tst_udf_self_load fails on static-only builds (--disable-shared)

[edhartnett]

Bug: tst_udf_self_load fails on static-only builds (--disable-shared)

Summary

tst_udf_self_load fails on any build configured with --disable-shared because
the test calls dlopen() on a shared library plugin (.so) that is never built
in a static-only configuration. The test was unconditionally included in the test
suite regardless of whether shared library support was enabled.

Observed failure

On the big-endian CI job (s390x, built with --disable-shared --enable-static):

ERR: dlopen failed for /home/runner/work/netcdf-c/netcdf-c/nc_test4/.libs/tst_udf_self_load_plugin.so: \
  /home/runner/work/netcdf-c/netcdf-c/nc_test4/.libs/tst_udf_self_load_plugin.so: \
  cannot open shared object file: No such file or directory

*** Testing UDF self-registration plugin loading.
*** testing plugin loader with NC_Dispatch*-returning init...Sorry! Unexpected result, tst_udf_self_load.c, line: 68
FAIL tst_udf_self_load (exit status: 2)

The .so file does not exist because libtool does not build shared modules when
--disable-shared is passed to configure.

Root cause

In nc_test4/Makefile.am, tst_udf_self_load was listed unconditionally in
NC4_TESTS:

tst_put_vars tst_elatefill tst_udf tst_put_vars_two_unlim_dim \
tst_bug1442 tst_quantize tst_h_transient_types tst_udf_self_load

The associated plugin (tst_udf_self_load_plugin.la) was also built
unconditionally via check_LTLIBRARIES. When --disable-shared is in effect,
libtool builds only a static archive for check_LTLIBRARIES targets and does
not install a .so into .libs/. The test binary is compiled and linked, but
at runtime dlopen() cannot find the plugin file, causing an immediate failure.

Proposed fix

Add an AM_CONDITIONAL for enable_shared in configure.ac:

AM_CONDITIONAL(ENABLE_SHARED, [test "x$enable_shared" = xyes])

Then guard the test and its plugin in nc_test4/Makefile.am under that
conditional:

# UDF self-registration plugin loading test.
# Requires a shared library build: the test dlopen()s a .so plugin.
# Skip entirely when built with --disable-shared.
if ENABLE_SHARED
NC4_TESTS += tst_udf_self_load
check_LTLIBRARIES = tst_udf_self_load_plugin.la
tst_udf_self_load_plugin_la_SOURCES = tst_udf_self_load_plugin.c
tst_udf_self_load_plugin_la_LDFLAGS = -module -avoid-version -shared -no-undefined -rpath /tmp
tst_udf_self_load_plugin_la_LIBADD = ${top_builddir}/liblib/libnetcdf.la
tst_udf_self_load_CPPFLAGS = $(AM_CPPFLAGS) -DTEST_PLUGIN_PATH=\"$(abs_builddir)/.libs/tst_udf_self_load_plugin@MODULE_EXT@\"
endif # ENABLE_SHARED

This ensures the test and plugin are only built and run when shared libraries
are enabled, which is the only configuration where dlopen() can succeed.

Affected files

• configure.ac — add AM_CONDITIONAL(ENABLE_SHARED, ...)
• nc_test4/Makefile.am — guard tst_udf_self_load and its plugin under if ENABLE_SHARED

Reproducer

Configure netCDF-C with shared libraries disabled and run the nc_test4 suite:

./configure --enable-hdf5 --disable-shared --enable-static
make check -C nc_test4 TESTS=tst_udf_self_load

The test can also be reproduced under QEMU emulation using the CI workflow
.github/workflows/run_tests_bigendian.yml, which passes --disable-shared.