Commit 675a137d authored by Alexey Spiridonov's avatar Alexey Spiridonov Committed by Praveen Kumar Ramakrishnan

Factor out pipe methods, add takeOwnershipOfPipes

Summary:
In order to create an EventBase'd Suprocess class, I'd like to be able to manage the lifetime of pipes independently of the lifetime of the process. To this effect, I factored out basic Pipe handling, and provided a function that detaches the pipe vector from the Subprocess object.

#6996492 a push-blocking test is broken in trunk

Test Plan: added a unit test, fbconfig -r folly && fbmake runtests && fbmake runtests_opt

Reviewed By: dancol@fb.com

Subscribers: yfeldblum, chalfant, dancol, wez, anarayanan, trunkagent, net-systems@, njormrod, folly-diffs@

FB internal diff: D1699969

Signature: t1:1699969:1430975299:30d291ab7fcc555edddf098b33095a5b29500e76
parent ec3c8f79
......@@ -96,6 +96,7 @@ std::string ProcessReturnCode::str() const {
(coreDumped() ? " (core dumped)" : ""));
}
CHECK(false); // unreached
return ""; // silence GCC warning
}
CalledProcessError::CalledProcessError(ProcessReturnCode rc)
......@@ -189,13 +190,9 @@ Subprocess::Subprocess(
Subprocess::~Subprocess() {
CHECK_NE(returnCode_.state(), ProcessReturnCode::RUNNING)
<< "Subprocess destroyed without reaping child";
closeAll();
}
namespace {
void closeChecked(int fd) {
checkUnixError(::close(fd), "close");
}
struct ChildErrorInfo {
int errCode;
......@@ -214,16 +211,9 @@ void childError(int errFd, int errCode, int errnoValue) {
} // namespace
void Subprocess::closeAll() {
for (auto& p : pipes_) {
closeChecked(p.parentFd);
}
pipes_.clear();
}
void Subprocess::setAllNonBlocking() {
for (auto& p : pipes_) {
int fd = p.parentFd;
int fd = p.pipe.fd();
int flags = ::fcntl(fd, F_GETFL);
checkUnixError(flags, "fcntl");
int r = ::fcntl(fd, F_SETFL, flags | O_NONBLOCK);
......@@ -244,12 +234,8 @@ void Subprocess::spawn(
// Make a copy, we'll mutate options
Options options(optionsIn);
// On error, close all of the pipes_
auto pipesGuard = makeGuard([&] {
for (auto& p : this->pipes_) {
CHECK_ERR(::close(p.parentFd));
}
});
// On error, close all pipes_ (ignoring errors, but that seems fine here).
auto pipesGuard = makeGuard([this] { pipes_.clear(); });
// Create a pipe to use to receive error information from the child,
// in case it fails before calling exec()
......@@ -325,6 +311,9 @@ void Subprocess::spawnInternal(
// doesn't need to reset the flag on its end, as we always dup2() the fd,
// and dup2() fds don't share the close-on-exec flag.
#if FOLLY_HAVE_PIPE2
// If possible, set close-on-exec atomically. Otherwise, a concurrent
// Subprocess invocation can fork() between "pipe" and "fnctl",
// causing FDs to leak.
r = ::pipe2(fds, O_CLOEXEC);
checkUnixError(r, "pipe2");
#else
......@@ -335,21 +324,21 @@ void Subprocess::spawnInternal(
r = fcntl(fds[1], F_SETFD, FD_CLOEXEC);
checkUnixError(r, "set FD_CLOEXEC");
#endif
PipeInfo pinfo;
pinfo.direction = p.second;
pipes_.emplace_back();
Pipe& pipe = pipes_.back();
pipe.direction = p.second;
int cfd;
if (p.second == PIPE_IN) {
// Child gets reading end
pinfo.parentFd = fds[1];
pipe.pipe = folly::File(fds[1], /*owns_fd=*/ true);
cfd = fds[0];
} else {
pinfo.parentFd = fds[0];
pipe.pipe = folly::File(fds[0], /*owns_fd=*/ true);
cfd = fds[1];
}
p.second = cfd; // ensure it gets dup2()ed
pinfo.childFd = p.first;
pipe.childFd = p.first;
childFds.push_back(cfd);
pipes_.push_back(pinfo);
}
}
......@@ -543,6 +532,8 @@ ProcessReturnCode Subprocess::poll() {
pid_t found = ::waitpid(pid_, &status, WNOHANG);
checkUnixError(found, "waitpid");
if (found != 0) {
// Though the child process had quit, this call does not close the pipes
// since its descendants may still be using them.
