backends: fix vala object filename for sources in subdirectories

When both_libraries() reuses object files from the shared library
for the static library via ExtractedObjects, the object filenames
must match those produced for the shared library. For non-built vala
source files in subdirectories, object_filename_from_source() was
using os.path.basename() which dropped the subdirectory component,
while the actual compilation preserved it.

Use os.path.relpath() against the target's source directory instead,
matching the logic in generate_vala_compile().

Fixes: #6960
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

mesonbuild/backend/backends.py

@@ -783,7 +783,7 @@ class Backend:
                     rel_src = rel_src[len(build_dir) + 1:]
                 rel_src = os.path.relpath(rel_src, self.get_target_private_dir(target))
             else:
-                rel_src = os.path.basename(rel_src)
+                rel_src = os.path.relpath(rel_src, os.path.join(self.build_to_src, target.get_subdir()))
             # A meson- prefixed directory is reserved; hopefully no-one creates a file name with such a weird prefix.
             gen_source = 'meson-generated_' + rel_src[:-5] + '.c'
         elif source.is_built:

test cases/vala/33 subdir sources and both_libraries/lib/meson.build

@@ -0,0 +1,10 @@
+args = []
+# https://github.com/mesonbuild/meson/issues/1969
+if get_option('unity') == 'on'
+  vala_args = ['-H', 'mylib.h']
+endif
+
+l = both_libraries('valalib', 'subdir/mylib.vala',
+                   vala_args : args,
+                   dependencies : valadeps,
+                   install_dir : [false, get_option('includedir'), false])

test cases/vala/33 subdir sources and both_libraries/lib/subdir/mylib.vala

@@ -0,0 +1,5 @@
+public class LibraryObject : Object {
+    public void func() {
+       stdout.printf("Method in library called.");
+    }
+}

test cases/vala/33 subdir sources and both_libraries/meson.build

@@ -0,0 +1,5 @@
+project('shared library', 'vala', 'c')
+
+valadeps = [dependency('glib-2.0'), dependency('gobject-2.0')]
+
+subdir('lib')