Fix default collation deserialization

We've run into an issue where the database default collation doesn't
necessarily exist as a "real" collation in pg_collation. This code
specializes the deserialization code to handle this: if we don't find
the collation we're trying to deserialize, check if the name matches
that of the default collation and if so assume it's the default
collation.

extension/src/serialization/collations.rs

@@ -9,6 +9,8 @@ use flat_serialize::{impl_flat_serializable, FlatSerializable, WrapErr};
 
 use serde::{Deserialize, Serialize};
 
+use once_cell::sync::Lazy;
+
 use pg_sys::{Datum, Oid};
 use pgx::*;
 
@@ -74,7 +76,21 @@ struct FormData_pg_database {
 #[allow(non_camel_case_types)]
 type Form_pg_database = *mut FormData_pg_database;
 
+static DEFAULT_COLLATION_NAME: Lazy<CString> = Lazy::new(|| unsafe {
+    let tuple = pg_sys::SearchSysCache1(pg_sys::SysCacheIdentifier_DATABASEOID as _, pg_sys::MyDatabaseId as _);
+    if tuple.is_null() {
+        pgx::error!("no database info");
+    }
 
+    let database_tuple: Form_pg_database = get_struct(tuple);
+    let collation_name = (*database_tuple).datcollate.data.as_ptr();
+    let collation_name_len = CStr::from_ptr(collation_name).to_bytes().len();
+    let collation_name = pg_sys::pg_server_to_any(collation_name, collation_name_len as _, pg_sys::pg_enc_PG_UTF8 as _);
+    let collation_name = CStr::from_ptr(collation_name);
+    let collation_name = collation_name.clone();
+    pg_sys::ReleaseSysCache(tuple);
+    CString::from(collation_name)
+});
 
 impl Serialize for PgCollationId {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
@@ -107,32 +123,21 @@ impl Serialize for PgCollationId {
 
             // the 'default' collation isn't really a collation, and we need to
             // look in pg_database to discover what real collation is
-            let mut db_tuple = None;
             let collation_name =
                 if self.0 == DEFAULT_COLLATION_OID {
-                    let tuple = pg_sys::SearchSysCache1(pg_sys::SysCacheIdentifier_DATABASEOID as _, pg_sys::MyDatabaseId as _);
-                    if tuple.is_null() {
-                        pgx::error!("no database info");
-                    }
-                    db_tuple = Some(tuple);
-
-                    let database_tuple: Form_pg_database = get_struct(tuple);
-                    (*database_tuple).datcollate.data.as_ptr()
+                    &*DEFAULT_COLLATION_NAME
                 } else {
-                    (*collation_tuple).collname.data.as_ptr()
+                    let collation_name = (*collation_tuple).collname.data.as_ptr();
+                    let collation_name_len = CStr::from_ptr(collation_name).to_bytes().len();
+                    let collation_name = pg_sys::pg_server_to_any(collation_name, collation_name_len as _, pg_sys::pg_enc_PG_UTF8 as _);
+                    CStr::from_ptr(collation_name)
                 };
-
-            let collation_name_len = CStr::from_ptr(collation_name).to_bytes().len();
-            let collation_name = pg_sys::pg_server_to_any(collation_name, collation_name_len as _, pg_sys::pg_enc_PG_UTF8 as _);
-            let collation_name = CStr::from_ptr(collation_name);
             let collation_name = collation_name.to_str().unwrap();
 
             let qualified_name: (&str, &str) = (namespace, collation_name);
             layout = Some(qualified_name);
             let res = layout.serialize(serializer);
-            if let Some(db_tuple) = db_tuple {
-                pg_sys::ReleaseSysCache(db_tuple);
-            }
+
             pg_sys::ReleaseSysCache(tuple);
             res
         }
@@ -201,6 +206,11 @@ impl<'de> Deserialize<'de> for PgCollationId {
             }
 
             if collation_id == pg_sys::InvalidOid {
+                // The default collation doesn't necessarily exist in the
+                // collations catalog, so check that specially
+                if name == &**DEFAULT_COLLATION_NAME {
+                    return Ok(PgCollationId(100))
+                }
                 return Err(D::Error::custom(format!(
                     "invalid collation {:?}.{:?}",
                     namespace, name