Using RTLD_DEEPBIND to handle symbol conflicts in loaded libraries

I had to make oracle_fdw work with PostgreSQL compiled using
--with-ldap. The issue there is that Oracle's client library has the
delightful property of linking against a ldap library they bundle that
has symbol conflicts with OpenLDAP. At PostgreSQL startup libldap is
loaded, so when libclntsh.so (the Oracle client) is loaded it gets
bound to OpenLDAP symbols, and unsurprisingly crashes with a segfault
when those functions get used.

glibc-2.3.4+ has a flag called RTLD_DEEPBIND for dlopen that prefers
symbols loaded by the library to those provided by the caller. Using
this flag fixes my issue, PostgreSQL gets the ldap functions from
libldap, Oracle client gets them from whatever it links to. Both work
fine.

Attached is a patch that enables this flag on Linux when available.
This specific case could also be fixed by rewriting oracle_fdw to use
dlopen for libclntsh.so and pass this flag, but I think it would be
better to enable it for all PostgreSQL loaded extension modules. I
can't think of a sane use case where it would be correct to prefer
PostgreSQL loaded symbols to those the library was actually linked
against.

Does anybody know of a case where this flag wouldn't be a good idea?
Are there any similar options for other platforms? Alternatively, does
anyone know of linker flags that would give a similar effect?

Regards,
Ants Aasma
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