There are not many ways to test alternate code paths having failure of
some function from standard library as a precondition.
For a starter, we need to test failing unlink{,at} functions in a
controlled manner to mimic client and server path of the IPC connection
having different privileges to validate the previous commit. But the
test suite cannot assume it has root privileges (so as to add artificial
user system-wide, which is a pretty stupid idea on its own), cannot
generally use stuff like chroot/namespacing (not to speak about
synergies of the former like docker). So what's left is to make our
own playground, or better yet, use existing playground but just to
modify the rules of the game a bit when it's desired -- a variation
of old good LD_PRELOAD trick.
Note that this concept was already used in syslog tests (see commit
642f74d) and is now further extended using dlsym(RTLD_NEXT, "symbol")
to resolve the standard library symbol being shadowed by our little
"module". This hence yields a customized wrapping we use to either
inject a call failure or to increase an invocation counter so as to
assure something has indeed been called. As the mechanisms used are
not supposed to be available everywhere, the build system is
conditionalized respectively.
Back to our test when unlink{,at} fails, with the help of the described
mechanism, it was actually easy to massage test_ipc_server_fail_shm
into test_ipcc_truncate_when_unlink_fails_shm desired addition, which
is also featured in this commit, together with a modification to
resources.test script so that it expects particular number of empty
file leftovers (see previous commit).
It's expected that the module for failure injections will keep growing
so as to enable better overall coverage of the code (on the platforms
where this provision is available).
