libcamera: ipc: unix: Make socket operation asynchronous

Blocking socket operation when receiving messages may lead to long
delays, and possibly a complete deadlock, if the remote side delays
sending of the payload after the header, or doesn't send the payload at
all. To avoid this, make the socket non-blocking and implement a simple
state machine to receive the header synchronously with the socket read
notification. The payload read is still synchronous with the receive()
method to avoid data copies.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/include/ipc_unixsocket.h

@@ -49,6 +49,8 @@ private:
 	void dataNotifier(EventNotifier *notifier);
 
 	int fd_;
+	bool headerReceived_;
+	struct Header header_;
 	EventNotifier *notifier_;
 };
 

src/libcamera/ipc_unixsocket.cpp

@@ -7,6 +7,7 @@
 
 #include "ipc_unixsocket.h"
 
+#include <poll.h>
 #include <string.h>
 #include <sys/socket.h>
 #include <unistd.h>
@@ -49,10 +50,10 @@ LOG_DEFINE_CATEGORY(IPCUnixSocket)
  * transporting entire payloads with guaranteed ordering.
  *
  * The IPC design is asynchronous, a message is queued to a receiver which gets
- * notified that a message is ready to be consumed by a signal. The queuer of
- * the message gets no notification when a message is delivered nor processed.
- * If such interactions are needed a protocol specific to the users use-case
- * should be implemented on top of the IPC objects.
+ * notified that a message is ready to be consumed by the \ref readyRead
+ * signal. The sender of the message gets no notification when a message is
+ * delivered nor processed. If such interactions are needed a protocol specific
+ * to the users use-case should be implemented on top of the IPC objects.
  *
  * Establishment of an IPC channel is asymmetrical. The side that initiates
  * communication first instantiates a local side socket and creates the channel
@@ -64,7 +65,7 @@ LOG_DEFINE_CATEGORY(IPCUnixSocket)
  */
 
 IPCUnixSocket::IPCUnixSocket()
-	: fd_(-1), notifier_(nullptr)
+	: fd_(-1), headerReceived_(false), notifier_(nullptr)
 {
 }
 
@@ -89,7 +90,7 @@ int IPCUnixSocket::create()
 	int sockets[2];
 	int ret;
 
-	ret = socketpair(AF_UNIX, SOCK_DGRAM, 0, sockets);
+	ret = socketpair(AF_UNIX, SOCK_DGRAM | SOCK_NONBLOCK, 0, sockets);
 	if (ret) {
 		ret = -errno;
 		LOG(IPCUnixSocket, Error)
@@ -142,6 +143,7 @@ void IPCUnixSocket::close()
 	::close(fd_);
 
 	fd_ = -1;
+	headerReceived_ = false;
 }
 
 /**
@@ -193,38 +195,38 @@ int IPCUnixSocket::send(const Payload &payload)
  * \param[out] payload Payload where to write the received message
  *
  * This method receives the message payload from the IPC channel and writes it
- * to the \a payload. It blocks until one message is received, if an
- * asynchronous behavior is desired this method should be called when the
- * readyRead signal is emitted.
+ * to the \a payload. If no message payload is available, it returns
+ * immediately with -EAGAIN. The \ref readyRead signal shall be used to receive
+ * notification of message availability.
  *
  * \todo Add state machine to make sure we don't block forever and that
  * a header is always followed by a payload.
  *
  * \return 0 on success or a negative error code otherwise
+ * \retval -EAGAIN No message payload is available
+ * \retval -ENOTCONN The socket is not connected (neither create() nor bind()
+ * has been called)
  */
 int IPCUnixSocket::receive(Payload *payload)
 {
-	Header hdr;
-	int ret;
-
 	if (!isBound())
 		return -ENOTCONN;
 
-	if (!payload)
-		return -EINVAL;
+	if (!headerReceived_)
+		return -EAGAIN;
 
-	ret = ::recv(fd_, &hdr, sizeof(hdr), 0);
-	if (ret < 0) {
-		ret = -errno;
-		LOG(IPCUnixSocket, Error)
-			<< "Failed to recv header: " << strerror(-ret);
+	payload->data.resize(header_.data);
+	payload->fds.resize(header_.fds);
+
+	int ret = recvData(payload->data.data(), header_.data,
+			   payload->fds.data(), header_.fds);
+	if (ret < 0)
 		return ret;
-	}
 
-	payload->data.resize(hdr.data);
-	payload->fds.resize(hdr.fds);
+	headerReceived_ = false;
+	notifier_->setEnabled(true);
 
-	return recvData(payload->data.data(), hdr.data, payload->fds.data(), hdr.fds);
+	return 0;
 }
 
 /**
@@ -232,7 +234,8 @@ int IPCUnixSocket::receive(Payload *payload)
  * \brief A Signal emitted when a message is ready to be read
  */
 
-int IPCUnixSocket::sendData(const void *buffer, size_t length, const int32_t *fds, unsigned int num)
+int IPCUnixSocket::sendData(const void *buffer, size_t length,
+			    const int32_t *fds, unsigned int num)
 {
 	struct iovec iov[1];
 	iov[0].iov_base = const_cast<void *>(buffer);
@@ -266,7 +269,8 @@ int IPCUnixSocket::sendData(const void *buffer, size_t length, const int32_t *fd
 	return 0;
 }
 
-int IPCUnixSocket::recvData(void *buffer, size_t length, int32_t *fds, unsigned int num)
+int IPCUnixSocket::recvData(void *buffer, size_t length,
+			    int32_t *fds, unsigned int num)
 {
 	struct iovec iov[1];
 	iov[0].iov_base = buffer;
@@ -291,6 +295,7 @@ int IPCUnixSocket::recvData(void *buffer, size_t length, int32_t *fds, unsigned
 
 	if (recvmsg(fd_, &msg, 0) < 0) {
 		int ret = -errno;
+		if (ret != -EAGAIN)
 			LOG(IPCUnixSocket, Error)
 				<< "Failed to recvmsg: " << strerror(-ret);
 		return ret;
@@ -303,6 +308,35 @@ int IPCUnixSocket::recvData(void *buffer, size_t length, int32_t *fds, unsigned
 
 void IPCUnixSocket::dataNotifier(EventNotifier *notifier)
 {
+	int ret;
+
+	if (!headerReceived_) {
+		/* Receive the header. */
+		ret = ::recv(fd_, &header_, sizeof(header_), 0);
+		if (ret < 0) {
+			ret = -errno;
+			LOG(IPCUnixSocket, Error)
+				<< "Failed to receive header: " << strerror(-ret);
+			return;
+		}
+
+		headerReceived_ = true;
+	}
+
+	/*
+	 * If the payload has arrived, disable the notifier and emit the
+	 * readyRead signal. The notifier will be reenabled by the receive()
+	 * method.
+	 */
+	struct pollfd fds = { fd_, POLLIN, 0 };
+	ret = poll(&fds, 1, 0);
+	if (ret < 0)
+		return;
+
+	if (!(fds.revents & POLLIN))
+		return;
+
+	notifier_->setEnabled(false);
 	readyRead.emit(this);
 }