ninja: Delete output static lib before calling `ar`

Otherwise if the list of sources changes on reconfigure after building,
the static library will contain both the old and new objects.

Closes https://github.com/mesonbuild/meson/issues/1355

mesonbuild/backend/ninjabackend.py

@@ -30,9 +30,11 @@ from collections import OrderedDict
 if mesonlib.is_windows():
     quote_char = '"'
     execute_wrapper = 'cmd /c'
+    rmfile_prefix = 'del /f /s /q {} &&'
 else:
     quote_char = "'"
     execute_wrapper = ''
+    rmfile_prefix = 'rm -f {} &&'
 
 def ninja_quote(text):
     return text.replace(' ', '$ ').replace(':', '$:')
@@ -1238,10 +1240,16 @@ int dummy;
 '''
         else:
             command_template = ' command = {executable} $LINK_ARGS {output_args} $in\n'
+        cmdlist = []
+        if isinstance(static_linker, compilers.ArLinker):
+            # `ar` has no options to overwrite archives. It always appends,
+            # which is never what we want. Delete an existing library first if
+            # it exists. https://github.com/mesonbuild/meson/issues/1355
+            cmdlist = [execute_wrapper, rmfile_prefix.format('$out')]
+        cmdlist += static_linker.get_exelist()
         command = command_template.format(
-            executable=' '.join(static_linker.get_exelist()),
-            output_args=' '.join(static_linker.get_output_args('$out'))
-        )
+            executable=' '.join(cmdlist),
+            output_args=' '.join(static_linker.get_output_args('$out')))
         description = ' description = Static linking library $out\n\n'
         outfile.write(rule)
         outfile.write(command)

run_unittests.py

@@ -328,6 +328,47 @@ class AllPlatformTests(BasePlatformTests):
         self.init(testdir)
         self.assertRaises(subprocess.CalledProcessError, self.setconf, '-Dlibdir=/opt', False)
 
+    def test_static_library_overwrite(self):
+        '''
+        Tests that static libraries are never appended to, always overwritten.
+        Has to be a unit test because this involves building a project,
+        reconfiguring, and building it again so that `ar` is run twice on the
+        same static library.
+        https://github.com/mesonbuild/meson/issues/1355
+        '''
+        testdir = os.path.join(self.common_test_dir, '3 static')
+        env = Environment(testdir, self.builddir, self.meson_command,
+                          get_fake_options(self.prefix), [])
+        cc = env.detect_c_compiler(False)
+        static_linker = env.detect_static_linker(cc)
+        if not isinstance(static_linker, mesonbuild.compilers.ArLinker):
+                raise unittest.SkipTest('static linker is not `ar`')
+        # Configure
+        self.init(testdir)
+        # Get name of static library
+        targets = self.introspect('--targets')
+        self.assertEqual(len(targets), 1)
+        libname = targets[0]['filename']
+        # Build and get contents of static library
+        self.build()
+        before = self._run(['ar', 't', os.path.join(self.builddir, libname)],
+                           return_output=True).split()
+        # Filter out non-object-file contents
+        before = [f for f in before if f.endswith((b'.o', b'.obj'))]
+        # Static library should contain only one object
+        self.assertEqual(len(before), 1, msg=before)
+        # Change the source to be built into the static library
+        self.setconf('-Dsource=libfile2.c')
+        self.build()
+        after = self._run(['ar', 't', os.path.join(self.builddir, libname)],
+                          return_output=True).split()
+        # Filter out non-object-file contents
+        after = [f for f in after if f.endswith((b'.o', b'.obj'))]
+        # Static library should contain only one object
+        self.assertEqual(len(after), 1, msg=after)
+        # and the object must have changed
+        self.assertNotEqual(before, after)
+
 
 class LinuxlikeTests(BasePlatformTests):
     '''

test cases/common/3 static/libfile2.c

@@ -0,0 +1,3 @@
+int libfunc2() {
+    return 4;
+}

test cases/common/3 static/meson.build

@@ -1,3 +1,4 @@
 project('static library test', 'c')
-lib = static_library('mylib', 'libfile.c',
+
+lib = static_library('mylib', get_option('source'),
   link_args : '-THISMUSTNOBEUSED') # Static linker needs to ignore all link args.

test cases/common/3 static/meson_options.txt

@@ -0,0 +1 @@
+option('source', type : 'combo', choices : ['libfile.c', 'libfile2.c'], value : 'libfile.c')