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README.org

Bazel And Compile Commands

1 Aims

I generally use CMake for my C++ developments, but I recently have a look at Bazel from Google. I want to use it a little bit to make my opinion. If you are like me and use tools like Clang, RTags, etc. you must generate a compile_commands.json file. This is trivial with CMake, but AFAIK Bazel does not provide such native support.

I have found this Basics of generating a compile_commands.json file with Bazel gist from bsilver8192. The comment of mmlac was also very useful to understand how to use this gist.

The aim of this post is to automate the compile_commands.json file generation. Two Shell scripts are used for this purpose.

These two scripts can be found in this GitHub Repository (do not panic, maybe you are already in the right place. This link is only useful when this README.org file is used elsewhere).

1.1 Dependencies

I am running under Linux, Debian testing distribution. In peculiar I have the following packages installed:

  • protobuf-compiler
  • python-protobuf

1.2 Usage

The two scripts are:

  • setup_compile_commands.sh: used to set up the Bazel project root directory,
  • create_compile_commands.sh: used to generate the compile_commands.json file.

I hope that they will work out of the box, at least for configurations similar to mine.

1.2.1 The setup_compile_commands.sh script

The setup_compile_commands.sh script must be run only once, it copies and generates all the required files, see Directories and files creation (Annex) for details.

In a Bazel root directory (where the WORKSPACE file is) run:

setup_compile_commands.sh

The script should print

Create tools/actions/BUILD
Create tools/actions/generate_compile_command.py
Create tools/actions/generate_compile_commands_json.py
Create third_party/bazel/protos/extra_actions_base.proto
Generate third_party/bazel/protos/extra_actions_base_pb2.py
Create third_party/bazel/BUILD

and generate the following files

.
├── third_party
│   └── bazel
│       ├── BUILD
│       └── protos
│           ├── extra_actions_base_pb2.py
│           └── extra_actions_base.proto
└── tools
    └── actions
        ├── BUILD
        ├── generate_compile_command.py
        └── generate_compile_commands_json.py

5 directories, 6 files

1.2.2 The create_compile_commands.sh script

After having successfully used the setup_compile_commands.sh script it is very easy to generate the compile_commands.json file.

Choose a A_BAZEL_TARGET like ... or //my:target and run:

create_compile_commands.sh A_BAZEL_TARGET

The script should generate your compile_commands.json file.

2 More details about the shell scripts

2.1 The setup_compile_commands.sh script

The setup_compile_commands.sh script is run only once, it copies and generates all the required the files as described in Basics of generating a compile_commands.json file with Bazel.

The main trick to use is:

#!/bin/sh

more > "a_file.txt" <<'//MY_CODE_STREAM' 
Whatever you want 
    Whatever you want 
//MY_CODE_STREAM

to perform verbatim copies.

Note that for usage safety we stop the script if any of the file to be created already exists. We also check if the current directory contains the WORKSPACE file. Finally, thanks to the set -e option any command with a non-zero status stops the script too.

If you want to overwrite files you can use the “-f” option:

setup_compile_commands.sh -f

2.2 The create_compile_commands.sh script

This script generates the compile_commands.json file. It invokes the two following commands:

set -e

if [ "$#" -ne 1 ]; then
    echo "Usage: $(basename $0) BAZEL_TARGET"
    exit 1
fi

bazel build --experimental_action_listener=//tools/actions:generate_compile_commands_listener $1
python3 ./tools/actions/generate_compile_commands_json.py
exit 0

We added an error message in case the caller did not define a Bazel target ($1=”“)

3 Directories and files creation (Annex)

This part lists all the copied or generated files.

3.1 The tools/actions/ directory

3.1.1 The BUILD file

This file is a direct copy of the Basics of generating a compile_commands.json file with Bazel gist file.

py_binary(
  name = 'generate_compile_command',
  srcs = [
    'generate_compile_command.py',
  ],
  deps = [
    '//third_party/bazel:extra_actions_proto_py',
  ],
)

action_listener(
  name = 'generate_compile_commands_listener',
  visibility = ['//visibility:public'],
  mnemonics = [
    'CppCompile',
  ],
  extra_actions = [':generate_compile_commands_action'],
)

extra_action(
  name = 'generate_compile_commands_action',
  tools = [
    ':generate_compile_command',
  ],
  out_templates = [
    '$(ACTION_ID)_compile_command',
  ],
  cmd = '$(location :generate_compile_command) $(EXTRA_ACTION_FILE)' +
        ' $(output $(ACTION_ID)_compile_command)',
)

3.1.2 The generate_compile_command.py file

This file is a direct copy of the Basics of generating a compile_commands.json file with Bazel gist file.

