Add meson.build files to build with meson

Provide both build systems in parallel for now, to ensure that we work
out all the differences between the two. Some time from now, we'll give
up on autotools.

Meson tends to be faster and probably easier to read/maintain. On my
machine, the full meson configure+build+install takes a little under
half as long as a similar autotools-based invocation.

Building with meson is a two step process. First, configure the build:

  meson build

Then, compile the project:

  ninja -C build

There's some mild differences in functionality between meson and
autotools.  specifically:

1) No singular update-po target. meson only generates individual
update-po targets for each textdomain (of which we have 3).  To make
this easier, there's a build-aux/update-po script which finds all
update-po targets and runs them.

2) No 'make dist' equivalent. Just run 'git archive' to generate a
suitable tarball for distribution.

1 files changed, 250 insertions, 0 deletions

diff --git a/.ycm_extra_conf.py b/.ycm_extra_conf.py
new file mode 100644
index 00000000..f297deef
--- /dev/null
+++ b/.ycm_extra_conf.py
@@ -0,0 +1,250 @@
+#!/usr/bin/env python
+
+# SPDX-License-Identifier: Unlicense
+#
+# Based on the template file provided by the 'YCM-Generator' project authored by
+# Reuben D'Netto.
+# Jiahui Xie has re-reformatted and expanded the original script in accordance
+# to the requirements of the PEP 8 style guide and 'systemd' project,
+# respectively.
+#
+# The original license is preserved as it is.
+#
+#
+# This is free and unencumbered software released into the public domain.
+#
+# Anyone is free to copy, modify, publish, use, compile, sell, or
+# distribute this software, either in source code form or as a compiled
+# binary, for any purpose, commercial or non-commercial, and by any
+# means.
+#
+# In jurisdictions that recognize copyright laws, the author or authors
+# of this software dedicate any and all copyright interest in the
+# software to the public domain. We make this dedication for the benefit
+# of the public at large and to the detriment of our heirs and
+# successors. We intend this dedication to be an overt act of
+# relinquishment in perpetuity of all present and future rights to this
+# software under copyright law.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+# IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+# ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+# OTHER DEALINGS IN THE SOFTWARE.
+#
+# For more information, please refer to <http://unlicense.org/>
+
+"""
+YouCompleteMe configuration file tailored to support the 'meson' build system
+used by the 'systemd' project.
+"""
+
+import glob
+import os
+import ycm_core
+
+
+SOURCE_EXTENSIONS = (".C", ".cpp", ".cxx", ".cc", ".c", ".m", ".mm")
+HEADER_EXTENSIONS = (".H", ".h", ".hxx", ".hpp", ".hh")
+
+
+def DirectoryOfThisScript():
+    """
+    Return the absolute path of the parent directory containing this
+    script.
+    """
+    return os.path.dirname(os.path.abspath(__file__))
+
+
+def GuessBuildDirectory():
+    """
+    Guess the build directory using the following heuristics:
+
+    1. Returns the current directory of this script plus 'build'
+    subdirectory in absolute path if this subdirectory exists.
+
+    2. Otherwise, probes whether there exists any directory
+    containing '.ninja_log' file two levels above the current directory;
+    returns this single directory only if there is one candidate.
+    """
+    result = os.path.join(DirectoryOfThisScript(), "build")
+
+    if os.path.exists(result):
+        return result
+
+    result = glob.glob(os.path.join(DirectoryOfThisScript(),
+                                    "..", "..", "*", ".ninja_log"))
+
+    if not result:
+        return ""
+
+    if 1 != len(result):
+        return ""
+
+    return os.path.split(result[0])[0]
+
+
+def TraverseByDepth(root, include_extensions):
+    """
+    Return a set of child directories of the 'root' containing file
+    extensions specified in 'include_extensions'.
+
+    NOTE:
+        1. The 'root' directory itself is excluded from the result set.
+        2. No subdirectories would be excluded if 'include_extensions' is left
+           to 'None'.
+        3. Each entry in 'include_extensions' must begin with string '.'.
+    """
+    is_root = True
+    result = set()
+    # Perform a depth first top down traverse of the given directory tree.
+    for root_dir, subdirs, file_list in os.walk(root):
+        if not is_root:
+            # print("Relative Root: ", root_dir)
+            # print(subdirs)
+            if include_extensions:
+                get_ext = os.path.splitext
+                subdir_extensions = {
+                    get_ext(f)[-1] for f in file_list if get_ext(f)[-1]
