions_destroy(&FileActions);
   close(FullPipe[1]);
   if (SeparateErrors) {
     close(FullErrorPipe[1]);
   }

   if (spawnErr != 0 || Pid == 0) {
     close(FullPipe[0]);
     if (SeparateErrors) {
       close(FullErrorPipe[0]);
     }
     State = Finished;
     return true;
   }
 #else
   Pid = fork();
   switch (Pid) {
   case -1: {
     close(FullPipe[0]);
     if (SeparateErrors) {
       close(FullErrorPipe[0]);
     }
     State = Finished;
     Pid = 0;
     break;
   }
   case 0: {
     // Child process: Execute the program.
     dup2(FullPipe[1], STDOUT_FILENO);
     if (SeparateErrors) {
       dup2(FullErrorPipe[1], STDERR_FILENO);
     } else {
       dup2(STDOUT_FILENO, STDERR_FILENO);
     }
     close(FullPipe[0]);
     if (SeparateErrors) {
       close(FullErrorPipe[0]);
     }
     execve(ExecPath, const_cast<char **>(argvp), const_cast<char **>(envp));

     // If the execve() failed, we should exit. Follow Unix protocol and
     // return 127 if the executable was not found, and 126 otherwise.
     // Use _exit rather than exit so that atexit functions and static
     // object destructors cloned from the parent process aren't
     // redundantly run, and so that any data buffered in stdio buffers
     // cloned from the parent aren't redundantly written out.
     _exit(errno == ENOENT ? 127 : 126);
   }
   default:
     // Parent process: Break out of the switch to do our processing.
     break;
   }

   close(FullPipe[1]);
   if (SeparateErrors) {
     close(FullErrorPipe[1]);
   }

   if (Pid == 0)
     return true;
 #endif

   return false;
 }

 static bool readFromAPipe(int Pipe, std::string &Output) {
   char outputBuffer[1024];
   ssize_t readBytes = 0;
   while ((readBytes = read(Pipe, outputBuffer, sizeof(outputBuffer))) != 0) {
     if (readBytes < 0) {
       if (errno == EINTR)
         // read() was interrupted, so try again.
         continue;
       return true;
     }

     Output.append(outputBuffer, readBytes);
   }

   return false;
 }

 bool Task::readFromPipes() {
   bool Ret = readFromAPipe(Pipe, Output);
   if (SeparateErrors) {
     Ret |= readFromAPipe(ErrorPipe, Errors);
   }
   return Ret;
 }

 void Task::finishExecution() {
   assert(State == Executing &&
          "This Task must be executing to finish execution!");

   State = Finished;

   // Read the output of the command, so we can use it later.
   readFromPipes();

   close(Pipe);
   if (SeparateErrors) {
     close(ErrorPipe);
   }
 }

 bool TaskQueue::supportsBufferingOutput() {
   // The Unix implementation supports buffering output.
   return true;
 }

 bool TaskQueue::supportsParallelExecution() {
   // The Unix implementation supports parallel execution.
   return true;
 }

 unsigned TaskQueue::getNumberOfParallelTasks() const {
   // TODO: add support for choosing a better default value for
   // MaxNumberOfParallelTasks if NumberOfParallelTasks is 0. (Optimally, this
   // should choose a value > 1 tailored to the current system.)
   return NumberOfParallelTasks > 0 ? NumberOfParallelTasks : 1;
 }

 void TaskQueue::addTask(const char *ExecPath, ArrayRef<const char *> Args,
                         ArrayRef<const char *> Env, void *Context,
                         bool SeparateErrors) {
   std::unique_ptr<Task> T(
       new Task(ExecPath, Args, Env, Context, SeparateErrors));
   QueuedTasks.push(std::move(T));
 }

 bool TaskQueue::execute(TaskBeganCallback Began, TaskFinishedCallback Finished,
                         TaskSignalledCallback Signalled) {
   typedef llvm::DenseMap<pid_t, std::unique_ptr<Task>> PidToTaskMap;

   // Stores the current executing Tasks, organized by pid.
   PidToTaskMap ExecutingTasks;

