Add MQTT 3.1 protocol support

This change adds:
- Support for MQTT 3.1 protocol (MQIsdp) alongside 3.1.1 (MQTT)
- Proper version detection in Connect packet
- Test coverage for MQTT 3.1 protocol
- Fix for protocol reading in specs

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: carlhoerberg

spec/packets_spec.cr

@@ -50,7 +50,7 @@ describe MQTT::Protocol::Packet do
           io.write_string "to long"
           mio.rewind
 
-          expect_raises(MQTT::Protocol::Error::PacketDecode, /invalid protocol length/) do
+          expect_raises(MQTT::Protocol::Error::PacketDecode, /invalid protocol/) do
             MQTT::Protocol::Packet.from_io(mio)
           end
         end
@@ -171,6 +171,52 @@ describe MQTT::Protocol::Packet do
           connect.password.should eq password
           connect.will.not_nil!.topic.should eq wtopic
         end
+
+        it "supports MQTT 3.1 protocol" do
+          # Test MQTT 3.1 protocol by directly writing a Connect packet with version 0x03
+          # and verifying it's read back correctly
+          mio = IO::Memory.new
+          io = MQTT::Protocol::IO.new(mio)
+
+          # Write packet header
+          io.write_byte(0b00010000u8) # Connect packet
+
+          # Build the packet content in a separate IO to calculate length
+          content = IO::Memory.new
+          content_io = MQTT::Protocol::IO.new(content)
+          content_io.write_string("MQIsdp")
+          content_io.write_byte(0x03u8)            # MQTT 3.1 version
+          content_io.write_byte(0b00000010u8)      # Connect flags - clean session
+          content_io.write_int(30u16)              # Keepalive
+          content_io.write_string("mqtt31-client") # Client ID
+
+          # Write the remaining length and content
+          io.write_remaining_length(content.size)
+          io.write_bytes_raw(content.to_slice)
+
+          # Rewind to read
+          mio.rewind
+
+          # Read back and verify
+          packet = MQTT::Protocol::Packet.from_io(io)
+          packet.should be_a MQTT::Protocol::Connect
+
+          # Reset and read again to verify protocol name and version
+          mio.rewind
+          header_byte = mio.read_byte      # Skip type/flags
+          remaining_length = mio.read_byte # Skip remaining length
+
+          # Read protocol name
+          protocol_len = mio.read_bytes(UInt16, IO::ByteFormat::NetworkEndian)
+          protocol_name_bytes = Bytes.new(protocol_len)
+          mio.read_fully(protocol_name_bytes)
+          protocol_name = String.new(protocol_name_bytes)
+          protocol_name.should eq "MQIsdp"
+
+          # Read protocol version
+          protocol_version = mio.read_byte
+          protocol_version.should eq 0x03
+        end
       end
     end
 

src/mqtt/protocol/packets/connect.cr

@@ -11,11 +11,12 @@ module MQTT
       @username : String?
       @password : Bytes?
       @will : Will?
+      @version : UInt8
 
-      getter client_id, keepalive, username, password, will
+      getter client_id, keepalive, username, password, will, version
       getter? clean_session
 
-      def initialize(@client_id, @clean_session, @keepalive, @username, @password, @will)
+      def initialize(@client_id, @clean_session, @keepalive, @username, @password, @will, @version = 0x04)
         # Remaining length is at least 10:
         # protocol name (str) + protocol version (byte) + connect flags (byte) + keep alive (int)
         @remaining_length = 10
@@ -41,13 +42,20 @@ module MQTT
         decode_assert flags.zero?, MQTT::Protocol::Error::InvalidFlags, flags
 
         protocol_len = io.read_int
-        decode_assert protocol_len == 4, "invalid protocol length: #{protocol_len}"
-
         protocol = io.read_string(protocol_len)
-        decode_assert protocol == "MQTT", "invalid protocol: #{protocol.inspect}"
+
+        # MQIsdp is for MQTT 3.1, MQTT is for MQTT 3.1.1
+        if protocol == "MQTT"
+          decode_assert protocol_len == 4, "invalid protocol length for MQTT 3.1.1: #{protocol_len}"
+        elsif protocol == "MQIsdp"
+          decode_assert protocol_len == 6, "invalid protocol length for MQTT 3.1: #{protocol_len}"
+        else
+          decode_assert false, "invalid protocol: #{protocol.inspect}"
+        end
 
         version = io.read_byte
-        decode_assert version == 0x04, Error::UnacceptableProtocolVersion
+        decode_assert version == 0x04 || version == 0x03, Error::UnacceptableProtocolVersion
+        # @version will be set when we create a new instance at the end of this method
 
         connect_flags = io.read_byte
         decode_assert connect_flags.bit(0) == 0, "reserved connect flag set"
@@ -80,7 +88,7 @@ module MQTT
         username = io.read_string if has_username
         password = io.read_bytes if has_password
 
-        self.new(client_id, clean_session, keepalive, username, password, will)
+        self.new(client_id, clean_session, keepalive, username, password, will, version)
       end
 
       def to_io(io)
@@ -101,8 +109,13 @@ module MQTT
         connect_flags |= 0b0000_0010u8 if clean_session?
         io.write_byte(TYPE << 4)
         io.write_remaining_length remaining_length
-        io.write_string "MQTT"
-        io.write_byte 4u8 # "protocol version"
+        if version == 0x03
+          io.write_string "MQIsdp"
+          io.write_byte 3u8 # "protocol version"
+        else
+          io.write_string "MQTT"
+          io.write_byte 4u8 # "protocol version"
+        end
         io.write_byte connect_flags
         io.write_int keepalive
         io.write_string client_id if client_id