Provide a way to create a compilation DB in the bazel project

This uses http://bant.build/ to create a compilation DB.

The change provides two useful scripts:

  * `etc/bazel-make-compilation-db.sh` creates a compilation db.
    To avoid a huge compile_commands.json, this generates the
    compile_flags.txt which is a much shorter file applied to all
    files. This keeps memory usage low for tools such as clang-tidy
  * `etc/run-clang-tidy.sh` a convenience script that creates the
    compilation DB and then calls `run-clang-tidy-cached.cc`.
    It uses `clang-tidy` from the llvm-toolchain configured in the MODULE.bazel

Issues #8586

Signed-off-by: Henner Zeller <[email protected]>

Author: hzeller

MODULE.bazel

@@ -75,6 +75,9 @@ bazel_dep(name = "tcmalloc", version = "0.0.0-20250331-43fcf6e")
 bazel_dep(name = "zlib", version = "1.3.1.bcr.5")
 bazel_dep(name = "yaml-cpp", version = "0.8.0")
 
+# Make compilation DB (and possibly build cleaner)
+bazel_dep(name = "bant", version = "0.2.4", dev_dependency = True)
+
 # Use HEAD of master upstream to fix bugs such as missing stdint include
 git_override(
     module_name = "yaml-cpp",

MODULE.bazel.lock

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+set -eu
+
+# Which bazel and bant to use. If unset, defaults are used.
+BAZEL=${BAZEL:-bazel}
+BANT="$($(dirname "$0")/get-bant-path.sh)"
+
+# Important to run with --remote_download_outputs=all to make sure generated
+# files are actually visible locally in case a remote cache (that includes
+# --disk_cache) is used ( https://github.com/hzeller/bazel-gen-file-issue )
+BAZEL_OPTS="-c opt --remote_download_outputs=all"
+
+# Tickle some build targets to fetch all dependencies and generate files,
+# so that they can be seen by the users of the compilation db.
+# Right now, comile everything (which should not be too taxing with the
+# bazel cache), but it could be made more specific to only trigger specific
+# targets that are sufficient to get and regenerate everything.
+"${BAZEL}" build -k ${BAZEL_OPTS} src/...
+
+# Create compilation DB. Command 'compilation-db' would create a huge *.json file,
+# but compile_flags.txt is perfectly sufficient and easier for tools to use as
+# these tools will require much less memory.
+
+"${BANT}" compile-flags -o compile_flags.txt
+
+# The QT headers are not properly picked up; add them manually.
+for f in bazel-out/../../../external/qt-bazel*/qt_source/qtbase*/build/include/Q* ; do echo "-I$f" ; done >> compile_flags.txt
+
+# If there are two styles of comp-dbs, tools might have issues. Warn user.
+if [ -r compile_commands.json ]; then
+  printf "\n\033[1;31mSuggest to remove old compile_commands.json to not interfere with compile_flags.txt\033[0m\n\n"
+fi

etc/bazel-make-compilation-db.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+set -eu
+
+# Print path to a bant binary. Can be provided by an environment variable
+# or built from our dependency.
+
+BAZEL=${BAZEL:-bazel}
+BANT=${BANT:-needs-to-be-compiled-locally}
+
+# Bant not given, compile from bzlmod dep.
+if [ "${BANT}" = "needs-to-be-compiled-locally" ]; then
+  # Suppress build output to not clutter terminal.
+  "${BAZEL}" build --noshow_progress -c opt @bant//bant:bant > /dev/null 2>&1
+
+  # Note: to NOT use cquery as we want to be robust against symbolic
+  # links vanishing
+  BANT=$(realpath bazel-bin/external/bant*/bant/bant | head -1)
+fi
+
+echo "$BANT"

etc/get-bant-path.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# Invocation without parameters simply uses the .clang-tidy config to run on
+# all *.{cc,h} files. Additional parameters passed to this script are passed
+# to clang-tidy as-is, e.g. if you only want to run a particular check and
+# the auto-fix
+#   run-clang-tidy.sh --checks="-*,modernize-loop-convert" --fix
+
+set -eu
+
+# Use either CLANG_TIDY provided by the user as environment variable or use
+# our own from the toolchain we configured in the MODULE.bazel
+export CLANG_TIDY="${CLANG_TIDY:-$(bazel run -c opt --run_under='echo' @llvm_toolchain//:clang-tidy 2>/dev/null)}"
+
+# The user should keep the compilation DB fresh, but refresh here
+# if substantial things changed or it is not there in the first place.
+if [ MODULE.bazel -nt compile_flags.txt -o BUILD.bazel -nt compile_flags.txt ] ; then
+   "$(dirname "$0")/bazel-make-compilation-db.sh"
+fi
+
+"$(dirname "$0")/run-clang-tidy-cached.cc" "$@"