   // Maintains the current fds we're checking with poll.
   std::vector<struct pollfd> PollFds;

   bool SubtaskFailed = false;

   unsigned MaxNumberOfParallelTasks = getNumberOfParallelTasks();

   if (MaxNumberOfParallelTasks == 0)
     MaxNumberOfParallelTasks = 1;

   while ((!QueuedTasks.empty() && !SubtaskFailed) ||
          !ExecutingTasks.empty()) {
     // Enqueue additional tasks, if we have additional tasks, we aren't
     // already at the parallel limit, and no earlier subtasks have failed.
     while (!SubtaskFailed && !QueuedTasks.empty() &&
            ExecutingTasks.size() < MaxNumberOfParallelTasks) {
       std::unique_ptr<Task> T(QueuedTasks.front().release());
       QueuedTasks.pop();
       if (T->execute())
         return true;

       pid_t Pid = T->getPid();

       if (Began) {
         Began(Pid, T->getContext());
       }

       PollFds.push_back({ T->getPipe(), POLLIN | POLLPRI | POLLHUP, 0 });
       // We should also poll T->getErrorPipe(), but this introduces timing
       // issues with shutting down the task after reading getPipe().
       ExecutingTasks[Pid] = std::move(T);
     }

     assert(PollFds.size() > 0 &&
            "We should only call poll() if we have fds to watch!");
     int ReadyFdCount = poll(PollFds.data(), PollFds.size(), -1);
     if (ReadyFdCount == -1) {
       // Recover from error, if possible.
       if (errno == EAGAIN || errno == EINTR)
         continue;
       return true;
     }

     // Holds all fds which have finished during this loop iteration.
     std::vector<int> FinishedFds;

     for (struct pollfd &fd : PollFds) {
       if (fd.revents & POLLIN || fd.revents & POLLPRI || fd.revents & POLLHUP ||
           fd.revents & POLLERR) {
         // An event which we care about occurred. Find the appropriate Task.
         auto predicate = [&fd](PidToTaskMap::value_type &value) -> bool {
           return value.second->getPipe() == fd.fd;
         };

         auto iter = std::find_if(ExecutingTasks.begin(), ExecutingTasks.end(),
                                  predicate);
         assert(iter != ExecutingTasks.end() &&
                "All outstanding fds must be associated with an executing Task");
         Task &T = *iter->second;
         if (fd.revents & POLLIN || fd.revents & POLLPRI) {
           // There's data available to read.
           T.readFromPipes();
         }

         if (fd.revents & POLLHUP || fd.revents & POLLERR) {
           // This fd was "hung up" or had an error, so we need to wait for the
           // Task and then clean up.
           pid_t Pid;
           int Status;
           do {
             Status = 0;
             Pid = waitpid(T.getPid(), &Status, 0);
             assert(Pid != 0 &&
                    "We do not pass WNOHANG, so we should always get a pid");
             if (Pid < 0 && (errno == ECHILD || errno == EINVAL))
               return true;
           } while (Pid < 0);

           assert(Pid == T.getPid() &&
                  "We asked to wait for this Task, but we got another Pid!");

           T.finishExecution();

           if (WIFEXITED(Status)) {
             int Result = WEXITSTATUS(Status);

             if (Finished) {
               // If we have a TaskFinishedCallback, only set SubtaskFailed to
               // true if the callback returns StopExecution.
               SubtaskFailed = Finished(T.getPid(), Result, T.getOutput(),
                                        T.getErrors(), T.getContext()) ==
                   TaskFinishedResponse::StopExecution;
             } else if (Result != 0) {
               // Since we don't have a TaskFinishedCallback, treat a subtask
               // which returned a nonzero exit code as having failed.
               SubtaskFailed = true;
             }
           } else if (WIFSIGNALED(Status)) {
             // The process exited due to a signal.
             int Signal = WTERMSIG(Status);