+                }
+                if subdir_extensions & include_extensions:
+                    result.add(root_dir)
+            else:
+                result.add(root_dir)
+        else:
+            is_root = False
+
+    return result
+
+
+_project_src_dir = os.path.join(DirectoryOfThisScript(), "src")
+_include_dirs_set = TraverseByDepth(_project_src_dir, frozenset({".h"}))
+flags = [
+    "-x",
+    "c"
+    # The following flags are partially redundant due to the existence of
+    # 'compile_commands.json'.
+    #    '-Wall',
+    #    '-Wextra',
+    #    '-Wfloat-equal',
+    #    '-Wpointer-arith',
+    #    '-Wshadow',
+    #    '-std=gnu99',
+]
+
+for include_dir in _include_dirs_set:
+    flags.append("-I" + include_dir)
+
+# Set this to the absolute path to the folder (NOT the file!) containing the
+# compile_commands.json file to use that instead of 'flags'. See here for
+# more details: http://clang.llvm.org/docs/JSONCompilationDatabase.html
+#
+# You can get CMake to generate this file for you by adding:
+#   set( CMAKE_EXPORT_COMPILE_COMMANDS 1 )
+# to your CMakeLists.txt file.
+#
+# Most projects will NOT need to set this to anything; you can just change the
+# 'flags' list of compilation flags. Notice that YCM itself uses that approach.
+compilation_database_folder = GuessBuildDirectory()
+
+if os.path.exists(compilation_database_folder):
+    database = ycm_core.CompilationDatabase(compilation_database_folder)
+else:
+    database = None
+
+
+def MakeRelativePathsInFlagsAbsolute(flags, working_directory):
+    """
+    Iterate through 'flags' and replace the relative paths prefixed by
+    '-isystem', '-I', '-iquote', '--sysroot=' with absolute paths
+    start with 'working_directory'.
+    """
+    if not working_directory:
+        return list(flags)
+    new_flags = []
+    make_next_absolute = False
+    path_flags = ["-isystem", "-I", "-iquote", "--sysroot="]
+    for flag in flags:
+        new_flag = flag
+
+        if make_next_absolute:
+            make_next_absolute = False
+            if not flag.startswith("/"):
+                new_flag = os.path.join(working_directory, flag)
+
+        for path_flag in path_flags:
+            if flag == path_flag:
+                make_next_absolute = True
+                break
+
+            if flag.startswith(path_flag):
+                path = flag[len(path_flag):]
+                new_flag = path_flag + os.path.join(working_directory, path)
+                break
+
+        if new_flag:
+            new_flags.append(new_flag)
+    return new_flags
+
+
+def IsHeaderFile(filename):
+    """
+    Check whether 'filename' is considered as a header file.
+    """
+    extension = os.path.splitext(filename)[1]
+    return extension in HEADER_EXTENSIONS
+
+
+def GetCompilationInfoForFile(filename):
+    """
+    Helper function to look up compilation info of 'filename' in the 'database'.
+    """
+    # The compilation_commands.json file generated by CMake does not have
+    # entries for header files. So we do our best by asking the db for flags for
+    # a corresponding source file, if any. If one exists, the flags for that
+    # file should be good enough.
+    if not database:
+        return None
+
+    if IsHeaderFile(filename):
+        basename = os.path.splitext(filename)[0]
+        for extension in SOURCE_EXTENSIONS:
+            replacement_file = basename + extension
+            if os.path.exists(replacement_file):
+                compilation_info = \
+                    database.GetCompilationInfoForFile(replacement_file)
+                if compilation_info.compiler_flags_:
+                    return compilation_info
+        return None
+    return database.GetCompilationInfoForFile(filename)
+
+
+def FlagsForFile(filename, **kwargs):
+    """
+    Callback function to be invoked by YouCompleteMe in order to get the
+    information necessary to compile 'filename'.
+
+    It returns a dictionary with a single element 'flags'. This element is a
+    list of compiler flags to pass to libclang for the file 'filename'.
+    """
+    if database:
+        # Bear in mind that compilation_info.compiler_flags_ does NOT return a
+        # python list, but a "list-like" StringVec object
+        compilation_info = GetCompilationInfoForFile(filename)
+        if not compilation_info:
+            return None
+
+        final_flags = MakeRelativePathsInFlagsAbsolute(
+            compilation_info.compiler_flags_,
+            compilation_info.compiler_working_dir_)
+
+    else:
+        relative_to = DirectoryOfThisScript()
+        final_flags = MakeRelativePathsInFlagsAbsolute(flags, relative_to)
+
+    return {
+        "flags": final_flags,
+        "do_cache": True
+    }