Add a COB_LOAD_GLOBAL config option to modify dlopen behavior

[florianschmidt1994]

This PR adds a config option called COB_LOAD_GLOBAL which allows the flags of dlopen used in call.c to be changed between RTLD_GLOBAL and RTLD_LOCAL (see https://linux.die.net/man/3/dlopen)

Motivation

This flag is motivated by an ambition to be able to write a multi-threaded c program, that then loads libcob with dlmopen in one namespace per thread, effectively allowing us to call the same libcob / Cobol module from multiple threads in a "thread-safe" way. The image below shows the a very simplified intended call sequence

Because there is a "bug" in dlopen that stops us from calling dlopen("module", RTLD_GLOBAL), this PR introduces an option to be able to change this flag to RTLD_LOCAL at runtime.

As at glibc 2.24, specifying the RTLD_GLOBAL flag when calling dlmopen() generates an error. Furthermore, specifying RTLD_GLOBAL when calling dlopen() results in a program crash (SIGSEGV) if the call is made from any object loaded in a namespace other than the initial namespace.

Docs

# Environment name:  COB_LOAD_GLOBAL
#   Parameter name:  load_global
#          Purpose:  tell the system loader to provide symbols in CALLed
#                    programs globally, allowing symbols to be found for linked
#                    libraries later
#             Type:  boolean
#             Note:  COBOL CALLs will always find symbols in already CALLed or
#                    pre-modules; this setting is mostly an advise to the system,
#                    not all systems are capable of loading libraries global/local
#          Default:  false
#          Example:  load_global true

Implementation

• Change lt_dlopen from a macro definition to an actual function, which internally calls dlopen with the flags based on the configuration
• Add cob_load_global / COB_LOAD_GLOBAL as a configuration
• Add documentation to runtime.cfg about COB_LOAD_GLOBAL

[GitMensch]

Apart from the other PR's review to that branch, the change here makes the current scenario even more confusing.
I'd like to fix this in this PR (or up-front upstream).

After the changes:

• instead of calling lt_dlopen we call a new static function cob_dlopen
• this has the current code of this PR in for the case of dlopen usage
• for the WIN32 variant it uses what is currently used there
• for the libtool variant it uses lt_dlopenadvise() with an advise matching the runtime setting

@florianschmidt1994 can you please inspect the libtool variant (to test this you can simply undefine USE_LIBDL in config.h)?
I think I'll work on the cob_dlopen stuff late-evening.

[florianschmidt1994]

Yup, I'll check it out. Basically something like this, right?

static void* cob_dlopen(const char* filename) {
	#ifdef _WIN32
	if (x == NULL) {
		return GetModuleHandle (NULL);
	}
	return LoadLibrary(x);	
	#elif	defined(USE_LIBDL)
	int flags = cobsetptr->cob_load_global 
		? RTLD_LAZY | RTLD_GLOBAL
		: RTLD_LAZY | RTLD_LOCAL;

	return dlopen(filename, flags);
	# else
	// TODO: initialise lt_dladvise
	lt_dlopenadvise(filename, /*advise here*/);
}

[florianschmidt1994]

@GitMensch I've tried out your suggestion in the latest commit and temporarily defined lt_dlopen as cob_dlopen to try it out. I've also added some indentation to the macros, does that conform with your style or should I remove them again?

[GitMensch]

nice update, we may be able to push that soon

Note: we still need the testcase addition - does not need to be something that benefit from this as this is hard to make portable - just do a simple CALL and CANCEL. We should have an easy one for COB_LOAD_CASE

[florianschmidt1994]

When developing, I've noticed some test failures, where some of them might be related to our change. On the latest commit of this repositories "main branch" (gcos4gnucobol-3.x), there are some test failures when the build is configured with --without-dl. These failures persist also with our changes applied.

Commit Hash	Loader	COB_LOAD_GLOBAL	Test Failures
731b81a	--without-dl	not applicable	47 49 50 784 failed
731b81a		not applicable	no test failure

In the first version of this PR (bd80e8c), there is an additional test failure (781) when using COB_LOAD_GLOBAL=false, i.e. the default, and preloading where symbols are overwritten in an unexpected manner.

Commit Hash	Loader	COB_LOAD_GLOBAL	Test Failures
bd80e8c		false	781 failed
bd80e8c		true	no test failure
bd80e8c	--without-dl	false	47 49 50 781 failed
bd80e8c	--without-dl	true	47 49 50 failed

In history up to the latest commit (62de625), there exists a fix (see 1f4d607), the same that is already applied in WIN32 environments, to fix this.

Commit Hash	Loader	COB_LOAD_GLOBAL	Test Failures
62de625		false	no test failure
62de625		true	no test failure
62de625	--without-dl	false	47 49 50 failed
62de625	--without-dl	true	47 49 50 failed