The page header struct does not currently have any versioning or extensibility information, which would make it difficult to make any changes to it in the future without major upgrade/downgrade planning.

To make the page header extensible in a backwards compatible way, we can claim the unused byte field and rename it to extra_header_length, which we would set to the length of the header minus the current header size (28 bytes). For example, if we add a new 4-byte field and make the new header 32 bytes, we would set the extra length to 32 – 28 = 4.

In this way, the current expected value for this field would be 28 – 28 = 0, which is exactly what should be stored in this unused field already across all of our existing deployments.

As a part of this ticket, we should also update all places in the code that assume a fixed length (derived using sizeof) to leverage the extra length field, validate the field upon read, and add a test.