shared: add generic IPC barrier

The "Barrier" object is a simple inter-process barrier implementation. It
allows placing synchronization points and waiting for the other side to
reach it. Additionally, it has an abortion-mechanism as second-layer
synchronization to send abortion-events asynchronously to the other side.

The API is usually used to synchronize processes during fork(). However,
it can be extended to pass state through execve() so you could synchronize
beyond execve().

Usually, it's used like this (error-handling replaced by assert() for
simplicity):

    Barrier b;

    r = barrier_init(&b);
    assert_se(r >= 0);

    pid = fork();
    assert_se(pid >= 0);
    if (pid == 0) {
            barrier_set_role(&b, BARRIER_CHILD);

            ...do child post-setup...
            if (CHILD_SETUP_FAILED)
                       exit(1);
            ...child setup done...

            barrier_place(&b);
            if (!barrier_sync(&b)) {
                    /* parent setup failed */
                    exit(1);
            }

            barrier_destroy(&b); /* redundant as execve() and exit() imply this */

            /* parent & child setup successful */
            execve(...);
    }

    barrier_set_role(&b, BARRIER_PARENT);

    ...do parent post-setup...
    if (PARENT_SETUP_FAILED) {
            barrier_abort(&b);          /* send abortion event */
            barrier_wait_abortion(&b);  /* wait for child to abort (exit() implies abortion) */
            barrier_destroy(&b);
           ...bail out...
    }
    ...parent setup done...

    barrier_place(&b);
    if (!barrier_sync(&b)) {
            ...child setup failed... ;
            barrier_destroy(&b);
            ...bail out...
    }

    barrier_destroy(&b);

    ...child setup successfull...

This is the most basic API. Using barrier_place() to place barriers and
barrier_sync() to perform a full synchronization between both processes.
barrier_abort() places an abortion barrier which superceeds any other
barriers, exit() (or barrier_destroy()) places an abortion-barrier that
queues behind existing barriers (thus *not* replacing existing barriers
unlike barrier_abort()).

This example uses hard-synchronization with wait_abortion(), sync() and
friends. These are all optional. Barriers are highly dynamic and can be
used for one-way synchronization or even no synchronization at all
(postponing it for later). The sync() call performs a full two-way
synchronization.

The API is documented and should be fairly self-explanatory. A test-suite
shows some special semantics regarding abortion, wait_next() and exit().

Internally, barriers use two eventfds and a pipe. The pipe is used to
detect exit()s of the remote side as eventfds do not allow that. The
eventfds are used to place barriers, one for each side. Barriers itself
are numbered, but the numbers are reused once both sides reached the same
barrier, thus you cannot address barriers by the index. Moreover, the
numbering is implicit and we only store a counter. This makes the
implementation itself very lightweight, which is probably negligible
considering that we need 3 FDs for a barrier..

Last but not least: This barrier implementation is quite heavy. It's
definitely not meant for fast IPC synchronization. However, it's very easy
to use. And given the *HUGE* overhead of fork(), the barrier-overhead
should be negligible.

