build: implement clang-tidy using our plugin

The principle of this is that you can either externally build it with
Nix (actual implementation will be in a future commit), or it can be
built with meson if the Nix one is not passed in.

The idea I have is that dev shells don't receive the one from Nix to
avoid having to build it, but CI can use the one from Nix and save some
gratuitous rebuilds.

The design of this is that you can run `ninja -C build clang-tidy` and
it will simply correctly clang-tidy the codebase in spite of PCH
bullshit caused by the cc-wrapper.

This is a truly horrendous number of hacks in a ball, caused by bugs in
several pieces of software, and I am not even getting started.

I don't consider this to fix the clang-tidy issue filing, since we still
have a fair number of issues to fix even on the existing minimal
configuration, and I have not yet implemented it in CI. Realistically we
will need to do something like https://github.com/Ericsson/codechecker
to be able to silence warnings without physically touching the code, or
at least *diff* reports between versions.

Also, the run-clang-tidy output design is rather atrocious and must
not be inflicted upon anyone I have respect for, since it buries the
diagnostics in a pile of invocation logs. We would do really well to
integrate with the Gerrit SARIF stuff so we can dump the reports on
people in a user-friendly manner.

Related: https://git.lix.systems/lix-project/lix/issues/147

Change-Id: Ifefe533f3b56874795de231667046b2da6ff2461

1 files changed, 98 insertions, 0 deletions

diff --git a/meson/clang-tidy/meson.build b/meson/clang-tidy/meson.build
new file mode 100644
index 000000000..9aba5fbdc
--- /dev/null
+++ b/meson/clang-tidy/meson.build
@@ -0,0 +1,98 @@
+# The clang-tidy target for Lix
+
+run_clang_tidy = find_program('run-clang-tidy', required : false)
+# Although this looks like it wants to be pkg-config, pkg-config does not
+# really work for *plugins*, which are executable-like .so files that also
+# cannot be found via find_program. Fun!
+if get_option('lix-clang-tidy-checks-path') != ''
+  lix_clang_tidy_so = get_option('lix-clang-tidy-checks-path')
+  lix_clang_tidy_so_found = true
+else
+  lix_clang_tidy_subproj = subproject(
+    'lix-clang-tidy',
+    required : false,
+    default_options : {'build-by-default': false}
+  )
+  if lix_clang_tidy_subproj.found()
+    lix_clang_tidy_so = lix_clang_tidy_subproj.get_variable('lix_clang_tidy')
+    lix_clang_tidy_so_found = true
+  else
+    lix_clang_tidy_so_found = false
+  endif
+endif
+
+# Due to numerous problems, such as:
+# - Meson does not expose pch targets, but *fine*, I can just ask Ninja for
+#   them with `ninja -t targets rule cpp_PCH` and build them manually:
+#   https://github.com/mesonbuild/meson/issues/13499
+# - Nixpkgs stdenv buries the cc-wrapper under a giant pile of assumptions
+#   about the cc-wrapper actually being used on the cc of a stdenv, rather than
+#   independently for clang-tidy, and we need to use cc-wrapper to get the
+#   correct hardening flags so that clang-tidy can actually parse the PCH file
+#
+# I give up. I am going to delete the damn PCH args and then it will work.
+meson.add_postconf_script(
+  python,
+  meson.current_source_dir() / 'clean_compdb.py',
+  meson.global_build_root() / 'compile_commands.json',
+  meson.current_build_dir() / 'compile_commands.json',
+)
+
+# Horrible hack to get around not being able to depend on another target's
+# generated headers in any way in the meson DSL
+# https://github.com/mesonbuild/meson/issues/12817 which was incorrectly
+# closed, if you *actually* need to generate the files once.
+# Also related: https://github.com/mesonbuild/meson/issues/3667
+#
+# Or we could ban meson generators because their design is broken.
+build_all_generated_headers = custom_target(
+  command : [
+    python,
+    meson.current_source_dir() / 'build_required_targets.py',
+    meson.global_build_root(),
+  ],
+  output : 'generated_headers.stamp',
+  build_by_default : false,
+  build_always_stale : true,
+)
+
+if lix_clang_tidy_so_found
+  run_clang_tidy_args = [
+    '-load',
+    lix_clang_tidy_so,
+    '-p',
+    # We have to workaround a run-clang-tidy bug too, so we must give the
+    # directory name rather than the actual compdb file.
+    # https://github.com/llvm/llvm-project/issues/101440
+    meson.current_build_dir(),
+    '-quiet',
+  ]
+  run_target(
+    'clang-tidy',
+    command : [
+      # XXX: This explicitly invokes it with python because of a nixpkgs bug
+      # where clang-unwrapped does not patch interpreters in run-clang-tidy.
+      # However, making clang-unwrapped depend on python is also silly, so idk.
+      python,
+      run_clang_tidy,
+      run_clang_tidy_args,
+      '-warnings-as-errors',
+      '*',
+    ],
+    depends : [
+      build_all_generated_headers,
+    ],
+  )
+  run_target(
+    'clang-tidy-fix',
+    command : [
+      python,
+      run_clang_tidy,
+      run_clang_tidy_args,
+      '-fix',
+    ],
+    depends : [
+      build_all_generated_headers,
+    ],
+  )
+endif