FAQ: Migration to Postgres 16

[workingjubilee]

There are some difficulties recently presented with migrating PGRX extensions to Postgres 16. Here I will attempt to answer these questions as they may arise.

Q. My build is failing with some sort of bindgen error, something about <emmintrin.h>?

A. You may have too many clangs. Basically, with rustup, rust-bindgen, and a system's clang packages, there can be a situation where there are "too many clangs" on the system. Essentially:

• rust-bindgen tries to use a clang of version M
• rust-bindgen attempts to build against the indicated postgres, dragging in its headers
• the postgres you want to build against is also bound against clang N's SIMD intrinsic headers (there is all of <[aeinpstw-z]mmintrin.h>).
• those SIMD headers are written in a way that may depend on specific internal details of the compiler in order to compile at all, thus any given clang M cannot be expected to compile clang N's headers, and clang N might not even be able to compile clang M's headers! They are more "compiler implementation details" than they are "regular" C code, which is why they are included using the same syntax as "system headers".
• thus the build fails when it uses clang M, but it will succeed if it uses clang N, but due to the vagaries of certain PATH lookups, rust-bindgen may default to clang M

This may not be something PGRX can solve (this is still unclear), but it is something that can be easily resolved as a configuration issue. It is usually sufficient to remove all but one clang package on the system, preferably using a more recent one (as this was initially discovered when using clang 11 and compile-erroring on clang 14's intrinsic headers).

Q. cargo pgrx install (or other commands) on macOS doesn't work?
A. Oops. Try pgrx v0.10.2! It should be patched now.

[workingjubilee]

My current hypothesis is that the Postgres 16 build errors might be induced by our conditional setting of detect_include_paths. This is possible to disable by setting PGRX_BINDGEN_NO_DETECT_INCLUDES=please.

[workingjubilee]

Most concerns about migration to Postgres 16 should be addressed by using pgrx v0.11.1 (I hope!)