clang-tidy: Avoid spawning too many threads

The clang-tidy-fix target uses run-clang-tidy to do the fixing, however
this script itself spawns `os.cpu_count()` threads as part of its
internal parallelism. When combined with Meson's parallelism this
results in the creation of potentially thousands of unecessary threads.

This commit rewrites the clang-tidy-fix to perform the same task
run-clang-tidy does but exclusively on Meson's thread pool. "Fix-it"
snippets are saved to `meson-private/clang-tidy-fix/` by a parallel
clang-tidy phase, afterwards (to avoid races) all collected fixes are
applied with a single call to clang-apply-replacements.

3 files changed, 72 insertions, 9 deletions

diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index d8995f031..6eb1f1d01 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -3666,10 +3666,11 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         self.generate_clangtool('format', 'check')
 
     def generate_clangtidy(self) -> None:
-        import shutil
-        if not shutil.which('clang-tidy'):
+        if not environment.detect_clangtidy():
             return
         self.generate_clangtool('tidy')
+        if not environment.detect_clangapply():
+            return
         self.generate_clangtool('tidy', 'fix')
 
     def generate_tags(self, tool: str, target_name: str) -> None:
diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index 90c9bb911..71a2f3afc 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -275,6 +275,32 @@ def detect_clangformat() -> T.List[str]:
             return [path]
     return []
 
+def detect_clangtidy() -> T.List[str]:
+    """ Look for clang-tidy binary on build platform
+
+    Return: a single-element list of the found clang-tidy binary ready to be
+        passed to Popen()
+    """
+    tools = get_llvm_tool_names('clang-tidy')
+    for tool in tools:
+        path = shutil.which(tool)
+        if path is not None:
+            return [path]
+    return []
+
+def detect_clangapply() -> T.List[str]:
+    """ Look for clang-apply-replacements binary on build platform
+
+    Return: a single-element list of the found clang-apply-replacements binary
+        ready to be passed to Popen()
+    """
+    tools = get_llvm_tool_names('clang-apply-replacements')
+    for tool in tools:
+        path = shutil.which(tool)
+        if path is not None:
+            return [path]
+    return []
+
 def detect_windows_arch(compilers: CompilersDict) -> str:
     """
     Detecting the 'native' architecture of Windows is not a trivial task. We
diff --git a/mesonbuild/scripts/clangtidy.py b/mesonbuild/scripts/clangtidy.py
index 1e0c4a5a3..a922f8514 100644
--- a/mesonbuild/scripts/clangtidy.py
+++ b/mesonbuild/scripts/clangtidy.py
@@ -6,15 +6,22 @@ from __future__ import annotations
 import argparse
 import subprocess
 from pathlib import Path
+import tempfile
+import os
+import shutil
+import sys
 
 from .run_tool import run_tool
+from ..environment import detect_clangtidy, detect_clangapply
 import typing as T
 
-def run_clang_tidy(fname: Path, builddir: Path) -> subprocess.CompletedProcess:
-    return subprocess.run(['clang-tidy', '-quiet', '-p', str(builddir), str(fname)])
-
-def run_clang_tidy_fix(fname: Path, builddir: Path) -> subprocess.CompletedProcess:
-    return subprocess.run(['run-clang-tidy', '-fix', '-format', '-quiet', '-p', str(builddir), str(fname)])
+def run_clang_tidy(fname: Path, tidyexe: list, builddir: Path, fixesdir: T.Optional[Path]) -> subprocess.CompletedProcess:
+    args = []
+    if fixesdir is not None:
+        handle, name = tempfile.mkstemp(prefix=fname.name + '.', suffix='.yaml', dir=fixesdir)
+        os.close(handle)
+        args.extend(['-export-fixes', name])
+    return subprocess.run(tidyexe + args + ['-quiet', '-p', str(builddir), str(fname)])
 
 def run(args: T.List[str]) -> int:
     parser = argparse.ArgumentParser()
@@ -26,5 +33,34 @@ def run(args: T.List[str]) -> int:
     srcdir = Path(options.sourcedir)
     builddir = Path(options.builddir)
 
-    run_func = run_clang_tidy_fix if options.fix else run_clang_tidy
-    return run_tool('clang-tidy', srcdir, builddir, run_func, builddir)
+    tidyexe = detect_clangtidy()
+    if not tidyexe:
+        print(f'Could not execute clang-tidy "{" ".join(tidyexe)}"')
+        return 1
+
+    fixesdir: T.Optional[Path] = None
+    if options.fix:
+        applyexe = detect_clangapply()
+        if not applyexe:
+            print(f'Could not execute clang-apply-replacements "{" ".join(applyexe)}"')
+            return 1
+
+        fixesdir = builddir / 'meson-private' / 'clang-tidy-fix'
+        if fixesdir.is_dir():
+            shutil.rmtree(fixesdir)
+        elif fixesdir.exists():
+            fixesdir.unlink()
+        fixesdir.mkdir(parents=True)
+
+    tidyret = run_tool('clang-tidy', srcdir, builddir, run_clang_tidy, tidyexe, builddir, fixesdir)
+    if fixesdir is not None:
+        print('Applying fix-its...')
+        applyret = subprocess.run(applyexe + ['-format', '-style=file', '-ignore-insert-conflict', fixesdir]).returncode
+
+    if tidyret != 0:
+        print('Errors encountered while running clang-tidy', file=sys.stderr)
+        return tidyret
+    if fixesdir is not None and applyret != 0:
+        print('Errors encountered while running clang-apply-replacements', file=sys.stderr)
+        return applyret
+    return 0