Default libdir is "lib" when cross compiling. Closes #2535.

docs/markdown/snippets/crosslib.md

@@ -0,0 +1,7 @@
+## Libdir defaults to `lib` when cross compiling
+
+Previously `libdir` defaulted to the value of the build machine such
+as `lib/x86_64-linux-gnu`, which is almost always incorrect when cross
+compiling. It now defaults to plain `lib` when cross compiling. Native
+builds remain unchanged and will point to the current system's library
+dir.

mesonbuild/coredata.py

@@ -299,6 +299,7 @@ class CoreData:
         # Only to print a warning if it changes between Meson invocations.
         self.pkgconf_envvar = os.environ.get('PKG_CONFIG_PATH', '')
         self.config_files = self.__load_config_files(options.native_file)
+        self.libdir_cross_fixup()
 
     @staticmethod
     def __load_config_files(filenames):
@@ -348,6 +349,13 @@ class CoreData:
 
         raise MesonException('Cannot find specified cross file: ' + filename)
 
+    def libdir_cross_fixup(self):
+        # By default set libdir to "lib" when cross compiling since
+        # getting the "system default" is always wrong on multiarch
+        # platforms as it gets a value like lib/x86_64-linux-gnu.
+        if self.cross_file is not None:
+            self.builtins['libdir'].value = 'lib'
+
     def sanitize_prefix(self, prefix):
         if not os.path.isabs(prefix):
             raise MesonException('prefix value {!r} must be an absolute path'

run_unittests.py

@@ -4528,6 +4528,7 @@ endian = 'little'
                 max_count = max(max_count, line.count(search_term))
         self.assertEqual(max_count, 1, 'Export dynamic incorrectly deduplicated.')
 
+
 class LinuxCrossArmTests(BasePlatformTests):
     '''
     Tests that cross-compilation to Linux/ARM works
@@ -4564,6 +4565,18 @@ class LinuxCrossArmTests(BasePlatformTests):
         self.assertRegex(compdb[0]['command'], '-D_FILE_OFFSET_BITS=64.*-U_FILE_OFFSET_BITS')
         self.build()
 
+    def test_cross_libdir(self):
+        # When cross compiling "libdir" should default to "lib"
+        # rather than "lib/x86_64-linux-gnu" or something like that.
+        testdir = os.path.join(self.common_test_dir, '1 trivial')
+        self.init(testdir)
+        for i in self.introspect('--buildoptions'):
+            if i['name'] == 'libdir':
+                self.assertEqual(i['value'], 'lib')
+                return
+        self.assertTrue(False, 'Option libdir not in introspect data.')
+
+
 class LinuxCrossMingwTests(BasePlatformTests):
     '''
     Tests that cross-compilation to Windows/MinGW works