gst-plugins-rs: Complete the NDK migration to fix linking against Rust

From !915 and !1191, the following bits were missing:

- Autotools expects the raw linker to perform detection for version
  scripting. If that parameter isn't detected in the `--help`, shared
  libraries are disabled (breaks libpng).
- Cargo, conversely, expects implicitly the clang executable itself
  because of a new dependency on libunwind starting with 1.68

To appease both, I pass the corresponding linkers separately, then
massage the flags into link-args entries in the TOML's rustflags entry.

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/1255>

2 files changed, 15 insertions, 12 deletions

diff --git a/cerbero/build/build.py b/cerbero/build/build.py
index 47cdb3f9..06b1e474 100644
--- a/cerbero/build/build.py
+++ b/cerbero/build/build.py
@@ -1277,19 +1277,15 @@ class Cargo(Build, ModifyEnvBase):
             self.append_config_toml(s)
 
         if self.config.target_platform == Platform.ANDROID:
-            linker = self.env['LD']
+            # Use the compiler's forwarding
+            # See https://android.googlesource.com/platform/ndk/+/master/docs/BuildSystemMaintainers.md#linkers
+            linker = self.get_env('RUSTC_LINKER')
             link_args = []
-            args = iter(shlex.split(self.env['LDFLAGS']))
+            args = iter(shlex.split(self.get_env('LDFLAGS', '')))
             # We need to extract necessary linker flags from LDFLAGS which is
             # passed to the compiler
             for arg in args:
-                if arg.startswith('-L'):
-                    link_args.append(arg)
-                elif arg == '-gcc-toolchain':
-                    arg = next(args)
-                    for l in glob.glob(f'{arg}/lib/gcc/*/*/libgcc.a'):
-                        d = os.path.dirname(l)
-                        link_args.append(f'-L{d}')
+                link_args += ['-C', f"link-args={arg}"]
             s = f'[target.{self.target_triple}]\n' \
                 f'linker = "{linker}"\n' \
                 f'rustflags = {link_args!r}\n'
diff --git a/config/android.config b/config/android.config
index 5b24585f..bdbc7dbb 100644
--- a/config/android.config
+++ b/config/android.config
@@ -104,6 +104,9 @@ env['CFLAGS'] = ''
 env['CXXFLAGS'] = ''
 env['OBJCFLAGS'] = ''
 
+# Make sure user's env doesn't mess up with our build.
+env.pop('RUSTFLAGS', None)
+
 # Android NDK path
 env['ANDROID_NDK_HOME'] = toolchain_prefix
 
@@ -152,7 +155,8 @@ if not target_arch in [Architecture.ARM64, Architecture.X86_64]:
    # nocopyreloc causes broken linking on arm64
    ldflags += ' -Wl,-z,nocopyreloc '
 
-ldflags += f' -L{sysroot}/usr/lib/{host}/{v} '
+# Initialize both the linker and the sysroot
+ldflags += f' -L{sysroot}/usr/lib/{host}/{v} -Wl,--sysroot={sysroot}'
 
 ldvariant = 'lld'
 ldflags += f' -fuse-ld={ldvariant} '
@@ -171,8 +175,11 @@ def llvm_cmd(command):
 env['CC']= ccache + llvm_cmd('clang') + f' -target {llvm_triple}' + f' --sysroot {sysroot}'
 env['CC_FOR_BUILD']= ccache + llvm_cmd('clang')
 env['CXX']= ccache + llvm_cmd('clang++') + f' -target {llvm_triple}' + f' --sysroot {sysroot}'
-# linker should be invoked through the clang/++ frontend
-env['LD']= ''
+# linker is needed for Autotools
+env['LD']= llvm_cmd(f'ld.lld')
+# Supply the linker separately to pick up the dependency on -lunwind
+# See https://blog.rust-lang.org/2023/01/09/android-ndk-update-r25.html
+env['RUSTC_LINKER']= llvm_cmd(f'{llvm_triple}-clang')
 env['CPP']= llvm_cmd('clang') + ' -E'
 env['RANLIB']= llvm_cmd('llvm-ranlib')
 env['AR']= llvm_cmd('llvm-ar')