shared module: Allow linking on Android

Android requires shared modules that use symbols from other shared
modules to be linked before they can be dlopen()ed in the correct
order. Not doing so leads to a missing symbol error:
android/ndk#201

We need to always allow linking for this. Also add a soname, although
it's not confirmed that it's needed, and it doesn't really hurt if it
isn't needed.

docs/yaml/functions/shared_module.yaml

@@ -15,12 +15,18 @@ description: |
 
 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
+   *Linking to a shared module on platforms other than Android is deprecated, and will be an error
+   in the future*.
+   It was previously 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.
+   [[build_target]] keyword argument can be used to create such a [[shared_library]] by passing
+   `override_options: 'b_lundef=false'`. Shared modules have other characteristics that make
+   them incompatible with linking, such as a lack of SONAME.
+   On macOS and iOS, linking to shared modules is disallowed by the linker, so we disallow it at
+   configure time.
+   On Android, if a shared module `foo` uses symbols from another shared module `bar`, `foo` must
+   also be linked to `bar`. Hence, linking one shared module to another will always be allowed when
+   building for Android.
 
 posargs_inherit: _build_target_base
 varargs_inherit: _build_target_base

mesonbuild/backend/ninjabackend.py

@@ -2792,7 +2792,7 @@ def get_target_type_link_args(self, target, linker):
                 commands += linker.get_std_shared_lib_link_args()
             # All shared libraries are PIC
             commands += linker.get_pic_args()
-            if not isinstance(target, build.SharedModule) or target.backwards_compat_want_soname:
+            if not isinstance(target, build.SharedModule) or target.force_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

@@ -1589,10 +1589,15 @@ def check_module_linking(self):
         Warn if shared modules are linked with target: (link_with) #2865
         '''
         for link_target in self.link_targets:
-            if isinstance(link_target, SharedModule) and not link_target.backwards_compat_want_soname:
+            if isinstance(link_target, SharedModule) and not link_target.force_soname:
                 if self.environment.machines[self.for_machine].is_darwin():
                     raise MesonException(
                         f'target {self.name} links against shared module {link_target.name}. This is not permitted on OSX')
+                elif self.environment.machines[self.for_machine].is_android() and isinstance(self, SharedModule):
+                    # Android requires shared modules that use symbols from other shared modules to
+                    # be linked before they can be dlopen()ed in the correct order. Not doing so
+                    # leads to a missing symbol error: https://github.com/android/ndk/issues/201
+                    link_target.force_soname = True
                 else:
                     mlog.deprecation(f'target {self.name} links against shared module {link_target.name}, which is incorrect.'
                             '\n             '
@@ -1601,7 +1606,7 @@ def check_module_linking(self):
                             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
+                    link_target.force_soname = True
 
 class Generator(HoldableObject):
     def __init__(self, exe: T.Union['Executable', programs.ExternalProgram],
@@ -2270,7 +2275,7 @@ def __init__(self, name, subdir, subproject, for_machine: MachineChoice, sources
         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
+        self.force_soname = False
 
     def get_default_install_dir(self, environment) -> T.Tuple[str, str]:
         return environment.get_shared_module_dir(), '{moduledir_shared}'