2 files changed, 532 insertions, 0 deletions

diff --git a/src/shared/barrier.c b/src/shared/barrier.c
new file mode 100644
index 0000000000..c198329cb0
--- /dev/null
+++ b/src/shared/barrier.c
@@ -0,0 +1,440 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 David Herrmann <dh.herrmann@gmail.com>
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd 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
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <poll.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/eventfd.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "barrier.h"
+#include "macro.h"
+#include "util.h"
+
+/**
+ * Barriers
+ * This barrier implementation provides a simple synchronization method based
+ * on file-descriptors that can safely be used between threads and processes. A
+ * barrier object contains 2 shared counters based on eventfd. Both processes
+ * can now place barriers and wait for the other end to reach a random or
+ * specific barrier.
+ * Barriers are numbered, so you can either wait for the other end to reach any
+ * barrier or the last barrier that you placed. This way, you can use barriers
+ * for one-way *and* full synchronization. Note that even-though barriers are
+ * numbered, these numbers are internal and recycled once both sides reached the
+ * same barrier (implemented as a simple signed counter). It is thus not
+ * possible to address barriers by their ID.
+ *
+ * Barrier-API: Both ends can place as many barriers via barrier_place() as
+ * they want and each pair of barriers on both sides will be implicitly linked.
+ * Each side can use the barrier_wait/sync_*() family of calls to wait for the
+ * other side to place a specific barrier. barrier_wait_next() waits until the
+ * other side calls barrier_place(). No links between the barriers are
+ * considered and this simply serves as most basic asynchronous barrier.
+ * barrier_sync_next() is like barrier_wait_next() and waits for the other side
+ * to place their next barrier via barrier_place(). However, it only waits for
+ * barriers that are linked to a barrier we already placed. If the other side
+ * already placed more barriers than we did, barrier_sync_next() returns
+ * immediately.
+ * barrier_sync() extends barrier_sync_next() and waits until the other end
+ * placed as many barriers via barrier_place() as we did. If they already placed
+ * as many as we did (or more), it returns immediately.
+ *
+ * Additionally to basic barriers, an abortion event is available.
+ * barrier_abort() places an abortion event that cannot be undone. An abortion
+ * immediately cancels all placed barriers and replaces them. Any running and
+ * following wait/sync call besides barrier_wait_abortion() will immediately
+ * return false on both sides (otherwise, they always return true).
+ * barrier_abort() can be called multiple times on both ends and will be a
+ * no-op if already called on this side.
+ * barrier_wait_abortion() can be used to wait for the other side to call
+ * barrier_abort() and is the only wait/sync call that does not return
+ * immediately if we aborted outself. It only returns once the other side
+ * called barrier_abort().
+ *
+ * Barriers can be used for in-process and inter-process synchronization.
+ * However, for in-process synchronization you could just use mutexes.
+ * Therefore, main target is IPC and we require both sides to *not* share the FD
+ * table. If that's given, barriers provide target tracking: If the remote side
+ * exit()s, an abortion event is implicitly queued on the other side. This way,
+ * a sync/wait call will be woken up if the remote side crashed or exited
+ * unexpectedly. However, note that these abortion events are only queued if the
+ * barrier-queue has been drained. Therefore, it is safe to place a barrier and
+ * exit. The other side can safely wait on the barrier even though the exit
+ * queued an abortion event. Usually, the abortion event would overwrite the
+ * barrier, however, that's not true for exit-abortion events. Those are only
+ * queued if the barrier-queue is drained (thus, the receiving side has placed
+ * more barriers than the remote side).
+ */
+
+/**
+ * barrier_init() - Initialize a barrier object
+ * @obj: barrier to initialize
+ *
+ * This initializes a barrier object. The caller is responsible of allocating
+ * the memory and keeping it valid. The memory does not have to be zeroed
+ * beforehand.
+ * Two eventfd objects are allocated for each barrier. If allocation fails, an
+ * error is returned.
+ *
+ * If this function fails, the barrier is reset to an invalid state so it is
+ * safe to call barrier_destroy() on the object regardless whether the
+ * initialization succeeded or not.
+ *
+ * The caller is responsible to destroy the object via barrier_destroy() before
+ * releasing the underlying memory.
+ *
+ * Returns: 0 on success, negative error code on failure.
+ */
+int barrier_init(Barrier *obj) {
+        _cleanup_(barrier_destroy) Barrier b = { };
+        int r;
+
+        assert_return(obj, -EINVAL);
+
+        b.me = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
+        if (b.me < 0)
+                return -errno;
+
+        b.them = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
+        if (b.them < 0)
+                return -errno;
+
+        r = pipe2(b.pipe, O_CLOEXEC | O_NONBLOCK);
+        if (r < 0)
+                return -errno;
+
+        memcpy(obj, &b, sizeof(b));
+        zero(b);
+        return 0;
+}
+
+/**
+ * barrier_destroy() - Destroy a barrier object
+ * @b: barrier to destroy or NULL
+ *
+ * This destroys a barrier object that has previously been initialized via
+ * barrier_init(). The object is released and reset to invalid state.
+ * Therefore, it is safe to call barrier_destroy() multiple times or even if
+ * barrier_init() failed. However, you must not call barrier_destroy() if you
+ * never called barrier_init() on the object before.
+ *
+ * It is safe to initialize a barrier via zero() / memset(.., 0, ...). Even
+ * though it has embedded FDs, barrier_destroy() can deal with zeroed objects
+ * just fine.
+ *
+ * If @b is NULL, this is a no-op.
+ */
+void barrier_destroy(Barrier *b) {
+        if (!b)
+                return;
+
+        /* @me and @them cannot be both FD 0. Lets be pedantic and check the
+         * pipes and barriers, too. If all are 0, the object was zero()ed and
+         * is invalid. This allows users to use zero(barrier) to reset the
+         * backing memory. */
+        if (b->me == 0 &&
+            b->them == 0 &&
+            b->pipe[0] == 0 &&
+            b->pipe[1] == 0 &&
+            b->barriers == 0)
+                return;
+
+        b->me = safe_close(b->me);
+        b->them = safe_close(b->them);
+        b->pipe[0] = safe_close(b->pipe[0]);
+        b->pipe[1] = safe_close(b->pipe[1]);
+        b->barriers = 0;
+}
+
+/**
+ * barrier_set_role() - Set the local role of the barrier
+ * @b: barrier to operate on
+ * @role: role to set on the barrier
+ *
+ * This sets the roles on a barrier object. This is needed to know which
+ * side of the barrier you're on. Usually, the parent creates the barrier via
+ * barrier_init() and then calls fork() or clone(). Therefore, the FDs are
+ * duplicated and the child retains the same barrier object.
+ *
+ * Both sides need to call barrier_set_role() after fork() or clone() are done.
+ * If this is not done, barriers will not work correctly.
+ *
+ * Note that barriers could be supported without fork() or clone(). However,
+ * this is currently not needed so it hasn't been implemented.
+ */
+void barrier_set_role(Barrier *b, unsigned int role) {
+        int fd;
+
+        assert(b);
+        assert(role == BARRIER_PARENT || role == BARRIER_CHILD);
+        /* make sure this is only called once */
+        assert(b->pipe[1] >= 0 && b->pipe[1] >= 0);
+
+        if (role == BARRIER_PARENT) {
+                b->pipe[1] = safe_close(b->pipe[1]);
+        } else {
+                b->pipe[0] = safe_close(b->pipe[0]);
+
+                /* swap me/them for children */
+                fd = b->me;
+                b->me = b->them;
+                b->them = fd;
+        }
+}
+
+/* places barrier; returns false if we aborted, otherwise true */
+static bool barrier_write(Barrier *b, uint64_t buf) {
+        ssize_t len;
+
+        /* prevent new sync-points if we already aborted */
+        if (barrier_i_aborted(b))
+                return false;
+
+        do {
+                len = write(b->me, &buf, sizeof(buf));
+        } while (len < 0 && (errno == EAGAIN || errno == EINTR));
+
+        if (len != sizeof(buf))
+                goto error;
+
+        /* lock if we aborted */
+        if (buf >= (uint64_t)BARRIER_ABORTION) {
+                if (barrier_they_aborted(b))
+                        b->barriers = BARRIER_WE_ABORTED;
+                else
+                        b->barriers = BARRIER_I_ABORTED;
+        } else if (!barrier_is_aborted(b)) {
+                b->barriers += buf;
+        }
+
+        return !barrier_i_aborted(b);
+
+error:
+        /* If there is an unexpected error, we have to make this fatal. There
