AP_HAL_Linux: Add Pollable/Poller

Add system's polling infrastructure to be notified whenever a
file descriptor is ready to be read from or written to.

Adds a few classes:
  * Poller, as an interface to epoll()
  * Pollable, as an interface to a file descriptor

libraries/AP_Common/missing/sys/epoll.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include_next <sys/epoll.h>
+
+#ifndef EPOLLWAKEUP
+#define EPOLLWAKEUP (1U<<29)
+#endif

libraries/AP_HAL_Linux/Poller.cpp

@@ -0,0 +1,334 @@
+/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+/*
+ * Copyright (C) 2016  Intel Corporation. All rights reserved.
+ *
+ * This file is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include <algorithm>
+#include <cstddef>
+#include <errno.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <stdint.h>
+#include <sys/epoll.h>
+#include <sys/types.h>
+#include <sys/uio.h>
+#include <unistd.h>
+
+#include "Poller.h"
+#include "Scheduler.h"
+
+extern const AP_HAL::HAL& hal;
+
+namespace Linux {
+
+uint32_t Poller::to_epoll_events(Poller::Event ev) {
+    // EPOLLWAKEUP prevents the system from hibernating or suspending when
+    // inside epoll_wait() for this particular event.  It is silently
+    // ignored if the process does not have the CAP_BLOCK_SUSPEND
+    // capability.
+    uint32_t op = EPOLLWAKEUP;
+
+    if (ev & Poller::Event::Read) {
+        op |= EPOLLIN;
+    }
+    if (ev & Poller::Event::Write) {
+        op |= EPOLLOUT;
+    }
+    if (ev & Poller::Event::Error) {
+        op |= EPOLLERR;
+    }
+
+    return op;
+}
+
+bool Poller::register_pollable(Pollable *p, const Poller::Event ev) {
+    if (_epfd < 0) {
+        return false;
+    }
+
+    struct epoll_event epev = {0};
+    epev.events = Poller::to_epoll_events(ev);
+    epev.data.ptr = static_cast<void *>(p);
+
+    return epoll_ctl(_epfd, EPOLL_CTL_ADD, p->get_fd(), &epev) == 0;
+}
+
+void Poller::unregister_pollable(const Pollable *p) {
+    if (_epfd >= 0) {
+        epoll_ctl(_epfd, EPOLL_CTL_DEL, p->get_fd(), NULL);
+    }
+}
+
+int Poller::poll(int timeout_ms) const {
+    const int max_events = 16;
+    epoll_event events[max_events];
+    const auto before_wait_us = AP_HAL::micros64();
+    const auto r = epoll_wait(_epfd, events, max_events, timeout_ms);
+    const auto delta_us = AP_HAL::micros64() - before_wait_us;
+    const auto delta_ms = delta_us / 1000;
+    const auto remaining_time_ms = timeout_ms - delta_ms;
+
+    if (r > 0) {
+        auto max_time_ms = remaining_time_ms / r;
+
+        if (!max_time_ms) {
+            max_time_ms = 1;
+        }
+
+        for (int i = 0; i < r; i++) {
+            Pollable *p = static_cast<Pollable *>(events[i].data.ptr);
+
+            if (events[i].events & EPOLLIN) {
+                p->on_can_read(max_time_ms);
+            }
+            if (events[i].events & EPOLLOUT) {
+                p->on_can_write(max_time_ms);
+            }
+            if (events[i].events & EPOLLERR) {
+                p->on_error(max_time_ms);
+            }
+            if (events[i].events & EPOLLHUP) {
+                p->on_hang_up(max_time_ms);
+            }
+        }
+    } else if (r < 0) {
+        if (errno == EINTR) {
+            // Try polling again with the remaining wait time.
+            return poll(remaining_time_ms);
+        }
+    }
+
+    return r;
+}
+
+Pollable::~Pollable() {
+    close(_fd);
+}
+
+bool Pollable::set_blocking(bool setting) {
+    auto curflags = fcntl(_fd, F_GETFL, 0);
+
+    if (curflags < 0) {
+        return false;
+    }
+
+    if (setting) {
+        curflags &= ~O_NONBLOCK;
+    } else {
+        curflags |= O_NONBLOCK;
+    }
+
+    return fcntl(_fd, F_SETFL, curflags) == 0;
+}
+
+void BufferedPollable::on_can_read(int max_time_ms) {
+    if (!_read_sem.take(max_time_ms)) {
+        return;
+    }
+
+    ByteBuffer::IoVec vec[2];
+    const auto n_vec = _read_buffer.reserve(vec, _read_buffer.space());
+    if (n_vec) {
+        struct iovec iovec[n_vec];
+        for (int i = 0; i < n_vec; i++) {
+            iovec[i].iov_base = static_cast<void *>(vec[i].data);
+            iovec[i].iov_len = static_cast<size_t>(vec[i].len);
+        }
+
+        (void) readv(_fd, iovec, n_vec);
+    }
+
+    _read_sem.give();
+}
+
+void BufferedPollable::on_hang_up(int max_time_ms) {
+    auto half_time_ms = max_time_ms / 2;
+
+    if (!_read_sem.take(half_time_ms)) {
+        return;
+    }
+
+    if (_write_sem.take(half_time_ms)) {
+        _read_buffer.advance(_read_buffer.available());
+        _write_buffer.advance(_write_buffer.available());