returnCode_ = ProcessReturnCode(status);
pid_ = -1;
}
......@@ -566,6 +557,8 @@ ProcessReturnCode Subprocess::wait() {
found = ::waitpid(pid_, &status, 0);
} while (found == -1 && errno == EINTR);
checkUnixError(found, "waitpid");
// Though the child process had quit, this call does not close the pipes
// since its descendants may still be using them.
DCHECK_EQ(found, pid_);
returnCode_ = ProcessReturnCode(status);
pid_ = -1;
......@@ -709,12 +702,14 @@ std::pair<IOBufQueue, IOBufQueue> Subprocess::communicateIOBuf(
void Subprocess::communicate(FdCallback readCallback,
FdCallback writeCallback) {
// This serves to prevent wait() followed by communicate(), but if you
// legitimately need that, send a patch to delete this line.
returnCode_.enforce(ProcessReturnCode::RUNNING);
setAllNonBlocking();
std::vector<pollfd> fds;
fds.reserve(pipes_.size());
std::vector<int> toClose;
std::vector<size_t> toClose; // indexes into pipes_
toClose.reserve(pipes_.size());
while (!pipes_.empty()) {
......@@ -723,7 +718,7 @@ void Subprocess::communicate(FdCallback readCallback,
for (auto& p : pipes_) {
pollfd pfd;
pfd.fd = p.parentFd;
pfd.fd = p.pipe.fd();
// Yes, backwards, PIPE_IN / PIPE_OUT are defined from the
// child's point of view.
if (!p.enabled) {
......@@ -746,13 +741,14 @@ void Subprocess::communicate(FdCallback readCallback,
for (size_t i = 0; i < pipes_.size(); ++i) {
auto& p = pipes_[i];
DCHECK_EQ(fds[i].fd, p.parentFd);
auto parentFd = p.pipe.fd();
DCHECK_EQ(fds[i].fd, parentFd);
short events = fds[i].revents;
bool closed = false;
if (events & POLLOUT) {
DCHECK(!(events & POLLIN));
if (writeCallback(p.parentFd, p.childFd)) {
if (writeCallback(parentFd, p.childFd)) {
toClose.push_back(i);
closed = true;
}
......@@ -762,7 +758,7 @@ void Subprocess::communicate(FdCallback readCallback,
// on) end of file
if (events & (POLLIN | POLLHUP)) {
DCHECK(!(events & POLLOUT));
if (readCallback(p.parentFd, p.childFd)) {
if (readCallback(parentFd, p.childFd)) {
toClose.push_back(i);
closed = true;
}
......@@ -777,7 +773,7 @@ void Subprocess::communicate(FdCallback readCallback,
// Close the fds in reverse order so the indexes hold after erase()
for (int idx : boost::adaptors::reverse(toClose)) {
auto pos = pipes_.begin() + idx;
closeChecked(pos->parentFd);
pos->pipe.close(); // Throws on error
pipes_.erase(pos);
}
}
......@@ -791,10 +787,10 @@ bool Subprocess::notificationsEnabled(int childFd) const {
return pipes_[findByChildFd(childFd)].enabled;
}
int Subprocess::findByChildFd(int childFd) const {
size_t Subprocess::findByChildFd(int childFd) const {
auto pos = std::lower_bound(
pipes_.begin(), pipes_.end(), childFd,
[] (const PipeInfo& info, int fd) { return info.childFd < fd; });
[] (const Pipe& pipe, int fd) { return pipe.childFd < fd; });
if (pos == pipes_.end() || pos->childFd != childFd) {
throw std::invalid_argument(folly::to<std::string>(
"child fd not found ", childFd));
......@@ -804,10 +800,19 @@ int Subprocess::findByChildFd(int childFd) const {
void Subprocess::closeParentFd(int childFd) {
int idx = findByChildFd(childFd);
closeChecked(pipes_[idx].parentFd);
pipes_[idx].pipe.close(); // May throw
pipes_.erase(pipes_.begin() + idx);
}
std::vector<Subprocess::ChildPipe> Subprocess::takeOwnershipOfPipes() {
std::vector<Subprocess::ChildPipe> pipes;
for (auto& p : pipes_) {
pipes.emplace_back(ChildPipe{p.childFd, std::move(p.pipe)});
}
pipes_.clear();
return pipes;
}
namespace {
class Initializer {
......
