Regression tests fail with musl libc because libpq.so can't be loaded

Running the regression tests when building with musl libc fails, with 
errors like the following:

ERROR:  could not load library 
"<builddir>/tmp_install/usr/lib/postgresql/libpqwalreceiver.so": Error 
loading shared library libpq.so.5: No such file or directory (needed by 
<builddir>/tmp_install/usr/lib/postgresql/libpqwalreceiver.so)

This was observed in Alpine Linux [1] and nixpkgs [2] a few years ago. I 
now looked at this a bit and this is what happens:

- The temporary install location is set via LD_LIBRARY_PATH in the 
regression tests, so that postgres can find those libs.

- All tests which load an extension / shared module via dlopen() fail, 
when the loaded library in turn depends on another library in 
tmp_install - I think in practice it's libpq.so all the time.

- LD_LIBRARY_PATH is used correctly to look for the direct dependency 
loaded in dlopen(...), but is not taken into account anymore when trying 
to locate libpq.so. This step only fails with musl, but works fine with 
glibc.


I can reproduce this with a simple Dockerfile (attached), which uses the 
library/postgres-alpine image, moves libpq.so to a different folder and 
points LD_LIBRARY_PATH at it. Build and run the dockerfile like this:

   docker build . -t pg-musl && docker run --rm pg-musl

This Dockerfile can easily be adjusted to work with the debian image - 
which shows that doing the same with glibc works just fine.


Even though this originated in "just" the regression tests, I'm filing 
this as a bug, because:
- The docs explicitly mention LD_LIBRARY_PATH support to point at a 
different /lib folder in [3].
- This can clearly break outside the test-suite as shown with the 
Dockerfile.


I tried a few more things:
- When I add an /etc/ld-musl-$(ARCH).path file and add the path to 
libpq.so's libdir to it, libpq.so can be found.
- When I add the path to libpq.so as an rpath to the postgres binary, 
libpq.so can be found.

Both is not surprising, but just confirms musl-ld actually works as 
expected. It's just LD_LIBRARY_PATH that seems to not be passed on.

To rule out a musl bug, I also put together a very simple test-case of 
an executable loading liba with dlopen(), which depends on libb and then 
constructing the same scenario with LD_LIBRARY_PATH. This works fine 
when compiled with glibc and musl, too. Thus, I believe the problem to 
be somewhere in how postgres loads those libraries.

Best,

Wolfgang

[1]: 

https://github.com/alpinelinux/aports/commit/d67ceb66a1ca9e1899071c9ef09fffba29fa0417#diff-2bd25b5172fc52319de1b09086ac0db6314d2e9fa73497979f5198f8caaec1b9

[2]: 
https://github.com/NixOS/nixpkgs/commit/09ffd722072291f00f2a54d7404eb568a15e562a

[3]: https://www.postgresql.org/docs/current/install-post.html