+        close(_fd);
+        _fd = -1;
+
+        _write_sem.give();
+    }
+
+    _read_sem.give();
+}
+
+void BufferedPollable::write_fd(uint32_t n_bytes) {
+    // NOTE: Must be called with _write_sem taken.
+
+    if (!n_bytes) {
+        return;
+    }
+
+    ByteBuffer::IoVec vec[2];
+    auto n_vec = _write_buffer.peekiovec(vec, n_bytes);
+    if (!n_vec) {
+        return;
+    }
+
+    struct iovec iovec[n_vec];
+    for (int i = 0; i < n_vec; i++) {
+        iovec[i].iov_base = static_cast<void *>(vec[i].data);
+        iovec[i].iov_len = static_cast<size_t>(vec[i].len);
+    }
+
+    auto written = writev(_fd, iovec, n_vec);
+    if (written > 0) {
+        _write_buffer.advance(static_cast<uint32_t>(written));
+    }
+}
+
+void BufferedPollable::on_can_write(int max_time_ms) {
+    if (_write_sem.take(max_time_ms)) {
+        write_fd(_write_buffer.available());
+        _write_sem.give();
+    }
+}
+
+bool BufferedPollable::is_write_buffer_empty() {
+    bool ret = false;
+
+    if (_write_sem.take_nonblocking()) {
+        ret = _write_buffer.available() > 0;
+        _write_sem.give();
+    }
+
+    return ret;
+}
+
+uint32_t BufferedPollable::get_read_buffer_available() {
+    uint32_t ret = 0;
+
+    if (_read_sem.take_nonblocking()) {
+        ret = _read_buffer.available();
+        _read_sem.give();
+    }
+
+    return ret;
+}
+
+uint32_t BufferedPollable::get_write_buffer_available() {
+    uint32_t ret = 0;
+
+    if (_write_sem.take_nonblocking()) {
+        ret = _write_buffer.available();
+        _write_sem.give();
+    }
+
+    return ret;
+}
+
+uint32_t BufferedPollable::buffered_read(uint8_t *ptr, uint32_t len) {
+    uint32_t ret = 0;
+    bool taken_sem;
+
+    if (_blocking_reads) {
+        taken_sem = _read_sem.take(100);
+    } else {
+        taken_sem = _read_sem.take_nonblocking();
+    }
+    if (taken_sem) {
+        ret = _read_buffer.read(ptr, len);
+        _read_sem.give();
+    }
+
+    return ret;
+}
+
+uint32_t BufferedPollable::buffered_write(const uint8_t *buf, uint32_t len) {
+    uint32_t ret = 0;
+    bool taken_sem;
+
+    if (_blocking_writes) {
+        taken_sem = _write_sem.take(100);
+    } else {
+        taken_sem = _write_sem.take_nonblocking();
+    }
+    if (taken_sem) {
+        if (_write_buffer.space() >= len) {
+            ret = _write_buffer.write(buf, len);
+        }
+        _write_sem.give();
+    }
+
+    return ret;
+}
+
+uint32_t PacketedBufferedPollable::to_mavlink_boundary(uint32_t available) {
+    // NOTE: Must be called with _write_sem taken.
+    const uint32_t mavlink_hdr_size = 8; // 6-byte header + 2-byte cksum.
+    const uint32_t mavlink_max_size = 256;
+    const uint8_t mavlink_marker = 254;
+
+    if (!available) {
+        return 0;
+    }
+
+    if (_write_buffer.peek(0) != mavlink_marker) {
+        // Non-mavlink packet at the start of the buffer.  Look ahead for a
+        // MAVLink start byte, up to 256 bytes ahead.
+        const auto limit = std::min(mavlink_max_size, available);
+        uint32_t contiguous_avail;
+        const uint8_t *ptr = _write_buffer.readptr(contiguous_avail);
+
+        if (contiguous_avail >= limit) {
+            // If there's enough contiguous data in the ring buffer, use a
+            // fast byte scan instead.  This should happen more often.
+            auto marker_pos = memchr(ptr, mavlink_marker, limit);
+            if (marker_pos) {
+                return static_cast<uint32_t>(
+                    static_cast<const uint8_t *>(marker_pos) - ptr);
+            }
+        } else {
+            for (uint32_t i = 0; i < limit; i++) {
+                if (_write_buffer.peek(i) == mavlink_marker) {
+                    return i;
+                }
+            }
+        }
+
+        // No MAVLink marker, limit the send size to mavlink_max_size.
+        return limit;
+    }
+
+    if (available < mavlink_hdr_size) {
+        return 0; // Not a full MAVLink packet yet.
+    }
+
+    // Possible MAVLink packet, just check if it is complete.
+    const auto pktlen = _write_buffer.peek(1); // Length is on 2nd byte.
+    if (pktlen == -1 || available < static_cast<uint8_t>(pktlen) + mavlink_hdr_size) {
+        return 0; // Not a full MAVLink packet yet.
+    }
+
+    // Packet seems complete. Send one at a time.
+    return pktlen + mavlink_hdr_size;
+}
+
+void PacketedBufferedPollable::on_can_write(int max_time_ms) {
+    if (_write_sem.take(max_time_ms)) {
+        write_fd(to_mavlink_boundary(_write_buffer.available()));
+        _write_sem.give();
+    }
+}
+
+}

