shared_module: Add soname when used as a link target

Emit a detailed deprecation warning that explains what to do instead.
Also add a unittest.

```
DEPRECATION: target prog links against shared module mymod, which is incorrect.
             This will be an error in the future, so please use shared_library() for mymod instead.
             If shared_module() was used for mymod because it has references to undefined symbols,
             use shared_libary() with `override_options: ['b_lundef=false']` instead.
```

Fixes https://github.com/mesonbuild/meson/issues/9492

docs/yaml/functions/shared_library.yaml

@@ -2,13 +2,6 @@ name: shared_library
 returns: lib
 description: Builds a shared library with the given sources.
 
-notes:
- - |
-  Linking to a shared module is not supported on some
-  platforms, notably OSX. Consider using a
-  [[shared_library]] instead, if you need to both
-  `dlopen()` and link with a library.
-
 posargs_inherit: _build_target_base
 varargs_inherit: _build_target_base
 kwargs_inherit: _build_target_base

docs/yaml/functions/shared_module.yaml

@@ -13,6 +13,15 @@ description: |
   you will need to set the `export_dynamic` argument of the executable to
   `true`.
 
+notes:
+ - |
+   *Linking to a shared module is deprecated, and will be an error in the future*.
+   It used to be allowed because it was the only way to have a shared-library-like target that
+   contained references to undefined symbols. However, since 0.40.0, the `override_options:`
+   [[build_target]] keyword argument can be used to create such a [[shared_library]], and shared
+   modules have other characteristics that make them incompatible with linking, such as a lack of
+   SONAME. Linking to shared modules also does not work on some platforms, such as on macOS / iOS.
+
 posargs_inherit: _build_target_base
 varargs_inherit: _build_target_base
 kwargs_inherit: _build_target_base

mesonbuild/backend/ninjabackend.py

@@ -2785,7 +2785,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
                 commands += linker.get_std_shared_lib_link_args()
             # All shared libraries are PIC
             commands += linker.get_pic_args()
-            if not isinstance(target, build.SharedModule):
+            if not isinstance(target, build.SharedModule) or target.backwards_compat_want_soname:
                 # Add -Wl,-soname arguments on Linux, -install_name on OS X
                 commands += linker.get_soname_args(
                     self.environment, target.prefix, target.name, target.suffix,

mesonbuild/build.py

@@ -1574,11 +1574,16 @@ You probably should put it in link_with instead.''')
             if isinstance(link_target, SharedModule):
                 if self.environment.machines[self.for_machine].is_darwin():
                     raise MesonException(
-                        'target links against shared modules. This is not permitted on OSX')
+                        f'target {self.name} links against shared module {link_target.name}. This is not permitted on OSX')
                 else:
-                    mlog.warning('target links against shared modules. This '
-                                 'is not recommended as it is not supported on some '
-                                 'platforms')
+                    mlog.deprecation(f'target {self.name} links against shared module {link_target.name}, which is incorrect.'
+                            '\n             '
+                            f'This will be an error in the future, so please use shared_library() for {link_target.name} instead.'
+                            '\n             '
+                            f'If shared_module() was used for {link_target.name} because it has references to undefined symbols,'
+                            '\n             '
+                            'use shared_libary() with `override_options: [\'b_lundef=false\']` instead.')
+                    link_target.backwards_compat_want_soname = True
                 return
 
 class Generator(HoldableObject):
@@ -2235,6 +2240,9 @@ class SharedModule(SharedLibrary):
             raise MesonException('Shared modules must not specify the soversion kwarg.')
         super().__init__(name, subdir, subproject, for_machine, sources, objects, environment, kwargs)
         self.typename = 'shared module'
+        # We need to set the soname in cases where build files link the module
+        # to build targets, see: https://github.com/mesonbuild/meson/issues/9492
+        self.backwards_compat_want_soname = False
 
     def get_default_install_dir(self, environment) -> T.Tuple[str, str]:
         return environment.get_shared_module_dir(), '{moduledir_shared}'

test cases/failing/75 link with shared module on osx/test.json

@@ -1,7 +1,7 @@
 {
   "stdout": [
     {
-      "line": "test cases/failing/75 link with shared module on osx/meson.build:8:0: ERROR: target links against shared modules. This is not permitted on OSX"
+      "line": "test cases/failing/75 link with shared module on osx/meson.build:8:0: ERROR: target prog links against shared module mymodule. This is not permitted on OSX"
     }
   ]
 }

test cases/unit/1 soname/main.c

@@ -0,0 +1,5 @@
+int versioned_func (void);
+
+int main (void) {
+  return versioned_func();
+}

test cases/unit/1 soname/meson.build

@@ -20,3 +20,16 @@ shared_library('settosame', 'versioned.c',
   install : true,
   soversion : '7.8.9',
   version : '7.8.9')
+
+shared_module('some_module', 'versioned.c',
+  install: true)
+
+module1 = shared_module('linked_module1', 'versioned.c',
+  install: true)
+
+module2 = shared_module('linked_module2', 'versioned.c',
+  install: true)
+module2_dep = declare_dependency(link_with: module2)
+
+executable('main1', 'main.c', link_with: module1)
+executable('main2', 'main.c', dependencies: module2_dep)

unittests/allplatformstests.py

@@ -1950,8 +1950,10 @@ class AllPlatformTests(BasePlatformTests):
         """
         tdir = os.path.join(self.unit_test_dir, '30 shared_mod linking')
         out = self.init(tdir)
-        msg = ('WARNING: target links against shared modules. This is not '
-               'recommended as it is not supported on some platforms')
+        msg = ('''DEPRECATION: target prog links against shared module mymod, which is incorrect.
+             This will be an error in the future, so please use shared_library() for mymod instead.
+             If shared_module() was used for mymod because it has references to undefined symbols,
+             use shared_libary() with `override_options: ['b_lundef=false']` instead.''')
         self.assertIn(msg, out)
 
     def test_mixed_language_linker_check(self):

unittests/linuxliketests.py

@@ -438,6 +438,24 @@ class LinuxlikeTests(BasePlatformTests):
         self.assertEqual(get_soname(bothset), 'libbothset.so.1.2.3')
         self.assertEqual(len(self.glob_sofiles_without_privdir(bothset[:-3] + '*')), 3)
 
+        # A shared_module that is not linked to anything
+        module = os.path.join(libpath, 'libsome_module.so')
+        self.assertPathExists(module)
+        self.assertFalse(os.path.islink(module))
+        self.assertEqual(get_soname(module), None)
+
+        # A shared_module that is not linked to an executable with link_with:
+        module = os.path.join(libpath, 'liblinked_module1.so')
+        self.assertPathExists(module)
+        self.assertFalse(os.path.islink(module))
+        self.assertEqual(get_soname(module), 'liblinked_module1.so')
+
+        # A shared_module that is not linked to an executable with dependencies:
+        module = os.path.join(libpath, 'liblinked_module2.so')
+        self.assertPathExists(module)
+        self.assertFalse(os.path.islink(module))
+        self.assertEqual(get_soname(module), 'liblinked_module2.so')
+
     def test_soname(self):
         self._test_soname_impl(self.builddir, False)