Implement mod communication channels (#6351)

Implement network communication for channels

* Implement ModChannel manager server side to route incoming messages from clients to other clients
* Add signal handler switch on client & ModChannelMgr on client to handle channels
* Add Lua API bindings + client packet sending + unittests
* Implement server message sending
* Add callback from received message handler to Lua API using registration method

3 files changed, 70 insertions, 1 deletions

diff --git a/doc/client_lua_api.md b/doc/client_lua_api.md
index 44c7c2787..4c48b6619 100644
--- a/doc/client_lua_api.md
+++ b/doc/client_lua_api.md
@@ -683,6 +683,12 @@ Call these functions only at load time!
     * Called when the local player uses an item.
     * Newest functions are called first.
     * If any function returns true, the item use is not sent to server.
+* `minetest.register_on_modchannel_message(func(channel_name, sender, message))`
+    * Called when an incoming mod channel message is received
+    * You must have joined some channels before, and server must acknowledge the
+      join request.
+    * If message comes from a server mod, `sender` field is an empty string.
+
 ### Sounds
 * `minetest.sound_play(spec, parameters)`: returns a handle
     * `spec` is a `SimpleSoundSpec`
@@ -754,6 +760,16 @@ Call these functions only at load time!
     * returns reference to mod private `StorageRef`
     * must be called during mod load time
 
+### Mod channels
+![Mod channels communication scheme](docs/mod channels.png)
+
+* `minetest.mod_channel_join(channel_name)`
+    * Client joins channel `channel_name`, and creates it, if necessary. You
+      should listen from incoming messages with `minetest.register_on_modchannel_message`
+      call to receive incoming messages. Warning, this function is asynchronous.
+    * You should use a minetest.register_on_connect(function() ... end) to perform
+      a successful channel join on client startup.
+
 ### Misc.
 * `minetest.parse_json(string[, nullvalue])`: returns something
     * Convert a string containing JSON data into the Lua equivalent
@@ -827,9 +843,25 @@ Call these functions only at load time!
 Class reference
 ---------------
 
+### ModChannel
+
+An interface to use mod channels on client and server
+
+#### Methods
+* `leave()`: leave the mod channel.
+    * Client leaves channel `channel_name`.
+    * No more incoming or outgoing messages can be sent to this channel from client mods.
+    * This invalidate all future object usage
+    * Ensure your set mod_channel to nil after that to free Lua resources
+* `is_writeable()`: returns true if channel is writeable and mod can send over it.
+* `send_all(message)`: Send `message` though the mod channel.
+    * If mod channel is not writeable or invalid, message will be dropped.
+    * Message size is limited to 65535 characters by protocol.
+
 ### Minimap
 An interface to manipulate minimap on client UI
 
+#### Methods
 * `show()`: shows the minimap (if not disabled by server)
 * `hide()`: hides the minimap
 * `set_pos(pos)`: sets the minimap position on screen
diff --git a/doc/lua_api.txt b/doc/lua_api.txt
index b71eb5477..812b857f7 100644
--- a/doc/lua_api.txt
+++ b/doc/lua_api.txt
@@ -1126,7 +1126,7 @@ The 2D perlin noise described by `noise_params` varies the Y co-ordinate of the
 stratum midpoint. The 2D perlin noise described by `np_stratum_thickness`
 varies the stratum's vertical thickness (in units of nodes). Due to being
 continuous across mapchunk borders the stratum's vertical thickness is
-unlimited. 
+unlimited.
 `y_min` and `y_max` define the limits of the ore generation and for performance
 reasons should be set as close together as possible but without clipping the
 stratum's Y variation.
@@ -2496,6 +2496,20 @@ Call these functions only at load time!
 * `minetest.register_can_bypass_userlimit(function(name, ip))`
     * Called when `name` user connects with `ip`.
     * Return `true` to by pass the player limit
+* `minetest.register_on_modchannel_message(func(channel_name, sender, message))`
+    * Called when an incoming mod channel message is received
+    * You should have joined  some channels to receive events.
+    * If message comes from a server mod, `sender` field is an empty string.
+* `minetest.register_on_modchannel_signal(func(channel_name, signal))`
+    * Called when a valid incoming mod channel signal is received
+    * Signal id permit to react to server mod channel events
+    * Possible values are:
+      0: join_ok
+      1: join_failed
+      2: leave_ok
+      3: leave_failed
+      4: event_on_not_joined_channel
+      5: state_changed
 
 ### Other registration functions
 * `minetest.register_chatcommand(cmd, chatcommand definition)`
@@ -2773,6 +2787,14 @@ and `minetest.auth_reload` call the authetification handler.
     * spread these updates to neighbours and can cause a cascade
       of nodes to fall.
 
+### Mod channels
+You can find mod channels communication scheme in `docs/mod_channels.png`.
+
+* `minetest.mod_channel_join(channel_name)`
+    * Server joins channel `channel_name`, and creates it if necessary. You
+      should listen from incoming messages with `minetest.register_on_modchannel_message`
+      call to receive incoming messages
+
 ### Inventory
 `minetest.get_inventory(location)`: returns an `InvRef`
 
@@ -3256,6 +3278,21 @@ These functions return the leftover itemstack.
 Class reference
 ---------------
 
+### ModChannel
+
+An interface to use mod channels on client and server
+
+#### Methods
+* `leave()`: leave the mod channel.
+    * Server leaves channel `channel_name`.
+    * No more incoming or outgoing messages can be sent to this channel from server mods.
+    * This invalidate all future object usage
+    * Ensure your set mod_channel to nil after that to free Lua resources
+* `is_writeable()`: returns true if channel is writeable and mod can send over it.
+* `send_all(message)`: Send `message` though the mod channel.
+    * If mod channel is not writeable or invalid, message will be dropped.
+    * Message size is limited to 65535 characters by protocol.
+
 ### `MetaDataRef`
 See `StorageRef`, `NodeMetaRef` and `ItemStackMetaRef`.
 
diff --git a/doc/mod_channels.png b/doc/mod_channels.png
new file mode 100644
index 000000000..08fdfca29
--- /dev/null
+++ b/doc/mod_channels.png