We were using a customized version which worked fine but was hard to merge with upstream changes. Now this code is maintained much like the types in static.auto.h that we copy and check with each release.
The goal is to eventually build directly against PostgreSQL (either source or libcommon) and this brings us one step closer.
pgPageChecksum() must modify the page header in order to calculate the checksum. The modification is temporary but make it clear that it happens by removing the const.
Also make a note about our non-entirely-kosher usage of a const Buffer in the PageChecksum filter. This is safe as currently coded but at the least we need to be aware of what is going on.
Page size is passed around a lot but in fact it can only have one value, PG_PAGE_SIZE_DEFAULT, which is checked when pg_control is loaded. There may be an argument for supporting multiple page sizes in the future but for now just use the constant to simplify the code.
There is also a significant performance benefit. Because pageSize was being used in pageChecksumBlock() the main loop was neither unrolled nor vectorized (-funroll-loops -ftree-vectorize) as it is now with a constant loop boundary.
This function made validation faster in Perl because fewer calls (and buffer transformations) were required when all checksums were valid.
In C calling pageChecksumTest() directly is just as efficient so there is no longer a need for pageChecksumBufferTest().
This filter exactly mimics the behavior of the Perl filter so is a drop-in replacement.
The filter is not integrated yet since it requires the Perl-to-C storage layer interface coming in a future commit.
