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

3 files changed, 164 insertions, 6 deletions

diff --git a/include/asterisk/alertpipe.h b/include/asterisk/alertpipe.h
new file mode 100644
index 000000000..5ff854ce8
--- /dev/null
+++ b/include/asterisk/alertpipe.h
@@ -0,0 +1,159 @@
+/*
+ * 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.
+ */
+
+#ifndef ASTERISK_ALERTPIPE_H
+#define ASTERISK_ALERTPIPE_H
+
+#include "asterisk/utils.h"
+
+typedef enum {
+	AST_ALERT_READ_SUCCESS = 0,
+	AST_ALERT_NOT_READABLE,
+	AST_ALERT_READ_FAIL,
+	AST_ALERT_READ_FATAL,
+} ast_alert_status_t;
+
+/*!
+ * \brief Initialize an alert pipe
+ * \since 13.16.0
+ *
+ * \param p a two-element array to hold the alert pipe's file descriptors
+ *
+ * \return non-zero if a failure occurred, zero otherwise.
+ */
+int ast_alertpipe_init(int alert_pipe[2]);
+
+/*!
+ * \brief Close an alert pipe
+ * \since 13.16.0
+ *
+ * \param p a two-element containing the alert pipe's file descriptors
+ */
+void ast_alertpipe_close(int alert_pipe[2]);
+
+/*!
+ * \brief Read an event from an alert pipe
+ * \since 13.16.0
+ *
+ * \param p a two-element array containing the alert pipe's file descriptors
+ *
+ * \retval AST_ALERT_READ_SUCCESS on success
+ * \retval AST_ALERT_NOT_READABLE if the alert pipe is not readable
+ * \retval AST_ALERT_READ_FATAL if the alert pipe's file descriptors are in
+ *         blocking mode, or a read error occurs.
+ */
+ast_alert_status_t ast_alertpipe_read(int alert_pipe[2]);
+
+/*!
+ * \brief Write an event to an alert pipe
+ * \since 13.16.0
+ *
+ * \param p a two-element array containing the alert pipe's file descriptors
+ *
+ * \return see write(2)
+ */
+ssize_t ast_alertpipe_write(int alert_pipe[2]);
+
+/*!
+ * \brief Consume all alerts written to the alert pipe
+ * \since 13.16.0
+ *
+ * \param p a two-element array containing the alert pipe's file descriptors
+ *
+ * \retval AST_ALERT_READ_SUCCESS on success
+ * \retval AST_ALERT_NOT_READABLE if the alert pipe is not readable
+ * \retval AST_ALERT_READ_FATAL if the alert pipe's file descriptors are in
+ *         blocking mode, or a read error occurs.
+ */
+ast_alert_status_t ast_alertpipe_flush(int alert_pipe[2]);
+
+/*!
+ * \brief Sets the alert pipe file descriptors to default values
+ * \since 13.16.0
+ *
+ * \param p a two-element array containing the alert pipe's file descriptors
+ */
+AST_INLINE_API(
+void ast_alertpipe_clear(int alert_pipe[2]),
+{
+	alert_pipe[0] = alert_pipe[1] = -1;
+}
+)
+
+/*!
+ * \brief Determine if the alert pipe is readable
+ * \since 13.16.0
+ *
+ * \param p a two-element array containing the alert pipe's file descriptors
+ *
+ * \return non-zero if the alert pipe is readable, zero otherwise.
+ */
+AST_INLINE_API(
+int attribute_pure ast_alertpipe_readable(int alert_pipe[2]),
+{
+	return alert_pipe[0] > -1;
+}
+)
+
+/*!
+ * \brief Determine if the alert pipe is writable
+ * \since 13.16.0
+ *
+ * \param p a two-element array containing the alert pipe's file descriptors
+ *
+ * \return non-zero if the alert pipe is writable, zero otherwise.
+ */
+AST_INLINE_API(
+int attribute_pure ast_alertpipe_writable(int alert_pipe[2]),
+{
+	return alert_pipe[1] > -1;
+}
+)
+
+/*!
+ * \brief Get the alert pipe's read file descriptor
+ * \since 13.16.0
+ *
+ * \param p a two-element array containing the alert pipe's file descriptors
+ *
+ * \return -1 if the file descriptor is not initialized, a non-negative value
+ *            otherwise.
+ */
+AST_INLINE_API(
+int attribute_pure ast_alertpipe_readfd(int alert_pipe[2]),
+{
+	return alert_pipe[0];
+}
+)
+
+/*!
+ * \brief Swap the file descriptors from two alert pipes
+ * \since 13.16.0
+ *
+ * \param p1 a two-element array containing an alert pipe's file descriptors
+ * \param p2 a two-element array containing an alert pipe's file descriptors
+ */
+AST_INLINE_API(
+void ast_alertpipe_swap(int alert_pipe_1[2], int alert_pipe_2[2]),
+{
+	SWAP(alert_pipe_1[0], alert_pipe_2[0]);
+	SWAP(alert_pipe_1[1], alert_pipe_2[1]);
+}
+)
+
+#endif /* ASTERISK_ALERTPIPE_H */
diff --git a/include/asterisk/autoconfig.h.in b/include/asterisk/autoconfig.h.in
index 5f8a9d37c..b39386b43 100644
--- a/include/asterisk/autoconfig.h.in
+++ b/include/asterisk/autoconfig.h.in
@@ -225,6 +225,10 @@
 /* Define to 1 if you have the `euidaccess' function. */
 #undef HAVE_EUIDACCESS
 
+/* Define to 1 if your system supports eventfd and the EFD_NONBLOCK and
+   EFD_SEMAPHORE flags. */
+#undef HAVE_EVENTFD
+
 /* Define to 1 if you have the `exp' function. */
 #undef HAVE_EXP
 
diff --git a/include/asterisk/channel.h b/include/asterisk/channel.h
index 32c9c7f67..3e04b5d0b 100644
--- a/include/asterisk/channel.h
+++ b/include/asterisk/channel.h
@@ -123,6 +123,7 @@ References:
 #ifndef _ASTERISK_CHANNEL_H
 #define _ASTERISK_CHANNEL_H
 
+#include "asterisk/alertpipe.h"
 #include "asterisk/abstract_jb.h"
 #include "asterisk/astobj2.h"
 #include "asterisk/poll-compat.h"
@@ -4267,12 +4268,6 @@ struct ast_namedgroups *ast_channel_named_pickupgroups(const struct ast_channel
 void ast_channel_named_pickupgroups_set(struct ast_channel *chan, struct ast_namedgroups *value);
 
 /* Alertpipe accessors--the "internal" functions for channel.c use only */
-typedef enum {
-	AST_ALERT_READ_SUCCESS = 0,
-	AST_ALERT_NOT_READABLE,
-	AST_ALERT_READ_FAIL,
-	AST_ALERT_READ_FATAL,
-} ast_alert_status_t;
 int ast_channel_alert_write(struct ast_channel *chan);
 int ast_channel_alert_writable(struct ast_channel *chan);
 ast_alert_status_t ast_channel_internal_alert_flush(struct ast_channel *chan);