Correctly handle the endiness and packedness of the the header reading and writing

src/saprus_message.zig

@@ -64,7 +64,7 @@ pub const SaprusMessage = union(SaprusPacketType) {
     }
 
     fn toBytesAux(
-        comptime Header: type,
+        Header: type,
         header: Header,
         payload: []const u8,
         buf: *std.ArrayList(u8),
@@ -81,19 +81,18 @@ pub const SaprusMessage = union(SaprusPacketType) {
 
         // At this point, payload_list contains the base64 encoded payload.
 
-        // Write the packet body to the output buf.
-        try buf.*.appendSlice(asBytes(&nativeToBig(u16, @intCast(payload_list.items.len))));
-
-        // Write the header bytes to the output buf.
-        const HeaderInt = @typeInfo(Header).@"struct".backing_integer.?;
-        std.mem.writePackedInt(
-            HeaderInt,
-            try buf.*.addManyAsSlice(@bitSizeOf(Header) / 8),
-            0,
-            @bitCast(header),
-            .little,
+        // Add the payload length to the output buf.
+        try buf.*.appendSlice(
+            asBytes(&nativeToBig(u16, @intCast(payload_list.items.len + @bitSizeOf(Header) / 8))),
         );
 
+        // Add the header bytes to the output buf.
+        var header_buf: [@sizeOf(Header)]u8 = undefined;
+        var header_buf_stream = std.io.fixedBufferStream(&header_buf);
+        try header_buf_stream.writer().writeStructEndian(header, .big);
+        // Add the exact number of bits in the header without padding.
+        try buf.*.appendSlice(header_buf[0 .. @bitSizeOf(Header) / 8]);
+
         try buf.*.appendSlice(payload_list.items);
     }
 
@@ -101,6 +100,7 @@ pub const SaprusMessage = union(SaprusPacketType) {
     pub fn toBytes(self: SaprusMessage, allocator: Allocator) ![]u8 {
         // Create a growable list of bytes to store the output in.
         var buf = std.ArrayList(u8).init(allocator);
+        errdefer buf.deinit();
 
         // Start with writing the message type, which is the first 16 bits of every Saprus message.
         try buf.appendSlice(asBytes(&nativeToBig(u16, @intFromEnum(self))));
@@ -116,17 +116,21 @@ pub const SaprusMessage = union(SaprusPacketType) {
         return buf.toOwnedSlice();
     }
 
-    inline fn fromBytesAux(
-        packet: SaprusPacketType,
+    fn fromBytesAux(
+        comptime packet: SaprusPacketType,
         Header: type,
         r: std.io.FixedBufferStream([]const u8).Reader,
         allocator: Allocator,
     ) !SaprusMessage {
-        // Read the header for the current message type.
-        const header = try r.readStructEndian(Header, .big);
         // Read the length of the base64 encoded payload.
         const len = try r.readInt(u16, .big);
 
+        // Read the header for the current message type.
+        var header_bytes: [@sizeOf(Header)]u8 = undefined;
+        _ = try r.read(header_bytes[0 .. @bitSizeOf(Header) / 8]);
+        var header_stream = std.io.fixedBufferStream(&header_bytes);
+        const header = try header_stream.reader().readStructEndian(Header, .big);
+
         // Read the base64 bytes into a list to be able to call the decoder on it.
         var payload_buf = std.ArrayList(u8).init(allocator);
         defer payload_buf.deinit();