I am making use of the new pipeline mode added to libpq in PostgreSQL 14. At the same time I would still like to support older libpq versions by not providing the extended functionality that depends on this mode.
The natural way to achieve this in C/C++ is to conditionally enable code that depends on the additional APIs based on the preprocessor macro. And I could easily do this if libpq-fe.h provided a macro containing its version. Now, such a macro (PG_VERSION_NUM) is provided by pg_config.h that normally accompanies libpq-fe.h. However, I don't believe the presence of this file is guaranteed. All the documentation says[1] about headers is this: "Client programs that use libpq must include the header file libpq-fe.h and must link with the libpq library." And there are good reasons why packagers of libpq may decide to omit this header (in a nutshell, it embeds target architecture- specific information, see this discussion for background[2]). And I may not want to include it in my code (it defines a lot of free- named macros that may clash with my names). So I am wondering if it would make sense to provide a better way to obtain the libpq version as a macro? To me, as a user, the simplest way would be to have such a macro defined by libpq-fe.h. This would also provide a reasonable fallback for previous versions: if this macro is not defined, I know I am dealing with version prior to 14 and if I need to know which exactly I can try to include pg_config.h (perhaps with the help of __has_include if I am using C++). If simply moving this macro to libpq-fe.h is not desirable (for example, because it is auto-generated), then perhaps we could move this (and a few other version-related macros[3]) to a separate header (for example, libpq-version.h) and either include it from libpq-fe.h or define a macro in libpq-fe.h that signals its presence (e.g., PG_HAS_VERSION or some such). What do you think? [1] https://www.postgresql.org/docs/9.3/libpq.html [2] https://bugzilla.redhat.com/show_bug.cgi?id=828467 [3] PG_MAJORVERSION PG_MAJORVERSION_NUM PG_MINORVERSION_NUM PG_VERSION PG_VERSION_NUM PG_VERSION_STR (this one includes target so maybe leave it in pg_config.h)