             StringRef ErrorMsg = strsignal(Signal);

             if (Signalled) {
               TaskFinishedResponse Response =
                   Signalled(T.getPid(), ErrorMsg, T.getOutput(), T.getErrors(),
                             T.getContext(), Signal);
               if (Response == TaskFinishedResponse::StopExecution)
                 // If we have a TaskCrashedCallback, only set SubtaskFailed to
                 // true if the callback returns StopExecution.
                 SubtaskFailed = true;
             } else {
               // Since we don't have a TaskCrashedCallback, treat a crashing
               // subtask as having failed.
               SubtaskFailed = true;
             }
           }

           ExecutingTasks.erase(Pid);
           FinishedFds.push_back(fd.fd);
         }
       } else if (fd.revents & POLLNVAL) {
         // We passed an invalid fd; this should never happen,
         // since we always mark fds as finished after calling
         // Task::finishExecution() (which closes the Task's fd).
         llvm_unreachable("Asked poll() to watch a closed fd");
       }

       fd.revents = 0;
     }

     // Remove any fds which we've closed from PollFds.
     for (int fd : FinishedFds) {
       auto predicate = [&fd] (struct pollfd &i) {
         return i.fd == fd;
       };

       auto iter = std::find_if(PollFds.begin(), PollFds.end(), predicate);
       assert(iter != PollFds.end() && "The finished fd must be in PollFds!");
       PollFds.erase(iter);
     }
   }

   return SubtaskFailed;
 }
	//===--- TaskQueue.inc - Unix-specific TaskQueue ----------------- C++ --===//
	//
	// This source file is part of the Swift.org open source project
	//
	// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
	// Licensed under Apache License v2.0 with Runtime Library Exception
	//
	// See https://swift.org/LICENSE.txt for license information
	// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
	//
	//===----------------------------------------------------------------------===//

	#include "swift/Basic/TaskQueue.h"

	#include "llvm/ADT/StringRef.h"
	#include "llvm/ADT/DenseMap.h"
	#include "llvm/ADT/DenseSet.h"
	#include "llvm/Support/ErrorHandling.h"

	#include <string>
	#include <cerrno>

	#if HAVE_POSIX_SPAWN
	#include <spawn.h>
	#endif

	#if HAVE_UNISTD_H
	#include <unistd.h>
	#endif

	#include <poll.h>
	#include <sys/types.h>
	#include <sys/wait.h>

	#if !defined(__APPLE__)
	extern char **environ;
	#else
	extern "C" {
	// _NSGetEnviron is from crt_externs.h which is missing in the iOS SDK.
	extern char ***_NSGetEnviron(void);
	}
	#endif

	namespace swift {
	namespace sys {

	class Task {
	/// The path to the executable which this Task will execute.
	const char *ExecPath;

	/// Any arguments which should be passed during execution.
	ArrayRef<const char *> Args;

	/// The environment which will be used during execution. If empty, uses
	/// this process's environment.
	ArrayRef<const char *> Env;

	/// Context which should be associated with this task.
	void *Context;

	/// True if the errors of the Task should be stored in Errors instead of Output.
	bool SeparateErrors;

	/// The pid of this Task when executing.
	pid_t Pid;

	/// A pipe for reading output from the child process.
	int Pipe;

	/// A pipe for reading errors from the child prcess, if SeparateErrors is true.
	int ErrorPipe;

	/// The current state of the Task.
	enum {
	Preparing,
	Executing,
	Finished
	} State;

	/// Once the Task has finished, this contains the buffered output of the Task.
	std::string Output;

	/// Once the Task has finished, if SeparateErrors is true, this contains the errors
	/// from the Task.
	std::string Errors;

	public:
	Task(const char ExecPath, ArrayRef<const char > Args,
	ArrayRef<const char > Env, void Context, bool SeparateErrors)
	: ExecPath(ExecPath), Args(Args), Env(Env), Context(Context),
	SeparateErrors(SeparateErrors), Pid(-1), Pipe(-1), ErrorPipe(-1),
	State(Preparing) {
	assert((Env.empty() \|\| Env.back() == nullptr) &&
	"Env must either be empty or null-terminated!");
	}

	const char *getExecPath() const { return ExecPath; }
	ArrayRef<const char *> getArgs() const { return Args; }
	StringRef getOutput() const { return Output; }
	StringRef getErrors() const { return Errors; }
	void *getContext() const { return Context; }
	pid_t getPid() const { return Pid; }
	int getPipe() const { return Pipe; }
	int getErrorPipe() const { return ErrorPipe; }