libraries/AP_HAL_Linux/Poller.h

@@ -0,0 +1,111 @@
+/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
+/*
+ * Copyright (C) 2016  Intel Corporation. All rights reserved.
+ *
+ * This file is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <sys/epoll.h>
+#include <unistd.h>
+
+#include "AP_HAL/utility/RingBuffer.h"
+#include "Semaphores.h"
+
+namespace Linux {
+
+class Pollable {
+public:
+    Pollable(int fd) : _fd(fd) { }
+    virtual ~Pollable();
+
+    int get_fd() const { return _fd; }
+
+    bool set_blocking(bool setting);
+
+    virtual void on_can_read(int max_time_ms) {}
+    virtual void on_can_write(int max_time_ms) {}
+    virtual void on_error(int max_time_ms) {}
+    virtual void on_hang_up(int max_time_ms) {}
+
+protected:
+    int _fd;
+};
+
+class BufferedPollable : public Pollable {
+public:
+    BufferedPollable(int fd) : Pollable(fd) { }
+    BufferedPollable(int fd, uint32_t read_buffer_size, uint32_t write_buffer_size)
+        : Pollable(fd)
+        , _read_buffer{read_buffer_size}
+        , _write_buffer{write_buffer_size} { }
+
+    bool is_write_buffer_empty();
+    uint32_t get_read_buffer_available();
+    uint32_t get_write_buffer_available();
+
+    uint32_t buffered_read(uint8_t *buf, uint32_t len);
+    uint32_t buffered_write(const uint8_t *buf, uint32_t len);
+
+    void set_blocking_writes(bool setting) { _blocking_writes = setting; }
+    void set_blocking_reads(bool setting) { _blocking_reads = setting; }
+
+    virtual void on_can_read(int max_time_ms) override;
+    virtual void on_can_write(int max_time_ms) override;
+    virtual void on_hang_up(int max_time_ms) override;
+
+protected:
+    ByteBuffer _read_buffer{1024}, _write_buffer{1024};
+    Linux::Semaphore _write_sem, _read_sem;
+
+    void write_fd(uint32_t n_bytes);
+
+private:
+    bool _blocking_writes{false}, _blocking_reads{false};
+};
+
+class PacketedBufferedPollable : public BufferedPollable {
+private:
+    uint32_t to_mavlink_boundary(uint32_t available);
+
+public:
+    PacketedBufferedPollable(int fd) : BufferedPollable(fd) { }
+
+    virtual void on_can_write(int max_time_ms) override;
+};
+
+class Poller {
+public:
+    Poller() : _epfd(epoll_create1(EPOLL_CLOEXEC)) { }
+    ~Poller() { close(_epfd); }
+
+    enum Event : uint8_t {
+        Read = 1<<0,
+        Write = 1<<1,
+        Error = 1<<2,
+        All = Read | Write | Error
+    };
+
+    bool register_pollable(Pollable*, const Event);
+    void unregister_pollable(const Pollable*);
+
+    int poll(int timeout_ms) const;
+
+private:
+    int _epfd;
+
+    static uint32_t to_epoll_events(Event events);
+};
+
+}