......@@ -26,30 +26,69 @@
* output, and standard error to/from child descriptors in the parent,
* or to create communication pipes between the child and the parent.
*
* The simplest example is a thread-safe version of the system() library
* The simplest example is a thread-safe [1] version of the system() library
* function:
* Subprocess(cmd).wait();
* which executes the command using the default shell and waits for it
* to complete, returning the exit status.
*
* A thread-safe version of popen() (type="r", to read from the child):
* A thread-safe [1] version of popen() (type="r", to read from the child):
* Subprocess proc(cmd, Subprocess::pipeStdout());
* // read from proc.stdout()
* proc.wait();
*
* A thread-safe version of popen() (type="w", to write to the child):
* A thread-safe [1] version of popen() (type="w", to write to the child):
* Subprocess proc(cmd, Subprocess::pipeStdin());
* // write to proc.stdin()
* proc.wait();
*
* If you want to redirect both stdin and stdout to pipes, you can, but
* note that you're subject to a variety of deadlocks. You'll want to use
* nonblocking I/O; look at the implementation of communicate() for an example.
* If you want to redirect both stdin and stdout to pipes, you can, but note
* that you're subject to a variety of deadlocks. You'll want to use
* nonblocking I/O, like the callback version of communicate().
*
* communicate() is a way to communicate to a child via its standard input,
* standard output, and standard error. It buffers everything in memory,
* so it's not great for large amounts of data (or long-running processes),
* but it insulates you from the deadlocks mentioned above.
* The string or IOBuf-based variants of communicate() are the simplest way
* to communicate with a child via its standard input, standard output, and
* standard error. They buffer everything in memory, so they are not great
* for large amounts of data (or long-running processes), but they are much
* simpler than the callback version.
*
* == A note on thread-safety ==
*
* [1] "thread-safe" refers ONLY to the fact that Subprocess is very careful
* to fork in a way that does not cause grief in multithreaded programs.
*
* Caveat: If your system does not have the atomic pipe2 system call, it is
* not safe to concurrently call Subprocess from different threads.
* Therefore, it is best to have a single thread be responsible for spawning
* subprocesses.
*
* A particular instances of Subprocess is emphatically **not** thread-safe.
* If you need to simultaneously communicate via the pipes, and interact
* with the Subprocess state, your best bet is to:
* - takeOwnershipOfPipes() to separate the pipe I/O from the subprocess.
* - Only interact with the Subprocess from one thread at a time.
*
* The current implementation of communicate() cannot be safely interrupted.
* To do so correctly, one would need to use EventFD, or open a dedicated
* pipe to be messaged from a different thread -- in particular, kill() will
* not do, since a descendant may keep the pipes open indefinitely.
*
* So, once you call communicate(), you must wait for it to return, and not
* touch the pipes from other threads. closeParentFd() is emphatically
* unsafe to call concurrently, and even sendSignal() is not a good idea.
* You can perhaps give the Subprocess's PID to a different thread before
* starting communicate(), and use that PID to send a signal without
* accessing the Subprocess object. In that case, you will need a mutex
* that ensures you don't wait() before you sent said signal. In a
* nutshell, don't do this.
*
* In fact, signals are inherently concurrency-unsafe on Unix: if you signal
* a PID, while another thread is in waitpid(), the signal may fire either
* before or after the process is reaped. This means that your signal can,
* in pathological circumstances, be delivered to the wrong process (ouch!).
* To avoid this, you should only use non-blocking waits (i.e. poll()), and
* make sure to serialize your signals (i.e. kill()) with the waits --
* either wait & signal from the same thread, or use a mutex.