	/// \brief Begins execution of this Task.
	/// \returns true on error, false on success
	bool execute();

	/// \brief Reads data from the pipes, if any is available.
	/// \returns true on error, false on success
	bool readFromPipes();

	/// \brief Performs any post-execution work for this Task, such as reading
	/// piped output and closing the pipe.
	void finishExecution();
	};

	} // end namespace sys
	} // end namespace swift

	bool Task::execute() {
	assert(State < Executing && "This Task cannot be executed twice!");
	State = Executing;

	// Construct argv.
	SmallVector<const char *, 128> Argv;
	Argv.push_back(ExecPath);
	Argv.append(Args.begin(), Args.end());
	Argv.push_back(0); // argv is expected to be null-terminated.

	// Set up the pipe.
	int FullPipe[2];
	pipe(FullPipe);
	Pipe = FullPipe[0];

	int FullErrorPipe[2];
	if (SeparateErrors) {
	pipe(FullErrorPipe);
	ErrorPipe = FullErrorPipe[0];
	}

	// Get the environment to pass down to the subtask.
	const char const envp = Env.empty() ? nullptr : Env.data();
	if (!envp) {
	#if __APPLE__
	envp = *_NSGetEnviron();
	#else
	envp = environ;
	#endif
	}

	const char **argvp = Argv.data();

	#if HAVE_POSIX_SPAWN
	posix_spawn_file_actions_t FileActions;
	posix_spawn_file_actions_init(&FileActions);

	posix_spawn_file_actions_adddup2(&FileActions, FullPipe[1], STDOUT_FILENO);

	if (SeparateErrors) {
	posix_spawn_file_actions_adddup2(&FileActions, FullErrorPipe[1],
	STDERR_FILENO);
	} else {
	posix_spawn_file_actions_adddup2(&FileActions, STDOUT_FILENO,
	STDERR_FILENO);
	}

	posix_spawn_file_actions_addclose(&FileActions, FullPipe[0]);
	if (SeparateErrors) {
	posix_spawn_file_actions_addclose(&FileActions, FullErrorPipe[0]);
	}

	// Spawn the subtask.
	int spawnErr = posix_spawn(&Pid, ExecPath, &FileActions, nullptr,
	const_cast<char **>(argvp),
	const_cast<char **>(envp));

	posix_spawn_file_actions_destroy(&FileActions);
	close(FullPipe[1]);
	if (SeparateErrors) {
	close(FullErrorPipe[1]);
	}

	if (spawnErr != 0 \|\| Pid == 0) {
	close(FullPipe[0]);
	if (SeparateErrors) {
	close(FullErrorPipe[0]);
	}
	State = Finished;
	return true;
	}
	#else
	Pid = fork();
	switch (Pid) {
	case -1: {
	close(FullPipe[0]);
	if (SeparateErrors) {
	close(FullErrorPipe[0]);
	}
	State = Finished;
	Pid = 0;
	break;
	}
	case 0: {
	// Child process: Execute the program.
	dup2(FullPipe[1], STDOUT_FILENO);
	if (SeparateErrors) {
	dup2(FullErrorPipe[1], STDERR_FILENO);
	} else {
	dup2(STDOUT_FILENO, STDERR_FILENO);
	}
	close(FullPipe[0]);
	if (SeparateErrors) {
	close(FullErrorPipe[0]);
	}
	execve(ExecPath, const_cast<char >(argvp), const_cast<char >(envp));

