FOV: Raise lower limit to avoid zoom-loading of distant world (#7234)

In the client, raise lower limit from 30 to 45 degrees, to avoid server
seeing this as a zoom and loading world beyond the server-set limit.
Add minimum in settingtypes.txt and enforce lower limit when set using
minetest.conf.

In the server, distrust the client-sent FOV if below the heuristic zoom
threshold and use the player object property 'zoom_fov' to check it, to
protect against hacked clients.

builtin/settingtypes.txt

@@ -574,7 +574,7 @@ fullscreen_bpp (Full screen BPP) int 24
 vsync (V-Sync) bool false
 
 #    Field of view in degrees.
-fov (Field of view) int 72 30 160
+fov (Field of view) int 72 45 160
 
 #    Adjust the gamma encoding for the light tables. Higher numbers are brighter.
 #    This setting is for the client only and is ignored by the server.

src/camera.cpp

@@ -71,7 +71,9 @@ Camera::Camera(MapDrawControl &draw_control, Client *client):
 	 */
 	m_cache_fall_bobbing_amount = g_settings->getFloat("fall_bobbing_amount");
 	m_cache_view_bobbing_amount = g_settings->getFloat("view_bobbing_amount");
-	m_cache_fov                 = g_settings->getFloat("fov");
+	// 45 degrees is the lowest FOV that doesn't cause the server to treat this
+	// as a zoom FOV and load world beyond the set server limits.
+	m_cache_fov                 = std::fmax(g_settings->getFloat("fov"), 45.0f);
 	m_arm_inertia               = g_settings->getBool("arm_inertia");
 	m_nametags.clear();
 }

src/clientiface.cpp

@@ -192,17 +192,35 @@ void RemoteClient::GetNextBlocks (
 	*/
 	s32 new_nearest_unsent_d = -1;
 
-	// get view range and camera fov from the client
+	// Get view range and camera fov (radians) from the client
 	s16 wanted_range = sao->getWantedRange() + 1;
 	float camera_fov = sao->getFov();
 
-	const s16 full_d_max = std::min(adjustDist(m_max_send_distance, camera_fov), wanted_range);
-	const s16 d_opt = std::min(adjustDist(m_block_optimize_distance, camera_fov), wanted_range);
+	// If below the heuristic zoom threshold (see adjustDist() in numeric.cpp)
+	// distrust client-sent FOV and get server-set player object property
+	// zoom FOV (degrees) as a check to avoid hacked clients using FOV to load
+	// distant world.
+	// 0.888 radians is slightly larger than the zoom threshold of 1.775 / 2
+	// radians.
+	if (camera_fov < 0.888f) {
+		float prop_zoom_fov = sao->getZoomFOV();
+		// If zoom is disabled by value 0
+		if (prop_zoom_fov < 0.001f)
+			camera_fov = 0.888f;
+		else
+			// Degrees -> radians
+			camera_fov = prop_zoom_fov * core::DEGTORAD;
+	}
+
+	const s16 full_d_max = std::min(adjustDist(m_max_send_distance, camera_fov),
+		wanted_range);
+	const s16 d_opt = std::min(adjustDist(m_block_optimize_distance, camera_fov),
+		wanted_range);
 	const s16 d_blocks_in_sight = full_d_max * BS * MAP_BLOCKSIZE;
-	//infostream << "Fov from client " << camera_fov << " full_d_max " << full_d_max << std::endl;
 
 	s16 d_max = full_d_max;
-	s16 d_max_gen = std::min(adjustDist(m_max_gen_distance, camera_fov), wanted_range);
+	s16 d_max_gen = std::min(adjustDist(m_max_gen_distance, camera_fov),
+		wanted_range);
 
 	// Don't loop very much at a time, adjust with distance,
 	// do more work per RTT with greater distances.

src/content_sao.cpp

@@ -1473,3 +1473,8 @@ bool PlayerSAO::getSelectionBox(aabb3f *toset) const
 
 	return true;
 }
+
+float PlayerSAO::getZoomFOV() const
+{
+	return m_prop.zoom_fov;
+}

src/content_sao.h

@@ -330,6 +330,7 @@ public:
 
 	v3f getEyePosition() const { return m_base_position + getEyeOffset(); }
 	v3f getEyeOffset() const;
+	float getZoomFOV() const;
 
 	inline Metadata &getMeta() { return m_meta; }