postgres_ffi: add WalStreamDecoder::complete_record() benchmark (#10158)

Touches #10097.

libs/postgres_ffi/src/wal_generator.rs

@@ -231,6 +231,22 @@ impl LogicalMessageGenerator {
         };
         [&header.encode(), prefix, message].concat().into()
     }
+
+    /// Computes how large a value must be to get a record of the given size. Convenience method to
+    /// construct records of pre-determined size. Panics if the record size is too small.
+    pub fn make_value_size(record_size: usize, prefix: &CStr) -> usize {
+        let xlog_header_size = XLOG_SIZE_OF_XLOG_RECORD;
+        let lm_header_size = size_of::<XlLogicalMessage>();
+        let prefix_size = prefix.to_bytes_with_nul().len();
+        let data_header_size = match record_size - xlog_header_size - 2 {
+            0..=255 => 2,
+            256..=258 => panic!("impossible record_size {record_size}"),
+            259.. => 5,
+        };
+        record_size
+            .checked_sub(xlog_header_size + lm_header_size + prefix_size + data_header_size)
+            .expect("record_size too small")
+    }
 }
 
 impl Iterator for LogicalMessageGenerator {