pkgconfig: Allow setting prefix in dataonly pc file

Some variables are reserved because meson set them automatically. But we
are not setting them for dataonly pc files, so there is no reason to
reserve them.

Fixes: #8583.

mesonbuild/modules/pkgconfig.py

@@ -469,10 +469,12 @@ class PkgConfigModule(ExtensionModule):
             raise mesonlib.MesonException('Too many positional arguments passed to Pkgconfig_gen.')
 
         dataonly = kwargs.get('dataonly', False)
+        if not isinstance(dataonly, bool):
+            raise mesonlib.MesonException('dataonly must be boolean.')
         if dataonly:
             default_subdirs = []
             blocked_vars = ['libraries', 'libraries_private', 'require_private', 'extra_cflags', 'subdirs']
-            if len(set(kwargs) & set(blocked_vars)) > 0:
+            if any(k in kwargs for k in blocked_vars):
                 raise mesonlib.MesonException(f'Cannot combine dataonly with any of {blocked_vars}')
 
         subdirs = mesonlib.stringlistify(kwargs.get('subdirs', default_subdirs))
@@ -519,7 +521,7 @@ class PkgConfigModule(ExtensionModule):
             reserved = ['prefix', 'libdir', 'includedir']
             variables = []
             for name, value in vardict.items():
-                if name in reserved:
+                if not dataonly and name in reserved:
                     raise mesonlib.MesonException(f'Variable "{name}" is reserved')
                 variables.append((name, value))
             return variables

run_unittests.py

@@ -6444,6 +6444,7 @@ class LinuxlikeTests(BasePlatformTests):
         self.assertEqual(libhello_nolib.get_link_args(), [])
         self.assertEqual(libhello_nolib.get_compile_args(), [])
         self.assertEqual(libhello_nolib.get_pkgconfig_variable('foo', {}), 'bar')
+        self.assertEqual(libhello_nolib.get_pkgconfig_variable('prefix', {}), self.prefix)
 
     def test_pkgconfig_gen_deps(self):
         '''

test cases/common/45 pkgconfig-gen/meson.build

@@ -65,7 +65,12 @@ pkgg.generate(
   description : 'A minimalistic pkgconfig file.',
   version : libver,
   dataonly: true,
-  variables : {'foo': 'bar'},
+  variables : {
+    'foo': 'bar',
+    # prefix is not set by default for dataonly pc files, but it is allowed to
+    # define it manually.
+    'prefix': get_option('prefix'),
+  },
 )
 
 # Regression test for 2 cases: