2 files changed, 212 insertions, 0 deletions
diff --git a/src/shared/eventfd-util.c b/src/shared/eventfd-util.c
new file mode 100644
index 0000000000..27b7cf788f
--- /dev/null
+++ b/src/shared/eventfd-util.c
@@ -0,0 +1,169 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 Djalal Harouni
+
+  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 <assert.h>
+#include <errno.h>
+#include <unistd.h>
+#include <sys/eventfd.h>
+#include <sys/syscall.h>
+
+#include "eventfd-util.h"
+#include "util.h"
+
+
+/*
+ * Use this to create processes that need to setup a full context
+ * and sync it with their parents using cheap mechanisms.
+ *
+ * This will create two blocking eventfd(s). A pair for the parent and
+ * the other for the child so they can be used as a notify mechanism.
+ * Each process will gets its copy of the parent and child eventfds.
+ *
+ * This is useful in case:
+ * 1) If the parent fails or dies, the child must die.
+ * 2) Child will install PR_SET_PDEATHSIG as soon as possible.
+ * 3) Parent and child need to sync using less resources.
+ * 4) If parent is not able to install a SIGCHLD handler:
+ *    parent will wait using a blocking eventfd_read() or
+ *    eventfd_child_succeeded() call on the child eventfd.
+ *
+ *    * If the child setup succeeded, child should notify with an
+ *      EVENTFD_CHILD_SUCCEEDED, parent will continue.
+ *    * If the child setup failed, child should notify with an
+ *      EVENTFD_CHILD_FAILED before any _exit(). This avoids blocking
+ *      the parent.
+ *
+ * 5) If parent is able to install a SIGCHLD handler:
+ *    An empty signal handler without SA_RESTART will do it, since the
+ *    blocking eventfd_read() or eventfd_parent_succeeded() of the
+ *    parent will be interrupted by SIGCHLD and the call will fail with
+ *    EINTR. This is useful in case the child dies abnormaly and did
+ *    not have a chance to notify its parent using EVENTFD_CHILD_FAILED.
+ *
+ * 6) Call wait*() in the main instead of the signal handler in order
+ *    to: 1) reduce side effects and 2) have a better handling for
+ *    child termination in order to reduce various race conditions.
+ *
+ *
+ * The return value of clone_with_eventfd() is the same of clone().
+ * On success the eventfds[] will contain the two eventfd(s). These
+ * file descriptors can be closed later with safe_close(). On failure,
+ * a negative value is returned in the caller's context, and errno will
+ * be set appropriately.
+ *
+ * Extra preliminary work:
+ * 1) Child can wait before starting its setup by using the
+ *    eventfd_recv_start() call on the parent eventfd, in that case the
+ *    parent must notify with EVENTFD_START, after doing any preliminary
+ *    work.
+ *
+ * Note: this function depends on systemd internal functions
+ * safe_close() and it should be used only by direct binaries, no
+ * libraries.
+ */
+pid_t clone_with_eventfd(int flags, int eventfds[2]) {
+        pid_t pid;
+
+        assert(eventfds);
+
+        eventfds[0] = eventfd(EVENTFD_INIT, EFD_CLOEXEC);
+        if (eventfds[0] < 0)
+                return -1;
+
+        eventfds[1] = eventfd(EVENTFD_INIT, EFD_CLOEXEC);
+        if (eventfds[1] < 0)
+                goto err_eventfd0;
+
+        pid = syscall(__NR_clone, flags, NULL);
+        if (pid < 0)
+                goto err_eventfd1;
+
+        return pid;
+
+err_eventfd1:
+        eventfds[1] = safe_close(eventfds[1]);
+err_eventfd0:
+        eventfds[0] = safe_close(eventfds[0]);
+        return -1;
+}
+
+int eventfd_send_state(int efd, eventfd_t s) {
+        return eventfd_write(efd, s);
+}
+
+/*
+ * Receive an eventfd state on the eventfd file descriptor.
+ *
+ * If the third argument is set to a value other than zero, then this
+ * function will compare the received value with this argument and set
+ * the return value.
+ *
+ * On success return 0. On error, -1 will be returned, and errno will
+ * be set appropriately.
+ */
+int eventfd_recv_state(int efd, eventfd_t *e, eventfd_t s) {
+        int ret;
+
+        ret = eventfd_read(efd, e);
+        if (ret < 0)
+                return ret;
+        else if (s != 0 && *e != s) {
+                errno = EINVAL;
+                return -1;
+        }
+
+        return 0;
+}
+
+/*
+ * Receive the EVENTFD_START state on the eventfd file descriptor.
+ *
+ * On Success return 0. On error, -1 will be returned, and errno will
+ * be set appropriately.
+ */
+int eventfd_recv_start(int efd) {
+        eventfd_t e = EVENTFD_INIT;
+        return eventfd_recv_state(efd, &e, EVENTFD_START);
+}
+
+/*
+ * Receive the EVENTFD_PARENT_SUCCEEDED state on the eventfd file
+ * descriptor.
+ *
+ * On Success return 0. On error, -1 will be returned, and errno will
+ * be set appropriately.
+ */
+int eventfd_parent_succeeded(int efd) {
+        eventfd_t e = EVENTFD_INIT;
+        return eventfd_recv_state(efd, &e, EVENTFD_PARENT_SUCCEEDED);
+}
+
+/*
+ * Receive the EVENTFD_CHILD_SUCCEEDED state on the eventfd file
+ * descriptor.
+ *
+ * On Success return 0. On error, -1 will be returned, and errno will
+ * be set appropriately.
+ */
+int eventfd_child_succeeded(int efd) {
+        eventfd_t e = EVENTFD_INIT;
+        return eventfd_recv_state(efd, &e, EVENTFD_CHILD_SUCCEEDED);
+}
diff --git a/src/shared/eventfd-util.h b/src/shared/eventfd-util.h
new file mode 100644
index 0000000000..0120f0409e
--- /dev/null
+++ b/src/shared/eventfd-util.h
@@ -0,0 +1,43 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 Djalal Harouni
+
+  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 <sys/types.h>
+#include <sys/eventfd.h>
+
+enum {
+        EVENTFD_INIT,
+        EVENTFD_START,
+        EVENTFD_PARENT_SUCCEEDED,
+        EVENTFD_PARENT_FAILED,
+        EVENTFD_CHILD_SUCCEEDED,
+        EVENTFD_CHILD_FAILED,
+};
+
+pid_t clone_with_eventfd(int flags, int eventfds[2]);
+
+int eventfd_send_state(int efd, eventfd_t s);
+int eventfd_recv_state(int efd, eventfd_t *e, eventfd_t s);
+
+int eventfd_recv_start(int efd);
+int eventfd_parent_succeeded(int efd);
+int eventfd_child_succeeded(int efd);