backends: add a new "none" backend

It can only be used for projects that don't have any rules at all, i.e.
they are purely using Meson to:

- configure files
- run (script?) tests
- install files that exist by the end of the setup stage

This can be useful e.g. for Meson itself, a pure python project.

docs/markdown/Builtin-options.md

@@ -75,7 +75,7 @@ machine](#specifying-options-per-machine) section for details.
 | Option                                 | Default value | Description                                                    | Is per machine | Is per subproject |
 | -------------------------------------- | ------------- | -----------                                                    | -------------- | ----------------- |
 | auto_features {enabled, disabled, auto} | auto       | Override value of all 'auto' features                          | no             | no                |
-| backend {ninja, vs,<br>vs2010, vs2012, vs2013, vs2015, vs2017, vs2019, vs2022, xcode} | ninja | Backend to use        | no             | no                |
+| backend {ninja, vs,<br>vs2010, vs2012, vs2013, vs2015, vs2017, vs2019, vs2022, xcode, none} | ninja | Backend to use  | no             | no                |
 | buildtype {plain, debug,<br>debugoptimized, release, minsize, custom} | debug |  Build type to use                    | no             | no                |
 | debug                                  | true          | Enable debug symbols and other information                     | no             | no                |
 | default_library {shared, static, both} | shared      | Default library type                                           | no             | yes               |
@@ -95,6 +95,16 @@ machine](#specifying-options-per-machine) section for details.
 | wrap_mode {default, nofallback,<br>nodownload, forcefallback, nopromote} | default | Wrap mode to use                 | no             | no                |
 | force_fallback_for                     | []            | Force fallback for those dependencies                          | no             | no                |
 
+#### Details for `backend`
+
+Several build file formats are supported as command runners to build the
+configured project. Meson prefers ninja by default, but platform-specific
+backends are also available for better IDE integration with native tooling:
+Visual Studio for Windows, and xcode for macOS. It is also possible to
+configure with no backend at all, which is an error if you have targets to
+build, but for projects that need configuration + testing + installation allows
+for a lighter automated build pipeline.
+
 #### Details for `buildtype`
 
 <a name="build-type-options"></a> For setting optimization levels and

docs/markdown/snippets/none-backend.md

@@ -0,0 +1,4 @@
+## New "none" backend
+
+The `--backend=none` option has been added, to configure a project that has no
+build rules, only install rules. This avoids depending on ninja.

mesonbuild/backend/backends.py

@@ -251,6 +251,9 @@ def get_backend_from_name(backend: str, build: T.Optional[build.Build] = None, i
     elif backend == 'xcode':
         from . import xcodebackend
         return xcodebackend.XCodeBackend(build, interpreter)
+    elif backend == 'none':
+        from . import nonebackend
+        return nonebackend.NoneBackend(build, interpreter)
     return None
 
 # This class contains the basic functionality that is needed by all backends.

mesonbuild/backend/nonebackend.py

@@ -0,0 +1,31 @@
+# Copyright 2022 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+from .backends import Backend
+from .. import mlog
+from ..mesonlib import MesonBugException
+
+
+class NoneBackend(Backend):
+
+    name = 'none'
+
+    def generate(self):
+        if self.build.get_targets():
+            raise MesonBugException('None backend cannot generate target rules, but should have failed earlier.')
+        mlog.log('Generating simple install-only backend')
+        self.serialize_tests()
+        self.create_install_data_files()

mesonbuild/coredata.py

@@ -61,7 +61,7 @@ if T.TYPE_CHECKING:
 # But the corresponding Git tag needs to be '0.1.0rc1'
 version = '1.0.99'
 
-backendlist = ['ninja', 'vs', 'vs2010', 'vs2012', 'vs2013', 'vs2015', 'vs2017', 'vs2019', 'vs2022', 'xcode']
+backendlist = ['ninja', 'vs', 'vs2010', 'vs2012', 'vs2013', 'vs2015', 'vs2017', 'vs2019', 'vs2022', 'xcode', 'none']
 
 DEFAULT_YIELDING = False
 

mesonbuild/interpreter/interpreter.py

@@ -3090,6 +3090,8 @@ class Interpreter(InterpreterBase, HoldableObject):
                                    "internal use. Please rename.")
 
     def add_target(self, name: str, tobj: build.Target) -> None:
+        if self.backend.name == 'none':
+            raise InterpreterException('Install-only backend cannot generate target rules, try using `--backend=ninja`.')
         if name == '':
             raise InterpreterException('Target name must not be empty.')
         if name.strip() == '':

mesonbuild/minstall.py

@@ -14,7 +14,6 @@
 from __future__ import annotations
 
 from glob import glob
-from pathlib import Path
 import argparse
 import errno
 import os
@@ -25,8 +24,7 @@ import subprocess
 import sys
 import typing as T
 
