if (BufferIsValid(buffer0))

UnlockReleaseBuffer(buffer0);

Note that we must only use PageSetLSN/PageGetLSN() when we know the action

is serialised. Only Startup process may modify data blocks during recovery,

so Startup process may execute PageGetLSN() without fear of serialisation

problems. All other processes must only call PageSet/GetLSN when holding

either an exclusive buffer lock or a shared lock plus buffer header lock,

or be writing the data block directly rather than through shared buffers

while holding AccessExclusiveLock on the relation.

Due to all these constraints, complex changes (such as a multilevel index

insertion) normally need to be described by a series of atomic-action WAL

records. What do you do if the intermediate states are not self-consistent?