fix segfault converting a null pg_sys::Datum to a String (#1853)

Looks like this was introduced in beb7901 and we didn't have test coverage around this specific case.

pgrx-tests/src/tests/from_into_datum_tests.rs

@@ -43,4 +43,17 @@ mod tests {
         assert_eq!(cstr, expected);
         Ok(())
     }
+
+    #[pg_test]
+    fn null_string_is_none() {
+        let val: pg_sys::Datum = pg_sys::Datum::null();
+        let is_null: bool = true;
+
+        unsafe {
+            if let Some(_) = String::from_datum(val, is_null) {
+                // <- this seg fault
+                panic!("converted a null Datum into a valid string")
+            }
+        }
+    }
 }

pgrx/src/datum/from.rs

@@ -401,23 +401,27 @@ impl FromDatum for String {
     #[inline]
     unsafe fn from_polymorphic_datum(
         datum: pg_sys::Datum,
-        _is_null: bool,
+        is_null: bool,
         _typoid: pg_sys::Oid,
     ) -> Option<String> {
-        let varlena = pg_sys::pg_detoast_datum_packed(datum.cast_mut_ptr());
-        let converted_varlena = convert_varlena_to_str_memoized(varlena);
-        let ret_string = converted_varlena.to_owned();
-
-        // If the datum is EXTERNAL or COMPRESSED, then detoasting creates a pfree-able chunk
-        // that needs to be freed. We can free it because `to_owned` above creates a Rust copy
-        // of the string.
-        if varlena::varatt_is_1b_e(datum.cast_mut_ptr())
-            || varlena::varatt_is_4b_c(datum.cast_mut_ptr())
-        {
-            pg_sys::pfree(varlena.cast());
-        }
+        if is_null || datum.is_null() {
+            return None;
+        } else {
+            let varlena = pg_sys::pg_detoast_datum_packed(datum.cast_mut_ptr());
+            let converted_varlena = convert_varlena_to_str_memoized(varlena);
+            let ret_string = converted_varlena.to_owned();
+
+            // If the datum is EXTERNAL or COMPRESSED, then detoasting creates a pfree-able chunk
+            // that needs to be freed. We can free it because `to_owned` above creates a Rust copy
+            // of the string.
+            if varlena::varatt_is_1b_e(datum.cast_mut_ptr())
+                || varlena::varatt_is_4b_c(datum.cast_mut_ptr())
+            {
+                pg_sys::pfree(varlena.cast());
+            }
 
-        Some(ret_string)
+            Some(ret_string)
+        }
     }
 }