A test for UDT/composite_type support (#329)

The test is essentially a cut-n-paste of the long example in the docs.

Author: eeeebbbbrrrr

plrust/src/tests.rs

@@ -1263,6 +1263,90 @@ mod tests {
 
         Ok(())
     }
+
+    #[pg_test]
+    fn test_udt() -> spi::Result<()> {
+        Spi::run(
+            r#"
+CREATE TYPE person AS (
+    name text,
+    age  float8
+);
+
+create function make_person(name text, age float8) returns person
+    strict parallel safe
+    language plrust as
+$$
+    // create the Heap Tuple representation of the SQL type `person`
+    let mut p = PgHeapTuple::new_composite_type("person")?;
+    
+    // set a few of its attributes
+    //
+    // Runtime errors can occur if the attribute name is invalid or if the Rust type of the value
+    // is not compatible with the backing SQL type for that attribute.  Hence the use of the `?` operator
+    p.set_by_name("name", name)?;
+    p.set_by_name("age", age)?;
+    
+    // return the `person`
+    Ok(Some(p))
+$$;
+
+create function get_person_name(p person) returns text
+    strict parallel safe
+    language plrust as
+$$
+   // `p` is a `PgHeapTuple` over the underlying data for `person`
+   Ok(p.get_by_name("name")?)
+$$;
+
+create function get_person_age(p person) returns float8
+    strict parallel safe
+    language plrust as
+$$
+   // `p` is a `PgHeapTuple` over the underlying data for `person`
+   Ok(p.get_by_name("age")?)
+$$;
+
+create function get_person_attribute(p person, attname text) returns text
+    strict parallel safe
+    language plrust as
+$$
+   match attname.to_lowercase().as_str() {
+    "age" => {
+        let age:Option<f64> = p.get_by_name("age")?;
+        Ok(age.map(|v| v.to_string()))
+    },
+    "name" => {
+        Ok(p.get_by_name("name")?)
+    },
+    _ => panic!("unknown attribute: `{attname}`")
+   }
+$$;
+
+create operator ->> (function = get_person_attribute, leftarg = person, rightarg = text);
+
+create table people
+(
+    id serial8 not null primary key,
+    p  person
+);
+
+insert into people (p) values (make_person('Johnny', 46.24));
+insert into people (p) values (make_person('Joe', 99.09));
+insert into people (p) values (make_person('Dr. Beverly Crusher of the Starship Enterprise', 32.0));
+            "#,
+        )?;
+
+        let johnny = Spi::get_one::<PgHeapTuple<AllocatedByRust>>(
+            "SELECT p FROM people WHERE p->>'name' = 'Johnny';",
+        )?
+        .expect("SPI result was null");
+
+        let age = johnny.get_by_name::<f64>("age")?.expect("age was null");
+        assert_eq!(age, 46.24);
+
+        Ok(())
+    }
 }
 
 #[cfg(any(test, feature = "pg_test"))]