# This is the implementation of a Bazel extra_action which generates
# _compile_command files for generate_compile_commands.py to consume.

import sys

import third_party.bazel.protos.extra_actions_base_pb2 as extra_actions_base_pb2

def _get_cpp_command(cpp_compile_info):
  compiler = cpp_compile_info.tool
  options = ' '.join(cpp_compile_info.compiler_option)
  source = cpp_compile_info.source_file
  output = cpp_compile_info.output_file
  return '%s %s -c %s -o %s' % (compiler, options, source, output), source

def main(argv):
  action = extra_actions_base_pb2.ExtraActionInfo()
  with open(argv[1], 'rb') as f:
    action.MergeFromString(f.read())
    command, source_file = _get_cpp_command(
      action.Extensions[extra_actions_base_pb2.CppCompileInfo.cpp_compile_info])
  with open(argv[2], 'w') as f:
    f.write(command)
    f.write('\0')
    f.write(source_file)

if __name__ == '__main__':
  sys.exit(main(sys.argv))

3.1.3 The generate_compile_commands_json.py file

This file is a direct copy of the Basics of generating a compile_commands.json file with Bazel gist file.

#!/usr/bin/python3

# This reads the _compile_command files :generate_compile_commands_action
# generates a outputs a compile_commands.json file at the top of the source
# tree for things like clang-tidy to read.

# Overall usage directions: run Bazel with
# --experimental_action_listener=//tools/actions:generate_compile_commands_listener
# for all the files you want to use clang-tidy with and then run this script.
# After that, `clang-tidy build_tests/gflags.cc` should work.

import sys
import pathlib
import os.path
import subprocess

'''
Args:
  path: The pathlib.Path to _compile_command file.
  command_directory: The directory commands are run from.
Returns a string to stick in compile_commands.json.
'''
def _get_command(path, command_directory):
  with path.open('r') as f:
    contents = f.read().split('\0')
    if len(contents) != 2:
      # Old/incomplete file or something; silently ignore it.
      return None
    return '''{
        "directory": "%s",
        "command": "%s",
        "file": "%s"
      }''' % (command_directory, contents[0].replace('"', '\\"'), contents[1])

'''
Args:
  path: A directory pathlib.Path to look for _compile_command files under.
  command_directory: The directory commands are run from.
Yields strings to stick in compile_commands.json.
'''
def _get_compile_commands(path, command_directory):
  for f in path.iterdir():
    if f.is_dir():
      yield from _get_compile_commands(f, command_directory)
    elif f.name.endswith('_compile_command'):
      command = _get_command(f, command_directory)
      if command:
        yield command

def main(argv):
  source_path = os.path.join(os.path.dirname(__file__), '../..')
  action_outs = os.path.join(source_path,
                             'bazel-bin/../extra_actions',
                             'tools/actions/generate_compile_commands_action')
  command_directory = subprocess.check_output(
    ('bazel', 'info', 'execution_root'),
    cwd=source_path).decode('utf-8').rstrip()
  commands = _get_compile_commands(pathlib.Path(action_outs), command_directory)
  with open(os.path.join(source_path, 'compile_commands.json'), 'w') as f:
    f.write('[{}]'.format(','.join(commands)))
    
if __name__ == '__main__':
  sys.exit(main(sys.argv))

3.2 The third_party/bazel directory

3.2.1 The protos/extra_actions_base_pb2.py file

This step requires the bazel/src/main/protobuf/extra_actions_base.proto file from the bazel source. Its last version can be downloaded using:

wget https://raw.githubusercontent.com/bazelbuild/bazel/master/src/main/protobuf/extra_actions_base.proto

This is a temporary file required to generate the protos/extra_actions_base_pb2.py file.

In the current script and in order to be consistent with the previous parts, I do not download this file. Instead I directly embed it in the shell script.

// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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.
//
// proto definitions for the blaze extra_action feature.

syntax = "proto2";

package blaze;

option java_multiple_files = true;
option java_package = "com.google.devtools.build.lib.actions.extra";

// A list of extra actions and metadata for the print_action command.
message ExtraActionSummary {
  repeated DetailedExtraActionInfo action = 1;
}

// An individual action printed by the print_action command.
message DetailedExtraActionInfo {
  // If the given action was included in the output due to a request for a
  // specific file, then this field contains the name of that file so that the
  // caller can correctly associate the extra action with that file.
  //
  // The data in this message is currently not sufficient to run the action on a
  // production machine, because not all necessary input files are identified,
  // especially for C++.
  //
  // There is no easy way to fix this; we could require that all header files
  // are declared and then add all of them here (which would be a huge superset
  // of the files that are actually required), or we could run the include
  // scanner and add those files here.
  optional string triggering_file = 1;
  // The actual action.
  required ExtraActionInfo action = 2;
}

