core: Use eventfd for alert pipes on Linux when possible

The primary win of switching to eventfd when possible is that it only
uses a single file descriptor while pipe() will use two. This means for
each bridge channel we're reducing the number of required file
descriptors by 1, and - if you're using timerfd - we also now have 1
less file descriptor per Asterisk channel.

The API is not ideal (passing int arrays), but this is the cleanest
approach I could come up with to maintain API/ABI.

I've also removed what I believe to be an erroneous code block that
checked the non-blocking flag on the pipe ends for each read. If the
file descriptor is 'losing' its non-blocking mode, it is because of a
bug somewhere else in our code.

In my testing I haven't seen any measurable difference in performance.

Change-Id: Iff0fb1573e7f7a187d5211ddc60aa8f3da3edb1d

1 files changed, 166 insertions, 0 deletions

diff --git a/main/alertpipe.c b/main/alertpipe.c
new file mode 100644
index 000000000..fa6ec7bcc
--- /dev/null
+++ b/main/alertpipe.c
@@ -0,0 +1,166 @@
+/*
+ * Asterisk -- An open source telephony toolkit.
+ *
+ * Copyright (C) 2017, Sean Bright
+ *
+ * Sean Bright <sean.bright@gmail.com>
+ *
+ * See http://www.asterisk.org for more information about
+ * the Asterisk project. Please do not directly contact
+ * any of the maintainers of this project for assistance;
+ * the project provides a web site, mailing lists and IRC
+ * channels for your use.
+ *
+ * This program is free software, distributed under the terms of
+ * the GNU General Public License Version 2. See the LICENSE file
+ * at the top of the source tree.
+ */
+
+/*! \file
+ *
+ * \brief Alert Pipe API
+ *
+ * \author Sean Bright
+ */
+
+#include "asterisk.h"
+
+#include <unistd.h>
+#include <fcntl.h>
+
+#ifdef HAVE_EVENTFD
+# include <sys/eventfd.h>
+#endif
+
+#include "asterisk/alertpipe.h"
+#include "asterisk/logger.h"
+
+int ast_alertpipe_init(int alert_pipe[2])
+{
+#ifdef HAVE_EVENTFD
+
+	int fd = eventfd(0, EFD_NONBLOCK | EFD_SEMAPHORE);
+	if (fd > -1) {
+		alert_pipe[0] = alert_pipe[1] = fd;
+		return 0;
+	}
+
+	ast_log(LOG_WARNING, "Failed to create alert pipe with eventfd(), falling back to pipe(): %s\n",
+		strerror(errno));
+	ast_alertpipe_clear(alert_pipe);
+
+#endif
+
+	if (pipe(alert_pipe)) {
+		ast_log(LOG_WARNING, "Failed to create alert pipe: %s\n", strerror(errno));
+		return -1;
+	} else {
+		int flags = fcntl(alert_pipe[0], F_GETFL);
+		if (fcntl(alert_pipe[0], F_SETFL, flags | O_NONBLOCK) < 0) {
+			ast_log(LOG_WARNING, "Failed to set non-blocking mode on alert pipe: %s\n",
+				strerror(errno));
+			ast_alertpipe_close(alert_pipe);
+			return -1;
+		}
+		flags = fcntl(alert_pipe[1], F_GETFL);
+		if (fcntl(alert_pipe[1], F_SETFL, flags | O_NONBLOCK) < 0) {
+			ast_log(LOG_WARNING, "Failed to set non-blocking mode on alert pipe: %s\n",
+				strerror(errno));
+			ast_alertpipe_close(alert_pipe);
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
+void ast_alertpipe_close(int alert_pipe[2])
+{
+#ifdef HAVE_EVENTFD
+
+	if (alert_pipe[0] == alert_pipe[1]) {
+		if (alert_pipe[0] > -1) {
+			close(alert_pipe[0]);
+			ast_alertpipe_clear(alert_pipe);
+		}
+		return;
+	}
+
+#endif
+
+	if (alert_pipe[0] > -1) {
+		close(alert_pipe[0]);
+	}
+	if (alert_pipe[1] > -1) {
+		close(alert_pipe[1]);
+	}
+	ast_alertpipe_clear(alert_pipe);
+}
+
+ast_alert_status_t ast_alertpipe_read(int alert_pipe[2])
+{
+	uint64_t tmp;
+
+	if (!ast_alertpipe_readable(alert_pipe)) {
+		return AST_ALERT_NOT_READABLE;
+	}
+
+	if (read(alert_pipe[0], &tmp, sizeof(tmp)) < 0) {
+		if (errno != EINTR && errno != EAGAIN) {
+			ast_log(LOG_WARNING, "read() failed: %s\n", strerror(errno));
+			return AST_ALERT_READ_FAIL;
+		}
+	}
+
+	return AST_ALERT_READ_SUCCESS;
+}
+
+ssize_t ast_alertpipe_write(int alert_pipe[2])
+{
+	uint64_t tmp = 1;
+
+	if (!ast_alertpipe_writable(alert_pipe)) {
+		errno = EBADF;
+		return 0;
+	}
+
+	/* preset errno in case returned size does not match */
+	errno = EPIPE;
+	return write(alert_pipe[1], &tmp, sizeof(tmp)) != sizeof(tmp);
+}
+
+ast_alert_status_t ast_alertpipe_flush(int alert_pipe[2])
+{
+	int bytes_read;
+	uint64_t tmp[16];
+
+	if (!ast_alertpipe_readable(alert_pipe)) {
+		return AST_ALERT_NOT_READABLE;
+	}
+
+	/* Read the alertpipe until it is exhausted. */
+	for (;;) {
+		bytes_read = read(alert_pipe[0], tmp, sizeof(tmp));
+		if (bytes_read < 0) {
+			if (errno == EINTR) {
+				continue;
+			}
+			if (errno == EAGAIN || errno == EWOULDBLOCK) {
+				/*
+				 * Would block so nothing left to read.
+				 * This is the normal loop exit.
+				 */
+				break;
+			}
+			ast_log(LOG_WARNING, "read() failed flushing alertpipe: %s\n",
+				strerror(errno));
+			return AST_ALERT_READ_FAIL;
+		}
+		if (!bytes_read) {
+			/* Read nothing so we are done */
+			break;
+		}
+	}
+
+	return AST_ALERT_READ_SUCCESS;
+}