	// If the execve() failed, we should exit. Follow Unix protocol and
	// return 127 if the executable was not found, and 126 otherwise.
	// Use _exit rather than exit so that atexit functions and static
	// object destructors cloned from the parent process aren't
	// redundantly run, and so that any data buffered in stdio buffers
	// cloned from the parent aren't redundantly written out.
	_exit(errno == ENOENT ? 127 : 126);
	}
	default:
	// Parent process: Break out of the switch to do our processing.
	break;
	}

	close(FullPipe[1]);
	if (SeparateErrors) {
	close(FullErrorPipe[1]);
	}

	if (Pid == 0)
	return true;
	#endif

	return false;
	}

	static bool readFromAPipe(int Pipe, std::string &Output) {
	char outputBuffer[1024];
	ssize_t readBytes = 0;
	while ((readBytes = read(Pipe, outputBuffer, sizeof(outputBuffer))) != 0) {
	if (readBytes < 0) {
	if (errno == EINTR)
	// read() was interrupted, so try again.
	continue;
	return true;
	}

	Output.append(outputBuffer, readBytes);
	}

	return false;
	}

	bool Task::readFromPipes() {
	bool Ret = readFromAPipe(Pipe, Output);
	if (SeparateErrors) {
	Ret \|= readFromAPipe(ErrorPipe, Errors);
	}
	return Ret;
	}

	void Task::finishExecution() {
	assert(State == Executing &&
	"This Task must be executing to finish execution!");

	State = Finished;

	// Read the output of the command, so we can use it later.
	readFromPipes();

	close(Pipe);
	if (SeparateErrors) {
	close(ErrorPipe);
	}
	}

	bool TaskQueue::supportsBufferingOutput() {
	// The Unix implementation supports buffering output.
	return true;
	}

	bool TaskQueue::supportsParallelExecution() {
	// The Unix implementation supports parallel execution.
	return true;
	}

	unsigned TaskQueue::getNumberOfParallelTasks() const {
	// TODO: add support for choosing a better default value for
	// MaxNumberOfParallelTasks if NumberOfParallelTasks is 0. (Optimally, this
	// should choose a value > 1 tailored to the current system.)
	return NumberOfParallelTasks > 0 ? NumberOfParallelTasks : 1;
	}

	void TaskQueue::addTask(const char ExecPath, ArrayRef<const char > Args,
	ArrayRef<const char > Env, void Context,
	bool SeparateErrors) {
	std::unique_ptr<Task> T(
	new Task(ExecPath, Args, Env, Context, SeparateErrors));
	QueuedTasks.push(std::move(T));
	}

	bool TaskQueue::execute(TaskBeganCallback Began, TaskFinishedCallback Finished,
	TaskSignalledCallback Signalled) {
	typedef llvm::DenseMap<pid_t, std::unique_ptr<Task>> PidToTaskMap;

	// Stores the current executing Tasks, organized by pid.
	PidToTaskMap ExecutingTasks;

	// Maintains the current fds we're checking with poll.
	std::vector<struct pollfd> PollFds;

	bool SubtaskFailed = false;

	unsigned MaxNumberOfParallelTasks = getNumberOfParallelTasks();

	if (MaxNumberOfParallelTasks == 0)
	MaxNumberOfParallelTasks = 1;

	while ((!QueuedTasks.empty() && !SubtaskFailed) \|\|
	!ExecutingTasks.empty()) {
	// Enqueue additional tasks, if we have additional tasks, we aren't
	// already at the parallel limit, and no earlier subtasks have failed.
	while (!SubtaskFailed && !QueuedTasks.empty() &&
	ExecutingTasks.size() < MaxNumberOfParallelTasks) {
	std::unique_ptr<Task> T(QueuedTasks.front().release());
	QueuedTasks.pop();
	if (T->execute())
	return true;

	pid_t Pid = T->getPid();

	if (Began) {
	Began(Pid, T->getContext());
	}

	PollFds.push_back({ T->getPipe(), POLLIN \| POLLPRI \| POLLHUP, 0 });
	// We should also poll T->getErrorPipe(), but this introduces timing
	// issues with shutting down the task after reading getPipe().
	ExecutingTasks[Pid] = std::move(T);
	}

	assert(PollFds.size() > 0 &&
	"We should only call poll() if we have fds to watch!");
	int ReadyFdCount = poll(PollFds.data(), PollFds.size(), -1);
	if (ReadyFdCount == -1) {
	// Recover from error, if possible.
	if (errno == EAGAIN \|\| errno == EINTR)
	continue;
	return true;
	}