+         * is no way we can recover from sync-errors. Therefore, we close the
+         * pipe-ends and treat this as abortion. The other end will notice the
+         * pipe-close and treat it as abortion, too. */
+
+        b->pipe[0] = safe_close(b->pipe[0]);
+        b->pipe[1] = safe_close(b->pipe[1]);
+        b->barriers = BARRIER_WE_ABORTED;
+        return false;
+}
+
+/* waits for barriers; returns false if they aborted, otherwise true */
+static bool barrier_read(Barrier *b, int64_t comp) {
+        uint64_t buf;
+        ssize_t len;
+        struct pollfd pfd[2] = { };
+        int r;
+
+        if (barrier_they_aborted(b))
+                return false;
+
+        while (b->barriers > comp) {
+                pfd[0].fd = (b->pipe[0] >= 0) ? b->pipe[0] : b->pipe[1];
+                pfd[0].events = POLLHUP;
+                pfd[0].revents = 0;
+                pfd[1].fd = b->them;
+                pfd[1].events = POLLIN;
+                pfd[1].revents = 0;
+
+                r = poll(pfd, 2, -1);
+                if (r < 0 && (errno == EAGAIN || errno == EINTR))
+                        continue;
+                else if (r < 0)
+                        goto error;
+
+                if (pfd[1].revents) {
+                        /* events on @them signal us new data */
+                        len = read(b->them, &buf, sizeof(buf));
+                        if (len < 0 && (errno == EAGAIN || errno == EINTR))
+                                continue;
+
+                        if (len != sizeof(buf))
+                                goto error;
+                } else if (pfd[0].revents & (POLLHUP | POLLERR | POLLNVAL)) {
+                        /* POLLHUP on the pipe tells us the other side exited.
+                         * We treat this as implicit abortion. But we only
+                         * handle it if there's no event on the eventfd. This
+                         * guarantees that exit-abortions do not overwrite real
+                         * barriers. */
+                        buf = BARRIER_ABORTION;
+                }
+
+                /* lock if they aborted */
+                if (buf >= (uint64_t)BARRIER_ABORTION) {
+                        if (barrier_i_aborted(b))
+                                b->barriers = BARRIER_WE_ABORTED;
+                        else
+                                b->barriers = BARRIER_THEY_ABORTED;
+                } else if (!barrier_is_aborted(b)) {
+                        b->barriers -= buf;
+                }
+        }
+
+        return !barrier_they_aborted(b);
+
+error:
+        /* If there is an unexpected error, we have to make this fatal. There
+         * is no way we can recover from sync-errors. Therefore, we close the
+         * pipe-ends and treat this as abortion. The other end will notice the
+         * pipe-close and treat it as abortion, too. */
+
+        b->pipe[0] = safe_close(b->pipe[0]);
+        b->pipe[1] = safe_close(b->pipe[1]);
+        b->barriers = BARRIER_WE_ABORTED;
+        return false;
+}
+
+/**
+ * barrier_place() - Place a new barrier
+ * @b: barrier object
+ *
+ * This places a new barrier on the barrier object. If either side already
+ * aborted, this is a no-op and returns "false". Otherwise, the barrier is
+ * placed and this returns "true".
+ *
+ * Returns: true if barrier was placed, false if either side aborted.
+ */
+bool barrier_place(Barrier *b) {
+        assert(b);
+
+        if (barrier_is_aborted(b))
+                return false;
+
+        barrier_write(b, BARRIER_SINGLE);
+        return true;
+}
+
+/**
+ * barrier_abort() - Abort the synchronization
+ * @b: barrier object to abort
+ *
+ * This aborts the barrier-synchronization. If barrier_abort() was already
+ * called on this side, this is a no-op. Otherwise, the barrier is put into the
+ * ABORT-state and will stay there. The other side is notified about the
+ * abortion. Any following attempt to place normal barriers or to wait on normal
+ * barriers will return immediately as "false".
+ *
+ * You can wait for the other side to call barrier_abort(), too. Use
+ * barrier_wait_abortion() for that.
+ *
+ * Returns: false if the other side already aborted, true otherwise.
+ */
+bool barrier_abort(Barrier *b) {
+        assert(b);
+
+        barrier_write(b, BARRIER_ABORTION);
+        return !barrier_they_aborted(b);
+}
+
+/**
+ * barrier_wait_next() - Wait for the next barrier of the other side
+ * @b: barrier to operate on
+ *
+ * This waits until the other side places its next barrier. This is independent
+ * of any barrier-links and just waits for any next barrier of the other side.
