scripts: make clang tools obey b_colorout

Right now, the clang-tidy and clang-format targets use the program default
and do not let b_colorout decide whether to colorize output.

However, the wrappers that run the tool are going to be changed to buffer
output, and that would disable colorization unconditionally.  So pass
a --color option to the tools and use it when building the command line.

clang-format's -fcolor-diagnostics option simply does not work, and the
"right" (or at least working) option is --color which is undocumented.
--color is present all the way back to clang 10, but I digged into
clang-format's source code to figure out what's happening.  The problem
is that -fcolor-diagnostics is a complete no-operation; in fact it is
a bool that is initialized to true.  gdb shows:

  (gdb) p ShowColors
  $2 = {<llvm::cl::Option> = {
    ... <llvm::cl::opt_storage<bool, false, false>> = {Value = true,  ... }, ...}

on entry to clang-format's main, meaning that specifying the option on
the command line does nothing at all.

To see how clang-format determines whether to use colors you need to look
at enters SMDiagnostic::print, which simply does

  ColorMode Mode = ShowColors ? ColorMode::Auto : ColorMode::Disable;

showing once more that in fact the option cannot force-on the colors (
-fno-color-diagnostics instead works).  Continuing in SMDiagnostic::print,
this RAII constructor would write the escape sequence to the terminal:

  WithColor S(OS, raw_ostream::SAVEDCOLOR, true, false, Mode);

It ends up in WithColor::changeColor, which does

  if (colorsEnabled())
    OS.changeColor(Color, Bold, BG);

Digging further down, colorsEnabled() is where the Mode member is consulted:

  bool WithColor::colorsEnabled() {
    switch (Mode) {
      case ColorMode::Enable:
        return true;
      case ColorMode::Disable:
        return false;
      case ColorMode::Auto:
        return AutoDetectFunction(OS);
      }
      llvm_unreachable("All cases handled above.");
    }

and the "AutoDetectFunction" is

  static bool DefaultAutoDetectFunction(const raw_ostream &OS) {
    return *UseColor == cl::BOU_UNSET ? OS.has_colors()
                                      : *UseColor == cl::BOU_TRUE;
  }

UseColor is controlled by the "--color" option, so if that option was
unset you go to OS.has_colors() even in the presence of -fcolor-diagnostics.
This has been around for over 5 years in clang-format, and it was present
even earlier, so use it in meson as well.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 13 insertions, 4 deletions

diff --git a/mesonbuild/scripts/clangformat.py b/mesonbuild/scripts/clangformat.py
index 9ce050458..88cc89071 100644
--- a/mesonbuild/scripts/clangformat.py
+++ b/mesonbuild/scripts/clangformat.py
@@ -6,6 +6,7 @@ from __future__ import annotations
 import argparse
 import subprocess
 from pathlib import Path
+import sys
 
 from .run_tool import run_tool
 from ..environment import detect_clangformat
@@ -13,12 +14,15 @@ from ..mesonlib import version_compare
 from ..programs import ExternalProgram
 import typing as T
 
-def run_clang_format(fname: Path, exelist: T.List[str], check: bool, cformat_ver: T.Optional[str]) -> subprocess.CompletedProcess:
+def run_clang_format(fname: Path, exelist: T.List[str], options: argparse.Namespace, cformat_ver: T.Optional[str]) -> subprocess.CompletedProcess:
     clangformat_10 = False
-    if check and cformat_ver:
+    if options.check and cformat_ver:
         if version_compare(cformat_ver, '>=10'):
             clangformat_10 = True
             exelist = exelist + ['--dry-run', '--Werror']
+            # The option is not documented but it exists in version 10
+            if options.color == 'always' or options.color == 'auto' and sys.stdout.isatty():
+                exelist += ['--color=1']
         else:
             original = fname.read_bytes()
     before = fname.stat().st_mtime
@@ -26,7 +30,7 @@ def run_clang_format(fname: Path, exelist: T.List[str], check: bool, cformat_ver
     after = fname.stat().st_mtime
     if before != after:
         print('File reformatted: ', fname)
-        if check and not clangformat_10:
+        if options.check and not clangformat_10:
             # Restore the original if only checking.
             fname.write_bytes(original)
             ret.returncode = 1
@@ -35,6 +39,7 @@ def run_clang_format(fname: Path, exelist: T.List[str], check: bool, cformat_ver
 def run(args: T.List[str]) -> int:
     parser = argparse.ArgumentParser()
     parser.add_argument('--check', action='store_true')
+    parser.add_argument('--color', default='always')
     parser.add_argument('sourcedir')
     parser.add_argument('builddir')
     options = parser.parse_args(args)
@@ -52,4 +57,4 @@ def run(args: T.List[str]) -> int:
     else:
         cformat_ver = None
 
-    return run_tool('clang-format', srcdir, builddir, run_clang_format, exelist, options.check, cformat_ver)
+    return run_tool('clang-format', srcdir, builddir, run_clang_format, exelist, options, cformat_ver)
diff --git a/mesonbuild/scripts/clangtidy.py b/mesonbuild/scripts/clangtidy.py
index a922f8514..fe34801e0 100644
--- a/mesonbuild/scripts/clangtidy.py
+++ b/mesonbuild/scripts/clangtidy.py
@@ -26,6 +26,7 @@ def run_clang_tidy(fname: Path, tidyexe: list, builddir: Path, fixesdir: T.Optio
 def run(args: T.List[str]) -> int:
     parser = argparse.ArgumentParser()
     parser.add_argument('--fix', action='store_true')
+    parser.add_argument('--color', default='always')
     parser.add_argument('sourcedir')
     parser.add_argument('builddir')
     options = parser.parse_args(args)
@@ -38,6 +39,9 @@ def run(args: T.List[str]) -> int:
         print(f'Could not execute clang-tidy "{" ".join(tidyexe)}"')
         return 1
 
+    if options.color == 'always' or options.color == 'auto' and sys.stdout.isatty():
+        tidyexe += ['--use-color']
+
     fixesdir: T.Optional[Path] = None
     if options.fix:
         applyexe = detect_clangapply()