Nullable string type (for disconnect explanation)

src/calloop.rs

@@ -140,7 +140,7 @@ fn handle_result(
         } else {
             eis::connection::DisconnectReason::Protocol
         };
-        connection.disconnected(reason, &err.to_string());
+        connection.disconnected(reason, Some(&err.to_string()));
         let _ = connection.flush();
     }
 
@@ -168,7 +168,7 @@ fn process_handshake(
             if !handle.has_interface("ei_seat") || !handle.has_interface("ei_device") {
                 handle.disconnected(
                     eis::connection::DisconnectReason::Protocol,
-                    "Need `ei_seat` and `ei_device`",
+                    Some("Need `ei_seat` and `ei_device`"),
                 );
                 let _ = request_converter.handle().flush();
                 return Err(HandshakeError::MissingInterface.into());

src/eiproto.rs.jinja

@@ -22,7 +22,7 @@
 
 // GENERATED FILE
 
-{# TODO handle context_type #}
+{# TODO More complete handling of nullable types? #}
 
 use crate::wire;
 
@@ -32,7 +32,9 @@ use crate::wire;
 
 {%- macro arg_type(arg, owned, generic) -%}
     {%- if arg.enum != None -%} {{ arg.enum.name|camel }}
+    {%- elif arg.protocol_type == 'string' and owned and arg.allow_null -%} Option<String>
     {%- elif arg.protocol_type == 'string' and owned -%} String
+    {%- elif arg.protocol_type == 'string' and arg.allow_null -%} Option<&str>
     {%- elif arg.protocol_type == 'string' -%} &str
     {%- elif arg.protocol_type == 'int32' -%} i32
     {%- elif arg.protocol_type == 'uint32' -%} u32
@@ -182,7 +184,7 @@ pub mod {{interface.plainname}} {
             let args = &[
             {%- for arg in outgoing.arguments %}
             {% if arg.interface_arg_for %}
-                wire::Arg::{{arg.protocol_type|camel}}({{arg.name|camel}}::NAME),
+                wire::Arg::{{arg.protocol_type|camel}}(Some({{arg.name|camel}}::NAME)),
             {% else %}
                 wire::Arg::{{arg.protocol_type|camel}}({{arg.name}}
                 {% if arg.protocol_type == 'new_id' %}

src/eiproto_ei.rs

@@ -542,7 +542,7 @@ pub mod connection {
             /// The reason for being disconnected.
             reason: DisconnectReason,
             /// An explanation for debugging purposes.
-            explanation: String,
+            explanation: Option<String>,
         },
         /// Seat presence information.
         ///

src/eiproto_eis.rs

@@ -476,7 +476,7 @@ pub mod connection {
             &self,
             last_serial: u32,
             reason: DisconnectReason,
-            explanation: &str,
+            explanation: Option<&str>,
         ) -> () {
             let args = &[
                 wire::Arg::Uint32(last_serial.into()),
@@ -1478,7 +1478,7 @@ pub mod device {
                 .new_object(InterfaceName::NAME.to_string(), version);
             let args = &[
                 wire::Arg::NewId(object.id().into()),
-                wire::Arg::String(InterfaceName::NAME),
+                wire::Arg::String(Some(InterfaceName::NAME)),
                 wire::Arg::Uint32(version.into()),
             ];
 

src/event.rs

@@ -976,7 +976,7 @@ pub struct Disconnected {
     /// Reason for disconnection.
     pub reason: ei::connection::DisconnectReason,
     /// Explanation for debugging purposes.
-    pub explanation: String,
+    pub explanation: Option<String>,
 }
 
 /// High-level translation of the seat description events ending with [`ei_seat.done`](ei::seat::Event::Done).

src/request.rs

@@ -75,8 +75,7 @@ impl Connection {
     ///
     /// Will panic if an internal Mutex is poisoned.
     // TODO(axka, 2025-07-08): rename to something imperative like `notify_disconnection`
-    // TODO(axka, 2025-07-08): `explanation` must support NULL: https://gitlab.freedesktop.org/libinput/libei/-/commit/267716a7609730914b24adf5860ec8d2cf2e7636
-    pub fn disconnected(&self, reason: DisconnectReason, explanation: &str) {
+    pub fn disconnected(&self, reason: DisconnectReason, explanation: Option<&str>) {
         let seats = self
             .0
             .seats
@@ -393,6 +392,7 @@ impl EisRequestConverter {
                 self.handle_touchscreen_request(touchscreen, request)?;
             }
         }
+
         Ok(())
     }
 

src/wire/arg.rs

@@ -19,7 +19,7 @@ pub enum Arg<'a> {
     Int64(i64),
     Float(f32),
     Fd(BorrowedFd<'a>),
-    String(&'a str),
+    String(Option<&'a str>),
     NewId(u64),
     Id(u64),
 }
@@ -56,7 +56,10 @@ impl Arg<'_> {
             Arg::Float(value) => buf.extend(value.to_ne_bytes()),
             // XXX unwrap?
             Arg::Fd(value) => fds.extend([value.try_clone_to_owned().unwrap()]),
-            Arg::String(value) => {
+            Arg::String(None) => {
+                buf.extend(0u32.to_ne_bytes());
+            }
+            Arg::String(Some(value)) => {
                 // Write 32-bit length, including NUL
                 let len = value.len() as u32 + 1;
                 buf.extend(len.to_ne_bytes());
@@ -145,11 +148,11 @@ impl OwnedArg for OwnedFd {
     }
 }
 
-impl OwnedArg for String {
+impl OwnedArg for Option<String> {
     fn parse(buf: &mut ByteStream) -> Result<Self, ParseError> {
         let mut len = u32::parse(buf)?;
         if len == 0 {
-            return Ok(String::new());
+            return Ok(None);
         }
         let bytes = buf.read_n(len as usize - 1)?; // Exclude NUL
         let string = String::from_utf8(bytes.collect())?;
@@ -159,11 +162,21 @@ impl OwnedArg for String {
             len += 1;
             buf.read::<1>()?;
         }
-        Ok(string)
+        Ok(Some(string))
+    }
+
+    fn as_arg(&self) -> Arg<'_> {
+        Arg::String(self.as_deref())
+    }
+}
+
+impl OwnedArg for String {
+    fn parse(buf: &mut ByteStream) -> Result<Self, ParseError> {
+        Option::<String>::parse(buf)?.ok_or(ParseError::InvalidNull)
     }
 
     fn as_arg(&self) -> Arg<'_> {
-        Arg::String(self)
+        Arg::String(Some(self))
     }
 }
 

src/wire/mod.rs

@@ -122,6 +122,8 @@ pub enum ParseError {
     HeaderLength(u32),
     /// Message length didn't match header.
     MessageLength(u32, u32),
+    /// NULL for non-nullable argument
+    InvalidNull,
 }
 
 impl fmt::Display for ParseError {
@@ -142,6 +144,9 @@ impl fmt::Display for ParseError {
             Self::MessageLength(a, b) => {
                 write!(f, "message length didn't match header ({a} != {b})")
             }
+            Self::InvalidNull => {
+                write!(f, "NULL value for non-nullable argument")
+            }
         }
     }
 }