vlib: add a new dl.loader module, to simplify dynamic library loading, when the DLLs may be in multiple customisable locations (#17161)

vlib/dl/loader/loader_test.v

@@ -0,0 +1,83 @@
+import dl.loader
+import dl
+
+fn test_dl_loader() ! {
+	$if linux {
+		run_test_invalid_lib_linux()!
+		return
+	}
+	$if windows {
+		run_test_invalid_lib_windows()!
+		run_test_valid_lib_windows()!
+		run_test_invalid_sym_windows()!
+		run_test_valid_sym_windows()!
+		return
+	} $else {
+		eprint('currently not implemented on this platform')
+	}
+}
+
+fn get_or_create_loader(name string, paths []string) !&loader.DynamicLibLoader {
+	return loader.get_or_create_dynamic_lib_loader(
+		key: name
+		paths: paths
+		flags: dl.rtld_now
+	)
+}
+
+fn run_test_invalid_lib_linux() ! {
+	// ensure a not-existing dl won't be loaded
+	mut dl_loader := get_or_create_loader(@MOD + '.' + @FN + '.' + 'lib', [
+		'not-existing-dynamic-link-library',
+	])!
+	defer {
+		dl_loader.unregister()
+	}
+	h := dl_loader.open() or { unsafe { nil } }
+	assert isnil(h)
+}
+
+fn run_test_invalid_lib_windows() ! {
+	// ensure a not-existing dl won't be loaded
+	mut dl_loader := get_or_create_loader(@MOD + '.' + @FN + '.' + 'lib', [
+		'not-existing-dynamic-link-library',
+	])!
+	defer {
+		dl_loader.unregister()
+	}
+	h := dl_loader.open() or { unsafe { nil } }
+	assert isnil(h)
+}
+
+fn run_test_valid_lib_windows() ! {
+	mut dl_loader := get_or_create_loader(@MOD + '.' + @FN + '.' + 'lib', [
+		'not-existing-dynamic-link-library',
+		'shell32',
+	])!
+	defer {
+		dl_loader.unregister()
+	}
+	h := dl_loader.open() or { unsafe { nil } }
+	assert !isnil(h)
+}
+
+fn run_test_invalid_sym_windows() ! {
+	mut dl_loader := get_or_create_loader(@MOD + '.' + @FN + '.' + 'lib', ['shell32'])!
+	defer {
+		dl_loader.unregister()
+	}
+	proc := dl_loader.get_sym('CommandLineToArgvW2') or { unsafe { nil } }
+	assert isnil(proc)
+}
+
+fn run_test_valid_sym_windows() ! {
+	mut dl_loader := get_or_create_loader(@MOD + '.' + @FN + '.' + 'lib', [
+		'not-existing-dynamic-link-library',
+		'shell32',
+	])!
+	defer {
+		dl_loader.unregister()
+	}
+	proc := dl_loader.get_sym('CommandLineToArgvW') or { unsafe { nil } }
+	assert !isnil(proc)
+}