[zaniebot]

PBS rewrites loader paths after installing CPython. On glibc, a wheel that links libpython and supplies its own DT_RUNPATH can still fail to import.

Backport the experimental --enable-relocatable flag proposed in python/cpython#160377 and implement it for Python 3.14, 3.15, and 3.16. CPython links shared libraries and standard-library extensions with paths relative to their installed locations. Its install rules generate relocatable scripts and embedding metadata, and sysconfig derives BINDIR, LIBDIR, and LIBPL from the running installation.

On glibc, the launcher that statically links libpython opens the bundled shared library at startup so a wheel can reuse it. On musl, the launcher carries a path to the library directory. Non-framework macOS builds give the dylib its @rpath install name at link time.

On one x86-64 Linux host with warm filesystem caches, minimal startup without site was 12–22% slower on glibc. The table gives median changes in milliseconds relative to the existing PBS builds. The import columns include startup and loading a C extension linked to libpython.

Python	glibc startup	glibc startup + import	musl startup	musl startup + import
3.14.9	+1.72	−0.57	−0.14	−0.44
3.15.1	+2.37	+0.02 (95% interval −0.03 to +0.06)	+0.11	+0.06
3.16.0a1	+2.09	−0.35	+0.23	+0.19

At startup, the preloaded libpython occupied 3.84–4.28 MiB of resident memory on glibc, including 1.31–1.49 MiB of private dirty pages per process. On musl, the linked import was the first use of the shared library.

On Python 3.15.1, a glibc prototype that retried a failed import saved 0.98 ms at startup and added 0.27 ms when a retry was needed. A different libpython on LD_LIBRARY_PATH can satisfy the first load and bypass the retry.