*/
#ifndef FOLLY_SUBPROCESS_H_
#define FOLLY_SUBPROCESS_H_
......@@ -70,6 +109,7 @@
#include <boost/operators.hpp>
#include <boost/noncopyable.hpp>
#include <folly/File.h>
#include <folly/FileUtil.h>
#include <folly/gen/String.h>
#include <folly/io/IOBufQueue.h>
......@@ -336,6 +376,67 @@ class Subprocess : private boost::noncopyable {
const Options& options = Options(),
const std::vector<std::string>* env = nullptr);
////
//// The methods below only manipulate the process state, and do not
//// affect its communication pipes.
////
/**
* Return the child's pid, or -1 if the child wasn't successfully spawned
* or has already been wait()ed upon.
*/
pid_t pid() const;
/**
* Return the child's status (as per wait()) if the process has already
* been waited on, -1 if the process is still running, or -2 if the
* process hasn't been successfully started. NOTE that this does not call
* waitpid() or Subprocess::poll(), but simply returns the status stored
* in the Subprocess object.
*/
ProcessReturnCode returnCode() const { return returnCode_; }
/**
* Poll the child's status and return it, return -1 if the process
* is still running. NOTE that it is illegal to call poll again after
* poll indicated that the process has terminated, or to call poll on a
* process that hasn't been successfully started (the constructor threw an
* exception).
*/
ProcessReturnCode poll();
/**
* Poll the child's status. If the process is still running, return false.
* Otherwise, return true if the process exited with status 0 (success),
* or throw CalledProcessError if the process exited with a non-zero status.
*/
bool pollChecked();
/**
* Wait for the process to terminate and return its status.
* Similarly to poll, it is illegal to call wait after the process
* has already been reaped or if the process has not successfully started.
*/
ProcessReturnCode wait();
/**
* Wait for the process to terminate, throw if unsuccessful.
*/
void waitChecked();
/**
* Send a signal to the child. Shortcuts for the commonly used Unix
* signals are below.
*/
void sendSignal(int signal);
void terminate() { sendSignal(SIGTERM); }
void kill() { sendSignal(SIGKILL); }
////
//// The methods below only affect the process's communication pipes, but
//// not its return code or state (they do not poll() or wait()).
////
/**
* Communicate with the child until all pipes to/from the child are closed.
*
......@@ -370,6 +471,8 @@ class Subprocess : private boost::noncopyable {
/**
* Communicate with the child until all pipes to/from the child are closed.
*
* == Semantics ==
*
* readCallback(pfd, cfd) will be called whenever there's data available
* on any pipe *from* the child (PIPE_OUT). pfd is the file descriptor
* in the parent (that you use to read from); cfd is the file descriptor
......@@ -387,25 +490,50 @@ class Subprocess : private boost::noncopyable {
* child, or make its writes fail with EPIPE, so you should generally
* avoid returning true unless you've reached end-of-file.
*
* NOTE that you MUST consume all data passed to readCallback (or return
* true to close the pipe). Similarly, you MUST write to a writable pipe
* (or return true to close the pipe). To do otherwise is an error that
* can result in a deadlock. You must do this even for pipes you are not
* communicate() returns when all pipes to/from the child are closed; the
* child might stay alive after that, so you must still wait().
* Conversely, the child may quit long before its pipes are closed, since
* its descendants can keep them alive forever.
*
* Most users won't need to use this callback version; the simpler version
* of communicate (which buffers data in memory) will probably work fine.
*
* == Things you must get correct ==
*
* 1) You MUST consume all data passed to readCallback (or return true to
* close the pipe). Similarly, you MUST write to a writable pipe (or
* return true to close the pipe). To do otherwise is an error that can
* result in a deadlock. You must do this even for pipes you are not
* interested in.
*
* Note that pfd is nonblocking, so be prepared for read() / write() to
* return -1 and set errno to EAGAIN (in which case you should return
* false). Use readNoInt() from FileUtil.h to handle interrupted reads
* for you
* 2) pfd is nonblocking, so be prepared for read() / write() to return -1
* and set errno to EAGAIN (in which case you should return false). Use
* readNoInt() from FileUtil.h to handle interrupted reads for you.
*
* Note that communicate() returns when all pipes to/from the child are
* closed; the child might stay alive after that, so you must still wait().
* 3) Your callbacks MUST NOT call any of the Subprocess methods that
* manipulate the pipe FDs. Check the docblocks, but, for example,
* neither closeParentFd (return true instead) nor takeOwnershipOfPipes
* are safe. Stick to reading/writing from pfd, as appropriate.