-from . import build
+from . import build, coredata, environment
-from . import environment
 from .backend.backends import InstallData
 from .mesonlib import MesonException, Popen_safe, RealPathAction, is_windows, setup_vsenv, pickle_load, is_osx
 from .scripts import depfixer, destdir_join
@@ -755,8 +753,11 @@ class Installer:
                 # file mode needs to be set last, after strip/depfixer editing
                 self.set_mode(outname, install_mode, d.install_umask)
 
-def rebuild_all(wd: str) -> bool:
+def rebuild_all(wd: str, backend: str) -> bool:
-    if not (Path(wd) / 'build.ninja').is_file():
+    if backend == 'none':
+        # nothing to build...
+        return True
+    if backend != 'ninja':
         print('Only ninja backend is supported to rebuild the project before installation.')
         return True
 
@@ -816,7 +817,8 @@ def run(opts: 'ArgumentType') -> int:
     if not opts.no_rebuild:
         b = build.load(opts.wd)
         setup_vsenv(b.need_vsenv)
-        if not rebuild_all(opts.wd):
+        backend = T.cast('str', b.environment.coredata.get_option(coredata.OptionKey('backend')))
+        if not rebuild_all(opts.wd, backend):
             sys.exit(-1)
     os.chdir(opts.wd)
     with open(os.path.join(log_dir, 'install-log.txt'), 'w', encoding='utf-8') as lf:

mesonbuild/mtest.py

@@ -43,7 +43,7 @@ import xml.etree.ElementTree as et
 from . import build
 from . import environment
 from . import mlog
-from .coredata import major_versions_differ, MesonVersionMismatchException
+from .coredata import MesonVersionMismatchException, OptionKey, major_versions_differ
 from .coredata import version as coredata_version
 from .mesonlib import (MesonException, OrderedSet, RealPathAction,
                        get_wine_shortpath, join_args, split_args, setup_vsenv)
@@ -2099,7 +2099,11 @@ def run(options: argparse.Namespace) -> int:
     setup_vsenv(b.need_vsenv)
 
     if not options.no_rebuild:
-        if not (Path(options.wd) / 'build.ninja').is_file():
+        backend = b.environment.coredata.get_option(OptionKey('backend'))
+        if backend == 'none':
+            # nothing to build...
+            options.no_rebuild = True
+        elif backend != 'ninja':
             print('Only ninja backend is supported to rebuild tests before running them.')
             # Disable, no point in trying to build anything later
             options.no_rebuild = True

test cases/python/7 install path/meson.build

@@ -17,4 +17,8 @@ py_plat.install_sources('test.py', pure: true, subdir: 'kwrevert')
 install_data('test.py', install_dir: py_plat.get_install_dir() / 'kw/data')
 install_data('test.py', install_dir: py_plat.get_install_dir(pure: true) / 'kwrevert/data')
 
+if get_option('backend') == 'none'
+    subdir('target')
+endif
+
 subdir('structured')

test cases/python/7 install path/target/meson.build

@@ -0,0 +1,3 @@
+testcase expect_error('Install-only backend cannot generate target rules, try using `--backend=ninja`.')
+    import('fs').copyfile('../test.py')
+endtestcase

unittests/baseplatformtests.py

@@ -70,6 +70,7 @@ class BasePlatformTests(TestCase):
             self.uninstall_command = get_backend_commands(self.backend)
         # Test directories
         self.common_test_dir = os.path.join(src_root, 'test cases/common')
+        self.python_test_dir = os.path.join(src_root, 'test cases/python')
         self.rust_test_dir = os.path.join(src_root, 'test cases/rust')
         self.vala_test_dir = os.path.join(src_root, 'test cases/vala')
         self.framework_test_dir = os.path.join(src_root, 'test cases/frameworks')

unittests/platformagnostictests.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import json
 import os
 import tempfile
 import subprocess
@@ -121,3 +122,19 @@ class PlatformAgnosticTests(BasePlatformTests):
                     '''))
             subprocess.check_call(self.wrap_command + ['update-db'], cwd=testdir)
             self.init(testdir, workdir=testdir)
+
+    def test_none_backend(self):
+        testdir = os.path.join(self.python_test_dir, '7 install path')
+
+        self.init(testdir, extra_args=['--backend=none'], override_envvars={'NINJA': 'absolutely false command'})
+        self.assertPathDoesNotExist(os.path.join(self.builddir, 'build.ninja'))
+
+        self.run_tests(inprocess=True, override_envvars={})
+
+        out = self._run(self.meson_command + ['install', f'--destdir={self.installdir}'], workdir=self.builddir)
+        self.assertNotIn('Only ninja backend is supported to rebuild the project before installation.', out)
+
+        with open(os.path.join(testdir, 'test.json'), 'rb') as f:
+            dat = json.load(f)
+        for i in dat['installed']:
+            self.assertPathExists(os.path.join(self.installdir, i['file']))