	// Holds all fds which have finished during this loop iteration.
	std::vector<int> FinishedFds;

	for (struct pollfd &fd : PollFds) {
	if (fd.revents & POLLIN \|\| fd.revents & POLLPRI \|\| fd.revents & POLLHUP \|\|
	fd.revents & POLLERR) {
	// An event which we care about occurred. Find the appropriate Task.
	auto predicate = [&fd](PidToTaskMap::value_type &value) -> bool {
	return value.second->getPipe() == fd.fd;
	};

	auto iter = std::find_if(ExecutingTasks.begin(), ExecutingTasks.end(),
	predicate);
	assert(iter != ExecutingTasks.end() &&
	"All outstanding fds must be associated with an executing Task");
	Task &T = *iter->second;
	if (fd.revents & POLLIN \|\| fd.revents & POLLPRI) {
	// There's data available to read.
	T.readFromPipes();
	}

	if (fd.revents & POLLHUP \|\| fd.revents & POLLERR) {
	// This fd was "hung up" or had an error, so we need to wait for the
	// Task and then clean up.
	pid_t Pid;
	int Status;
	do {
	Status = 0;
	Pid = waitpid(T.getPid(), &Status, 0);
	assert(Pid != 0 &&
	"We do not pass WNOHANG, so we should always get a pid");
	if (Pid < 0 && (errno == ECHILD \|\| errno == EINVAL))
	return true;
	} while (Pid < 0);

	assert(Pid == T.getPid() &&
	"We asked to wait for this Task, but we got another Pid!");

	T.finishExecution();

	if (WIFEXITED(Status)) {
	int Result = WEXITSTATUS(Status);

	if (Finished) {
	// If we have a TaskFinishedCallback, only set SubtaskFailed to
	// true if the callback returns StopExecution.
	SubtaskFailed = Finished(T.getPid(), Result, T.getOutput(),
	T.getErrors(), T.getContext()) ==
	TaskFinishedResponse::StopExecution;
	} else if (Result != 0) {
	// Since we don't have a TaskFinishedCallback, treat a subtask
	// which returned a nonzero exit code as having failed.
	SubtaskFailed = true;
	}
	} else if (WIFSIGNALED(Status)) {
	// The process exited due to a signal.
	int Signal = WTERMSIG(Status);

	StringRef ErrorMsg = strsignal(Signal);

	if (Signalled) {
	TaskFinishedResponse Response =
	Signalled(T.getPid(), ErrorMsg, T.getOutput(), T.getErrors(),
	T.getContext(), Signal);
	if (Response == TaskFinishedResponse::StopExecution)
	// If we have a TaskCrashedCallback, only set SubtaskFailed to
	// true if the callback returns StopExecution.
	SubtaskFailed = true;
	} else {
	// Since we don't have a TaskCrashedCallback, treat a crashing
	// subtask as having failed.
	SubtaskFailed = true;
	}
	}

	ExecutingTasks.erase(Pid);
	FinishedFds.push_back(fd.fd);
	}
	} else if (fd.revents & POLLNVAL) {
	// We passed an invalid fd; this should never happen,
	// since we always mark fds as finished after calling
	// Task::finishExecution() (which closes the Task's fd).
	llvm_unreachable("Asked poll() to watch a closed fd");
	}

	fd.revents = 0;
	}

	// Remove any fds which we've closed from PollFds.
	for (int fd : FinishedFds) {
	auto predicate = [&fd] (struct pollfd &i) {
	return i.fd == fd;
	};

	auto iter = std::find_if(PollFds.begin(), PollFds.end(), predicate);
	assert(iter != PollFds.end() && "The finished fd must be in PollFds!");
	PollFds.erase(iter);
	}
	}

	return SubtaskFailed;
	}