*
* Most users won't need to use this; the simpler version of communicate
* (which buffers data in memory) will probably work fine.
* == Good to know ==
*
* See ReadLinesCallback for an easy way to consume the child's output
* 1) See ReadLinesCallback for an easy way to consume the child's output
* streams line-by-line (or tokenized by another delimiter).
*
* 2) "Wait until the descendants close the pipes" is usually the behavior
* you want, since the descendants may have something to say even if the
* immediate child is dead. If you need to be able to force-close all
* parent FDs, communicate() will NOT work for you. Do it your own way by
* using takeOwnershipOfPipes().
*
* Why not? You can return "true" from your callbacks to sever active
* pipes, but inactive ones can remain open indefinitely. It is
* impossible to safely close inactive pipes while another thread is
* blocked in communicate(). This is BY DESIGN. Racing communicate()'s
* read/write callbacks can result in wrong I/O and data corruption. This
* class would need internal synchronization and timeouts, a poor and
* expensive implementation choice, in order to make closeParentFd()
* thread-safe.
*/
typedef std::function<bool(int, int)> FdCallback;
void communicate(FdCallback readCallback, FdCallback writeCallback);
......@@ -510,61 +638,39 @@ class Subprocess : private boost::noncopyable {
}
/**
* Enable notifications (callbacks) for one pipe to/from child. By default,
* all are enabled. Useful for "chatty" communication -- you want to disable
* write callbacks until you receive the expected message.
* communicate() callbacks can use this to temporarily enable/disable
* notifications (callbacks) for a pipe to/from the child. By default,
* all are enabled. Useful for "chatty" communication -- you want to
* disable write callbacks until you receive the expected message.
*
* Disabling a pipe does not free you from the requirement to consume all
* incoming data. Failing to do so will easily create deadlock bugs.
*
* Throws if the childFd is not known.
*/
void enableNotifications(int childFd, bool enabled);
/**
* Are notifications for one pipe to/from child enabled?
* Are notifications for one pipe to/from child enabled? Throws if the
* childFd is not known.
*/
bool notificationsEnabled(int childFd) const;
/**
* Return the child's pid, or -1 if the child wasn't successfully spawned
* or has already been wait()ed upon.
*/
pid_t pid() const;
/**
* Return the child's status (as per wait()) if the process has already
* been waited on, -1 if the process is still running, or -2 if the process
* hasn't been successfully started. NOTE that this does not poll, but
* returns the status stored in the Subprocess object.
*/
ProcessReturnCode returnCode() const { return returnCode_; }
/**
* Poll the child's status and return it, return -1 if the process
* is still running. NOTE that it is illegal to call poll again after
* poll indicated that the process has terminated, or to call poll on a
* process that hasn't been successfully started (the constructor threw an
* exception).
*/
ProcessReturnCode poll();
/**
* Poll the child's status. If the process is still running, return false.
* Otherwise, return true if the process exited with status 0 (success),
* or throw CalledProcessError if the process exited with a non-zero status.
*/
bool pollChecked();
/**
* Wait for the process to terminate and return its status.
* Similarly to poll, it is illegal to call wait after the process
* has already been reaped or if the process has not successfully started.
*/
ProcessReturnCode wait();
////
//// The following methods are meant for the cases when communicate() is
//// not suitable. You should not need them when you call communicate(),
//// and, in fact, it is INHERENTLY UNSAFE to use closeParentFd() or
//// takeOwnershipOfPipes() from a communicate() callback.
////
/**
* Wait for the process to terminate, throw if unsuccessful.
* Close the parent file descriptor given a file descriptor in the child.
* DO NOT USE from communicate() callbacks; make them return true instead.
*/
void waitChecked();
void closeParentFd(int childFd);
/**
* Set all pipes from / to child non-blocking. communicate() does
* Set all pipes from / to child to be non-blocking. communicate() does
* this for you.
*/
void setAllNonBlocking();
......@@ -572,27 +678,33 @@ class Subprocess : private boost::noncopyable {
/**
* Get parent file descriptor corresponding to the given file descriptor
* in the child. Throws if childFd isn't a pipe (PIPE_IN / PIPE_OUT).
* Do not close() the return file descriptor; use closeParentFd, below.
* Do not close() the returned file descriptor; use closeParentFd, above.