// Provides information to an extra_action on the original action it is
// shadowing.
message ExtraActionInfo {
  extensions 1000 to max;

  // The label of the ActionOwner of the shadowed action.
  optional string owner = 1;

  // Only set if the owner is an Aspect.
  // Corresponds to AspectValue.AspectKey.getAspectClass.getName()
  // This field is deprecated as there might now be
  // multiple aspects applied to the same target.
  // This is the aspect name of the last aspect
  // in 'aspects' (8) field.
  optional string aspect_name = 6 [deprecated = true];

  // Only set if the owner is an Aspect.
  // Corresponds to AspectValue.AspectKey.getParameters()
  // This field is deprecated as there might now be
  // multiple aspects applied to the same target.
  // These are the aspect parameters of the last aspect
  // in 'aspects' (8) field.
  map<string, StringList> aspect_parameters = 7 [deprecated = true];
  message StringList {
    option deprecated = true;
    repeated string value = 1;
  }

  message AspectDescriptor {
    // Corresponds to AspectDescriptor.getName()
    optional string aspect_name = 1;
    // Corresponds to AspectDescriptor.getParameters()
    map<string, StringList> aspect_parameters = 2;
    message StringList {
      repeated string value = 1;
    }
  }

  // If the owner is an aspect, all aspects applied to the target
  repeated AspectDescriptor aspects = 8;

  // An id uniquely describing the shadowed action at the ActionOwner level.
  optional string id = 2;

  // The mnemonic of the shadowed action. Used to distinguish actions with the
  // same ActionType.
  optional string mnemonic = 5;
}

message EnvironmentVariable {
  // It is possible that this name is not a valid variable identifier.
  required string name = 1;
  // The value is unescaped and unquoted.
  required string value = 2;
}

// Provides access to data that is specific to spawn actions.
// Usually provided by actions using the "Spawn" & "Genrule" Mnemonics.
message SpawnInfo {
  extend ExtraActionInfo {
    optional SpawnInfo spawn_info = 1003;
  }

  repeated string argument = 1;
  // A list of environment variables and their values. No order is enforced.
  repeated EnvironmentVariable variable = 2;
  repeated string input_file = 4;
  repeated string output_file = 5;
}

// Provides access to data that is specific to C++ compile actions.
// Usually provided by actions using the "CppCompile" Mnemonic.
message CppCompileInfo {
  extend ExtraActionInfo {
    optional CppCompileInfo cpp_compile_info = 1001;
  }

  optional string tool = 1;
  repeated string compiler_option = 2;
  optional string source_file = 3;
  optional string output_file = 4;
  // Due to header discovery, this won't include headers unless the build is
  // actually performed. If set, this field will include the value of
  // "source_file" in addition to the headers.
  repeated string sources_and_headers = 5;
  // A list of environment variables and their values. No order is enforced.
  repeated EnvironmentVariable variable = 6;
}

// Provides access to data that is specific to C++ link  actions.
// Usually provided by actions using the "CppLink" Mnemonic.
message CppLinkInfo {
  extend ExtraActionInfo {
    optional CppLinkInfo cpp_link_info = 1002;
  }

  repeated string input_file = 1;
  optional string output_file = 2;
  optional string interface_output_file = 3;
  optional string link_target_type = 4;
  optional string link_staticness = 5;
  repeated string link_stamp = 6;
  repeated string build_info_header_artifact = 7;
  // The list of command line options used for running the linking tool.
  repeated string link_opt = 8;
}

// Provides access to data that is specific to java compile actions.
// Usually provided by actions using the "Javac" Mnemonic.
message JavaCompileInfo {
  extend ExtraActionInfo {
    optional JavaCompileInfo java_compile_info = 1000;
  }

  optional string outputjar = 1;
  repeated string classpath = 2;
  repeated string sourcepath = 3;
  repeated string source_file = 4;
  repeated string javac_opt = 5;
  repeated string processor = 6;
  repeated string processorpath = 7;
  repeated string bootclasspath = 8;
}

// Provides access to data that is specific to python rules.
// Usually provided by actions using the "Python" Mnemonic.
message PythonInfo {
  extend ExtraActionInfo {
    optional PythonInfo python_info = 1005;
  }

  repeated string source_file = 1;
  repeated string dep_file = 2;
}

The command to generate extra_actions_base_pb2.py from the extra_actions_base.proto file is:

echo "Generate third_party/bazel/protos/extra_actions_base_pb2.py" 1>&2
protoc third_party/bazel/protos/extra_actions_base.proto --python_out=.

3.2.2 The BUILD file

We register this generated file thanks to a simple BUILD file:

licenses(["notice"])

py_library(
    name = "extra_actions_proto_py",
    srcs = ["protos/extra_actions_base_pb2.py"],
    visibility = ["//visibility:public"],
)
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