libcamera: device_enumerator_udev: Use std::string_view

In `udevNotify()`, constructing an std::string from the device's
associated action is unnecessary as it is only compared against static
strings, and for that purpose an std::string_view works just as well,
while being cheaper to construct.

In the same vein, an std::string_view can be used to store the device's
devnode initially, and the string construction can be deferred until it
is needed.

Furthermore, previously `udev_device_get_devnode()` was called twice.
The extra call is now removed.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/device_enumerator_udev.cpp

@@ -13,6 +13,7 @@
 #include <list>
 #include <map>
 #include <string.h>
+#include <string_view>
 #include <sys/ioctl.h>
 #include <sys/sysmacros.h>
 #include <unistd.h>
@@ -331,18 +332,18 @@ int DeviceEnumeratorUdev::addV4L2Device(dev_t devnum)
 void DeviceEnumeratorUdev::udevNotify()
 {
 	struct udev_device *dev = udev_monitor_receive_device(monitor_);
-	std::string action(udev_device_get_action(dev));
-	std::string deviceNode(udev_device_get_devnode(dev));
+	std::string_view action(udev_device_get_action(dev));
+	std::string_view deviceNode(udev_device_get_devnode(dev));
 
 	LOG(DeviceEnumerator, Debug)
-		<< action << " device " << udev_device_get_devnode(dev);
+		<< action << " device " << deviceNode;
 
 	if (action == "add") {
 		addUdevDevice(dev);
 	} else if (action == "remove") {
 		const char *subsystem = udev_device_get_subsystem(dev);
 		if (subsystem && !strcmp(subsystem, "media"))
-			removeDevice(deviceNode);
+			removeDevice(std::string(deviceNode));
 	}
 
 	udev_device_unref(dev);