libcamera: media_device: Add functions to lock device for other processes

Add lock() and unlock() which are backed by lockf() and allow an
instance of libcamera to claim exclusive access to a media device.
These functions are the base of allowing multiple user of libcamera to
coexist in the system without stepping on each other's toes.

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

src/libcamera/include/media_device.h

@@ -30,6 +30,9 @@ public:
 	void release();
 	bool busy() const { return acquired_; }
 
+	bool lock();
+	void unlock();
+
 	int populate();
 	bool valid() const { return valid_; }
 
@@ -58,6 +61,7 @@ private:
 	int fd_;
 	bool valid_;
 	bool acquired_;
+	bool lockOwner_;
 
 	int open();
 	void close();

src/libcamera/media_device.cpp

@@ -63,7 +63,8 @@ LOG_DEFINE_CATEGORY(MediaDevice)
  * populate() before the media graph can be queried.
  */
 MediaDevice::MediaDevice(const std::string &deviceNode)
-	: deviceNode_(deviceNode), fd_(-1), valid_(false), acquired_(false)
+	: deviceNode_(deviceNode), fd_(-1), valid_(false), acquired_(false),
+	  lockOwner_(false)
 {
 }
 
@@ -118,6 +119,62 @@ void MediaDevice::release()
 	acquired_ = false;
 }
 
+/**
+ * \brief Lock the device to prevent it from being used by other instances of
+ * libcamera
+ *
+ * Multiple instances of libcamera might be running on the same system, at the
+ * same time. To allow the different instances to coexist, system resources in
+ * the form of media devices must be accessible for enumerating the cameras
+ * they provide at all times, while still allowing an instance to lock a
+ * resource while it prepares to actively use a camera from the resource.
+ *
+ * This method shall not be called from a pipeline handler implementation
+ * directly, as the base PipelineHandler implementation handles this on the
+ * behalf of the specified implementation.
+ *
+ * \return True if the device could be locked, false otherwise
+ * \sa unlock()
+ */
+bool MediaDevice::lock()
+{
+	if (fd_ == -1)
+		return false;
+
+	/* Do not allow nested locking in the same libcamera instance. */
+	if (lockOwner_)
+		return false;
+
+	if (lockf(fd_, F_TLOCK, 0))
+		return false;
+
+	lockOwner_ = true;
+
+	return true;
+}
+
+/**
+ * \brief Unlock the device and free it for use for libcamera instances
+ *
+ * This method shall not be called from a pipeline handler implementation
+ * directly, as the base PipelineHandler implementation handles this on the
+ * behalf of the specified implementation.
+ *
+ * \sa lock()
+ */
+void MediaDevice::unlock()
+{
+	if (fd_ == -1)
+		return;
+
+	if (!lockOwner_)
+		return;
+
+	lockOwner_ = false;
+
+	lockf(fd_, F_ULOCK, 0);
+}
+
 /**
  * \fn MediaDevice::busy()
  * \brief Check if a device is in use