*/
int parentFd(int childFd) const {
return pipes_[findByChildFd(childFd)].parentFd;
return pipes_[findByChildFd(childFd)].pipe.fd();
}
int stdin() const { return parentFd(0); }
int stdout() const { return parentFd(1); }
int stderr() const { return parentFd(2); }
/**
* Close the parent file descriptor given a file descriptor in the child.
*/
void closeParentFd(int childFd);
/**
* Send a signal to the child. Shortcuts for the commonly used Unix
* signals are below.
* The child's pipes are logically separate from the process metadata
* (they may even be kept alive by the child's descendants). This call
* lets you manage the pipes' lifetime separetely from the lifetime of the
* child process.
*
* After this call, the Subprocess instance will have no knowledge of
* these pipes, and the caller assumes responsibility for managing their
* lifetimes. Pro-tip: prefer to explicitly close() the pipes, since
* folly::File would otherwise silently suppress I/O errors.
*
* No, you may NOT call this from a communicate() callback.
*/
void sendSignal(int signal);
void terminate() { sendSignal(SIGTERM); }
void kill() { sendSignal(SIGKILL); }
struct ChildPipe {
int childFd;
folly::File pipe; // Owns the parent FD
};
std::vector<ChildPipe> takeOwnershipOfPipes();
private:
static const int RV_RUNNING = ProcessReturnCode::RV_RUNNING;
......@@ -629,34 +741,40 @@ class Subprocess : private boost::noncopyable {
*/
void readChildErrorPipe(int pfd, const char* executable);
/**
* Close all file descriptors.
*/
void closeAll();
// Returns an index into pipes_. Throws std::invalid_argument if not found.
size_t findByChildFd(const int childFd) const;
// return index in pipes_
int findByChildFd(int childFd) const;
pid_t pid_;
ProcessReturnCode returnCode_;
// The number of pipes between parent and child is assumed to be small,
// so we're happy with a vector here, even if it means linear erase.
// sorted by childFd
struct PipeInfo : private boost::totally_ordered<PipeInfo> {
int parentFd = -1;
int childFd = -1;
int direction = PIPE_IN; // one of PIPE_IN / PIPE_OUT
bool enabled = true;
bool operator<(const PipeInfo& other) const {
/**
* Represents a pipe between this process, and the child process (or its
* descendant). To interact with these pipes, you can use communicate(),
* or use parentFd() and related methods, or separate them from the
* Subprocess instance entirely via takeOwnershipOfPipes().
*/
struct Pipe : private boost::totally_ordered<Pipe> {
folly::File pipe; // Our end of the pipe, wrapped in a File to auto-close.
int childFd = -1; // Identifies the pipe: what FD is this in the child?
int direction = PIPE_IN; // one of PIPE_IN / PIPE_OUT
bool enabled = true; // Are notifications enabled in communicate()?
bool operator<(const Pipe& other) const {
return childFd < other.childFd;
}
bool operator==(const PipeInfo& other) const {
bool operator==(const Pipe& other) const {
return childFd == other.childFd;
}
};
std::vector<PipeInfo> pipes_;
// Populated at process start according to fdActions, empty after
// takeOwnershipOfPipes(). Sorted by childFd. Can only have elements
// erased, but not inserted, after being populated.
//
// The number of pipes between parent and child is assumed to be small,
// so we're happy with a vector here, even if it means linear erase.
std::vector<Pipe> pipes_;
};
inline Subprocess::Options& Subprocess::Options::operator|=(
......
......@@ -444,3 +444,21 @@ TEST(CommunicateSubprocessTest, Chatty) {
EXPECT_EQ(0, proc.wait().exitStatus());
});
}
TEST(CommunicateSubprocessTest, TakeOwnershipOfPipes) {
std::vector<Subprocess::ChildPipe> pipes;
{
Subprocess proc(
"echo $'oh\\nmy\\ncat' | wc -l &", Subprocess::pipeStdout()
);
pipes = proc.takeOwnershipOfPipes();
proc.waitChecked();
}
EXPECT_EQ(1, pipes.size());
EXPECT_EQ(1, pipes[0].childFd);
char buf[10];
EXPECT_EQ(2, readFull(pipes[0].pipe.fd(), buf, 10));
buf[2] = 0;
EXPECT_EQ("3\n", std::string(buf));
}
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