Fix unity builds (#12452)

* unity builds: correct integer ceiling division

* edge case failure with unity builds:
  - static archive bar that gets installed, that links with another static
    archive foo that does not get installed
  - the number of files in static archive foo is divisible by unity_size

would yield an error with ninja:

  ninja: error: 'subprojects/foo/src/libfoo.a.p/meson-generated_foo-unity1.cpp.o', needed by 'src/libbar.a', missing and no known rule to make it

* unity builds: test for build failure when #files is divisible by unity_size

mesonbuild/backend/backends.py

@@ -920,7 +920,7 @@ class Backend:
                 if comp.language in LANGS_CANT_UNITY:
                     sources += srcs
                     continue
-                for i in range(len(srcs) // unity_size + 1):
+                for i in range((len(srcs) + unity_size - 1) // unity_size):
                     _src = self.get_unity_source_file(extobj.target,
                                                       comp.get_default_suffix(), i)
                     sources.append(_src)

test cases/common/272 unity/meson.build

@@ -0,0 +1,13 @@
+project('unity', 'c',
+  default_options : [
+    'unity_size=2'])
+
+if get_option('unity') != 'on'
+  error('MESON_SKIP_TEST: unity builds not enabled')
+endif
+
+slib_notinstalled = static_library('slib_notinstalled',
+  # test depends on the number of files being divisible by unity_size
+  ['slib1.c', 'slib2.c'])
+
+slib_installed = static_library('slib_installed', ['slib1.c', 'slib2.c'], link_with : slib_notinstalled, install : true)

test cases/common/272 unity/slib.c

@@ -0,0 +1,6 @@
+int func1(void);
+int func2(void);
+
+int static_lib_func(void) {
+    return func1() + func2();
+}

test cases/common/272 unity/slib1.c

@@ -0,0 +1,3 @@
+int func1(void) {
+    return 1;
+}

test cases/common/272 unity/slib2.c

@@ -0,0 +1,3 @@
+int func2(void) {
+    return 2;
+}

test cases/common/272 unity/test.json

@@ -0,0 +1,5 @@
+{
+  "installed": [
+    {"type": "file", "file": "usr/lib/libslib_installed.a"}
+  ]
+}