ycm: add initial support for the meson build system

The existing configuration file of 'YouCompleteMe' is outdated as it is
still based on 'Makefile' generated by the previous build system
(autoconf); the new script should be able to provide proper semantic
completion for definitions exposed in the project's header files.

.ycm_extra_conf.py

@@ -1,32 +1,146 @@
-import itertools
+#!/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 subprocess
-
-def GetFlagsFromMakefile(varname):
-  return subprocess.check_output([
-      "make", "-s", "print-%s" % varname]).decode().split()
-
-
-def Flatten(lists):
-  return list(itertools.chain.from_iterable(lists))
+import ycm_core
 
 
 def DirectoryOfThisScript():
     return os.path.dirname(os.path.abspath(__file__))
 
 
+def GuessBuildDirectory():
+    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):
+    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
+
+SOURCE_EXTENSIONS = [".C", ".cpp", ".cxx", ".cc", ".c", ".m", ".mm"]
+
+
 def MakeRelativePathsInFlagsAbsolute(flags, working_directory):
     if not working_directory:
-    return flags
+        return list(flags)
     new_flags = []
     make_next_absolute = False
-  path_flags = [ '-isystem', '-I', '-iquote', '--sysroot=' ]
+    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('/'):
+            if not flag.startswith("/"):
                 new_flag = os.path.join(working_directory, flag)
 
         for path_flag in path_flags:
@@ -44,24 +158,49 @@ def MakeRelativePathsInFlagsAbsolute(flags, working_directory):
     return new_flags
 
 
-def FlagsForFile(filename):
+def IsHeaderFile(filename):
+    extension = os.path.splitext(filename)[1]
+    return extension in [".H", ".h", ".hxx", ".hpp", ".hh"]
+
+
+def GetCompilationInfoForFile(filename):
+    # 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):
+    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': MakeRelativePathsInFlagsAbsolute(flags, relative_to),
-    'do_cache': True
+        "flags": final_flags,
+        "do_cache": True
     }
-
-flags = Flatten(map(GetFlagsFromMakefile, [
-  'AM_CPPFLAGS',
-  'CPPFLAGS',
-  'AM_CFLAGS',
-  'CFLAGS',
-]))
-
-# these flags cause crashes in libclang, so remove them
-flags.remove('-Wlogical-op')
-flags.remove('-Wsuggest-attribute=noreturn')
-flags.remove('-Wdate-time')
-
-# vim: set et ts=2 sw=2: