src/fuchsia/fuchsia-compat.c

// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include <assert.h>
#include <fcntl.h>
#include <pthread.h>
#include <pwd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>

#include <lib/fdio/fd.h>
#include <lib/fdio/io.h>
#include <lib/fdio/spawn.h>
#include <lib/fdio/unsafe.h>
#include <zircon/syscalls.h>
#include <zircon/syscalls/port.h>

// This is a very inefficient way to emulate select() by creating a port, adding all of the fds
// of interest as async waits, and blocking until we get a port packet back.
// The callers of this (like serverloop) generally have a static set of fds they care about, so
// it'd be much more efficient for them to register async waits on a port object that persists
// across blocking calls.
int fuchsia_select(int nfds, void* readfds, void* writefds, struct timeval *timeout) {
	fd_set* readfds_fd_set = (fd_set*) readfds;
	fd_set* writefds_fd_set = (fd_set*) writefds;

	int ret = 0;
	zx_handle_t port = ZX_HANDLE_INVALID;
	fdio_t* ios[FD_SETSIZE] = { 0 };

	// Create a fresh port for this wait.
	zx_status_t st = zx_port_create(0, &port);

	if (st != ZX_OK) {
		fprintf(stderr, "Can't allocate new port.\n");
		errno = EINVAL;
		ret = -1;
		goto cleanup;
	}


	// Register port waits for file descriptors in the read and write sets.
	for (int fd = 0; fd < nfds; ++fd) {
		uint32_t events = 0;
		if (readfds_fd_set && FD_ISSET(fd, readfds_fd_set)) {
			events |= POLLIN;
		}
		if (writefds_fd_set && FD_ISSET(fd, writefds_fd_set)) {
			events |= POLLOUT;
		}
		if (!events)
			continue;

		fdio_t* io;
		// This acquires a reference to the fdio which is released in the cleanup path below.
		if ((io = fdio_unsafe_fd_to_io(fd)) == NULL) {
			errno = EBADF;
			ret = -1;
			goto cleanup;
		}
		ios[fd] = io;
		zx_handle_t h;
		zx_signals_t sigs;
		// Translate the poll-style events to fdio-specific signal bits to wait on.
		fdio_unsafe_wait_begin(io, events, &h, &sigs);
		if (h == ZX_HANDLE_INVALID) {
			errno = EBADF;
			ret = -1;
			goto cleanup;
		}
		uint64_t key = fd;
		st = zx_object_wait_async(h, port, key, sigs, ZX_WAIT_ASYNC_ONCE);
		if (st != ZX_OK) {
			fprintf(stderr, "Can't wait on object %d.\n", st);
			errno = EINVAL;
			ret = -1;
			goto cleanup;
		}
	}

	zx_time_t deadline = (timeout == NULL) ? ZX_TIME_INFINITE :
			zx_deadline_after(ZX_SEC(timeout->tv_sec) + ZX_USEC(timeout->tv_usec));

	for (;;) {
		zx_port_packet_t packet;
		st = zx_port_wait(port, deadline, &packet);

		// We expect zx_port_wait to return either ZX_ERR_TIMED_OUT if nothing happened, or ZX_OK
		// if at least one thing happened.
		if (st == ZX_OK) {
			if (packet.type != ZX_PKT_TYPE_SIGNAL_ONE) {
				fprintf(stderr, "Unexpected port packet type %u\n", packet.type);
				errno = EINVAL;
				ret = -1;
				goto cleanup;
			}
			// We've heard about an fd in the set we care about. Update the read/write
			// sets to reflect this information, then remove them from the set we are
			// listening to.
			int fd = (int)packet.key;
			uint32_t events = 0;
			fdio_t* io = ios[fd];
			if (!io) {
				fprintf(stderr, "Can't find fd for packet key %d.\n", fd);
				errno = EINVAL;
				ret = -1;
				goto cleanup;
			}
			// fdio_unsafe_wait_end translates the signals back to poll-style flags.
			fdio_unsafe_wait_end(io, packet.signal.observed, &events);
			if (readfds_fd_set && FD_ISSET(fd, readfds_fd_set)) {
				if (events & POLLIN)
					ret++;
				else
					FD_CLR(fd, readfds_fd_set);
			}
			if (writefds_fd_set && FD_ISSET(fd, writefds_fd_set)) {
				if (events & POLLOUT)
					ret++;
				else
					FD_CLR(fd, writefds_fd_set);
			}
			// The read and write sets for this fd are now updated, and our wait has expired, so
			// remove this fd from the set of things we care about.
			ios[fd] = NULL;
			fdio_unsafe_release(io);
		} else if (st == ZX_ERR_TIMED_OUT) {
			break;
		} else {
			fprintf(stderr, "Port wait return unexpected error %d.\n", st);
			errno = EINVAL;
			ret = -1;
			goto cleanup;
		}

		// After pulling the first packet out, poll without blocking by doing another wait with a
		// deadline in the past. This will populate any other members of the read/write set that
		// are ready to go now.
		deadline = 0;
	}

	// If there are any entries left in ios at this point, we have not received a port packet
	// indicating that those fds are readable or writable and so we should clear those from the
	// read/write sets.
	for (int fd = 0; fd < nfds; ++fd) {
		if (ios[fd]) {
			if (readfds_fd_set && FD_ISSET(fd, readfds_fd_set)) {
				FD_CLR(fd, readfds_fd_set);
			}
			if (writefds_fd_set && FD_ISSET(fd, writefds_fd_set)) {
				FD_CLR(fd, writefds_fd_set);
			}
		}
	}

cleanup:
	// Release reference to any fdio objects we acquired with fdio_unsafe_fd_to_io().
	for (int fd = 0; fd < nfds; ++fd) {
		if (ios[fd]) {
			fdio_unsafe_release(ios[fd]);
		}
	}

	if (port != ZX_HANDLE_INVALID) {
		zx_handle_close(port);
	}
	return ret;
}