feat: GuiPresence (chat-service presence backend, gateway route, mod module); restore TODO history

backend/chat-service/src/presence/payload.rs

@@ -0,0 +1,46 @@
+//! Validation of the opaque GUI payload and server key. The payload is a
+//! base64 string produced by the mod (header + up to 8 cursor samples); the
+//! service never decodes it, it only bounds what it stores and relays.
+
+/// Header (4 bytes) + 8 samples x 4 bytes = 36 bytes = 48 base64 characters.
+pub const MAX_PAYLOAD_CHARS: usize = 48;
+pub const MAX_SERVER_CHARS: usize = 64;
+pub const MAX_WANT: usize = 40;
+
+pub fn valid_payload(s: &str) -> bool {
+    !s.is_empty()
+        && s.len() <= MAX_PAYLOAD_CHARS
+        && s.bytes().all(|b| b.is_ascii_alphanumeric() || matches!(b, b'+' | b'/' | b'='))
+}
+
+/// Normalised server key: trimmed, lower-case, no control characters, bounded.
+pub fn normalize_server(s: &str) -> Option<String> {
+    let s = s.trim().to_ascii_lowercase();
+    if s.is_empty() || s.chars().count() > MAX_SERVER_CHARS || s.chars().any(|c| c.is_control()) {
+        return None;
+    }
+    Some(s)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn payload_charset_and_length() {
+        assert!(valid_payload("AQEAAAA="));
+        assert!(!valid_payload(""));
+        assert!(!valid_payload("has space"));
+        assert!(!valid_payload("<script>"));
+        assert!(!valid_payload(&"A".repeat(MAX_PAYLOAD_CHARS + 1)));
+        assert!(valid_payload(&"A".repeat(MAX_PAYLOAD_CHARS)));
+    }
+
+    #[test]
+    fn server_is_normalised() {
+        assert_eq!(normalize_server("  Play.Example.COM ").as_deref(), Some("play.example.com"));
+        assert_eq!(normalize_server(""), None);
+        assert_eq!(normalize_server("a\nb"), None);
+        assert_eq!(normalize_server(&"x".repeat(65)), None);
+    }
+}