+ *
+ * If either side aborted, this returns false.
+ *
+ * Returns: false if either side aborted, true otherwise.
+ */
+bool barrier_wait_next(Barrier *b) {
+        assert(b);
+
+        if (barrier_is_aborted(b))
+                return false;
+
+        barrier_read(b, b->barriers - 1);
+        return !barrier_is_aborted(b);
+}
+
+/**
+ * barrier_wait_abortion() - Wait for the other side to abort
+ * @b: barrier to operate on
+ *
+ * This waits until the other side called barrier_abort(). This can be called
+ * regardless whether the local side already called barrier_abort() or not.
+ *
+ * If the other side has already aborted, this returns immediately.
+ *
+ * Returns: false if the local side aborted, true otherwise.
+ */
+bool barrier_wait_abortion(Barrier *b) {
+        assert(b);
+
+        barrier_read(b, BARRIER_THEY_ABORTED);
+        return !barrier_i_aborted(b);
+}
+
+/**
+ * barrier_sync_next() - Wait for the other side to place a next linked barrier
+ * @b: barrier to operate on
+ *
+ * This is like barrier_wait_next() and waits for the other side to call
+ * barrier_place(). However, this only waits for linked barriers. That means, if
+ * the other side already placed more barriers than (or as much as) we did, this
+ * returns immediately instead of waiting.
+ *
+ * If either side aborted, this returns false.
+ *
+ * Returns: false if either side aborted, true otherwise.
+ */
+bool barrier_sync_next(Barrier *b) {
+        assert(b);
+
+        if (barrier_is_aborted(b))
+                return false;
+
+        barrier_read(b, MAX((int64_t)0, b->barriers - 1));
+        return !barrier_is_aborted(b);
+}
+
+/**
+ * barrier_sync() - Wait for the other side to place as many barriers as we did
+ * @b: barrier to operate on
+ *
+ * This is like barrier_sync_next() but waits for the other side to call
+ * barrier_place() as often as we did (in total). If they already placed as much
+ * as we did (or more), this returns immediately instead of waiting.
+ *
+ * If either side aborted, this returns false.
+ *
+ * Returns: false if either side aborted, true otherwise.
+ */
+bool barrier_sync(Barrier *b) {
+        assert(b);
+
+        if (barrier_is_aborted(b))
+                return false;
+
+        barrier_read(b, 0);
+        return !barrier_is_aborted(b);
+}
diff --git a/src/shared/barrier.h b/src/shared/barrier.h
new file mode 100644
index 0000000000..7f76ec7910
--- /dev/null
+++ b/src/shared/barrier.h
@@ -0,0 +1,92 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 David Herrmann <dh.herrmann@gmail.com>
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd 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
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <errno.h>
+#include <inttypes.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+
+#include "macro.h"
+#include "util.h"
+
+/* See source file for an API description. */
+
+typedef struct Barrier Barrier;
+
+enum {
+        BARRIER_SINGLE                  = 1LL,
+        BARRIER_ABORTION                = INT64_MAX,
+
+        /* bias values to store state; keep @WE < @THEY < @I */
+        BARRIER_BIAS                    = INT64_MIN,
+        BARRIER_WE_ABORTED              = BARRIER_BIAS + 1LL,
+        BARRIER_THEY_ABORTED            = BARRIER_BIAS + 2LL,
+        BARRIER_I_ABORTED               = BARRIER_BIAS + 3LL,
+};
+
+enum {
+        BARRIER_PARENT,
+        BARRIER_CHILD,
+};
+
+struct Barrier {
+        int me;
+        int them;
+        int pipe[2];
+        int64_t barriers;
+};
+
+int barrier_init(Barrier *obj);
+void barrier_destroy(Barrier *b);
+
+void barrier_set_role(Barrier *b, unsigned int role);
+
+bool barrier_place(Barrier *b);
+bool barrier_abort(Barrier *b);
+
+bool barrier_wait_next(Barrier *b);
+bool barrier_wait_abortion(Barrier *b);
+bool barrier_sync_next(Barrier *b);
+bool barrier_sync(Barrier *b);
+
+static inline bool barrier_i_aborted(Barrier *b) {
+        return b->barriers == BARRIER_I_ABORTED || b->barriers == BARRIER_WE_ABORTED;
+}
+
+static inline bool barrier_they_aborted(Barrier *b) {
+        return b->barriers == BARRIER_THEY_ABORTED || b->barriers == BARRIER_WE_ABORTED;
+}
+
+static inline bool barrier_we_aborted(Barrier *b) {
+        return b->barriers == BARRIER_WE_ABORTED;
+}
+
+static inline bool barrier_is_aborted(Barrier *b) {
+        return b->barriers == BARRIER_I_ABORTED || b->barriers == BARRIER_THEY_ABORTED || b->barriers == BARRIER_WE_ABORTED;
+}
+
+static inline bool barrier_place_and_sync(Barrier *b) {
+        barrier_place(b);
+        return barrier_sync(b);
+}