feat: empty statement implementation for server-api

CHANGELOG.md

@@ -23,8 +23,40 @@ Versioning](https://semver.org/spec/v2.0.0.html).
   by PostgreSQL 18+ and the minor-only form used by older servers) and adopts
   the negotiated version for the rest of the connection.
 
-### Fixed
+### Changed
 
+- Breaking: `QueryParser::parse_sql` now returns
+  `PgWireResult<Option<Self::Statement>>`, and `StoredStatement::parse`
+  correspondingly returns `Option<StoredStatement<S>>`. `None` denotes an
+  empty query: it is stored as an empty statement and executes to
+  `EmptyQueryResponse`. This allows a query parser to report its own notion
+  of empty query; syntactically empty queries (semicolons and whitespace
+  only) are still never passed to the parser.
+- Breaking: `PortalStore` now represents empty statements and portals.
+  `get_statement` returns `Option<Entry<StoredStatement<S>>>` and
+  `get_portal` returns `Option<Entry<Portal<S>>>`: the new `Entry::Empty`
+  variant marks a name under which an empty prepared statement or portal is
+  stored, alongside the new `put_empty_statement`/`put_empty_portal`
+  methods. Like every `put_*`, storing an empty entry replaces whatever was
+  previously stored under that name, and `rm_*`/`clear_portals` remove
+  empty entries along with regular ones. `StoredStatement` and `Portal`
+  themselves are unchanged — the impact is limited to `PortalStore`
+  implementors and code calling `get_statement`/`get_portal` directly
+  (`Entry::value` helps with the migration).
+
+### Fixed
+
+- Extended query protocol: empty queries (a query string without any
+  statement, such as `""` or `";;"`) are now handled like PostgreSQL instead
+  of being dispatched to the query parser: `Parse` succeeds without calling
+  `QueryParser` and stores an empty statement, `Describe` answers
+  `ParameterDescription` (no parameters) + `NoData`, `Bind` succeeds
+  (rejecting bound parameters with `08P01`) and stores an empty portal, and
+  `Execute` returns `EmptyQueryResponse` without reaching `do_query`. An
+  empty `Parse` replaces any statement previously stored under the same
+  name, and `Close`/`Sync` drop empty statements and the unnamed empty
+  portal like real ones. Behavior verified message-for-message against
+  PostgreSQL 18.
 - Client API: backend messages are now decoded with the rules of the protocol
   version the client actually advertised, instead of always 3.2. Previously a
   4-byte protocol 3.0 cancel key was decoded as `SecretKey::Bytes` instead of

examples/cursor.rs

@@ -11,7 +11,7 @@ use pgwire::api::portal::Portal;
 use pgwire::api::query::SimpleQueryHandler;
 use pgwire::api::results::{DataRowEncoder, FieldFormat, FieldInfo, QueryResponse, Response, Tag};
 use pgwire::api::stmt::StoredStatement;
-use pgwire::api::store::{MemPortalStore, PortalStore};
+use pgwire::api::store::{Entry, MemPortalStore, PortalStore};
 use pgwire::api::{ClientInfo, ClientPortalStore, PgWireServerHandlers, Type};
 use pgwire::error::{ErrorInfo, PgWireError, PgWireResult};
 use pgwire::messages::response::NoticeResponse;
@@ -210,7 +210,7 @@ async fn handle_fetch(
 ) -> PgWireResult<Vec<Response>> {
     println!("FETCH {} FROM {}", count, cursor_name);
 
-    let Some(portal) = portal_store.get_portal(cursor_name) else {
+    let Some(Entry::Value(portal)) = portal_store.get_portal(cursor_name) else {
         return Err(PgWireError::UserError(Box::new(ErrorInfo::new(
             "ERROR".to_owned(),
             "34000".to_owned(),