Check in CI that Meson and pkg-config offer the CMake options

tools/check_build_options/check_build_options.py takes the compile
definitions of the CMake target as the reference and checks that the
CMake pkg-config block, meson_options.txt (name and default), meson.build,
and the Meson section of the package manager docs have each option with
the same definition and condition. It runs as make check_build_options
in the ci_meson_install job, so a new CMake option cannot be forgotten
in the Meson build again.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tools/check_build_options/README.md

@@ -0,0 +1,21 @@
+# check_build_options
+
+Checks that the Meson build and the pkg-config files offer the same options as the CMake target, so that a new CMake
+option is not forgotten in one of them.
+
+The compile definitions of the CMake target (`target_compile_definitions` in [`CMakeLists.txt`](../../CMakeLists.txt))
+are the reference. When you add an option there, also add it to
+
+- the pkg-config block in `CMakeLists.txt` (`NLOHMANN_JSON_PKGCONFIG_CFLAGS`),
+- [`meson_options.txt`](../../meson_options.txt), named without the `JSON_` prefix and with the same default,
+- [`meson.build`](../../meson.build) (`json_defines`), and
+- the list of Meson options in
+  [`docs/mkdocs/docs/integration/package_managers.md`](../../docs/mkdocs/docs/integration/package_managers.md).
+
+Run the check with
+
+```shell
+make check_build_options
+```
+
+It needs only Python 3 and runs in the `ci_meson_install` CI job.

tools/check_build_options/check_build_options.py

@@ -0,0 +1,144 @@
+#!/usr/bin/env python3
+"""Check that the Meson build and the pkg-config files offer the CMake options.
+
+The compile definitions of the CMake target (target_compile_definitions in
+CMakeLists.txt) are the reference. For every option used there, the script
+checks that
+
+- the CMake pkg-config file adds the same definition under the same condition,
+- meson_options.txt has a boolean option of the same name without the "JSON_"
+  prefix and with the same default,
+- meson.build adds the same definition under the same condition, and
+- the Meson section of the package manager documentation lists the option.
+
+Meson's MultipleHeaders option selects the include directory and adds no
+definition; it is the only Meson option without a definition.
+"""
+
+import argparse
+import os
+import re
+import sys
+
+REPO_ROOT = os.path.normpath(os.path.join(sys.path[0], '..', '..'))
+DOCS = os.path.join('docs', 'mkdocs', 'docs', 'integration', 'package_managers.md')
+
+# Meson options that add no compile definition, with their default
+MESON_ONLY = {'MultipleHeaders': 'false'}
+
+
+def read(root, path):
+    with open(os.path.join(root, path), encoding='utf-8') as f:
+        return f.read()
+
+
+def cmake_target_definitions(cmake):
+    """Return {option: (definition, add_if_on)} from target_compile_definitions."""
+    block = re.search(r'target_compile_definitions\(\s*\$\{NLOHMANN_JSON_TARGET_NAME\}\s*INTERFACE(.*?)\n\)', cmake, re.S)
+    if not block:
+        sys.exit('CMakeLists.txt: target_compile_definitions of the target not found')
+    result = {}
+    for line in block.group(1).split('\n'):
+        line = line.strip()
+        if not line:
+            continue
+        m = re.fullmatch(r'\$<\$<NOT:\$<BOOL:\$\{JSON_(\w+)\}>>:(\w+=\w+)>', line)
+        if m:
+            result[m.group(1)] = (m.group(2), False)
+            continue
+        m = re.fullmatch(r'\$<\$<BOOL:\$\{JSON_(\w+)\}>:(\w+=\w+)>', line)
+        if m:
+            result[m.group(1)] = (m.group(2), True)
+            continue
+        sys.exit(f'CMakeLists.txt: unexpected line in target_compile_definitions: {line}')
+    return result
+
+
+def cmake_defaults(cmake):
+    """Return {option: 'true'/'false'} for the option() calls of the form JSON_<name>."""
+    return {m.group(1): 'true' if m.group(2) == 'ON' else 'false'
+            for m in re.finditer(r'^option\(JSON_(\w+)\s+"[^"]*"\s+(ON|OFF)\)', cmake, re.M)}
+
+
+def cmake_pkgconfig_definitions(cmake):
+    """Return {option: (definition, add_if_on)} from the pkg-config block."""
+    return {m.group(2): (m.group(3), m.group(1) is None)
+            for m in re.finditer(r'if \((NOT )?JSON_(\w+)\)\s*\n\s*string\(APPEND NLOHMANN_JSON_PKGCONFIG_CFLAGS " -D(\w+=\w+)"\)', cmake)}
+
+
+def meson_options(options):
+    """Return {option: default} for the boolean options in meson_options.txt."""
+    result = {}
+    for block in re.findall(r'option\((.*?)\)', options, re.S):
+        name = re.search(r"'(\w+)'", block).group(1)
+        kind = re.search(r"type\s*:\s*'(\w+)'", block)
+        value = re.search(r'value\s*:\s*(\w+)', block)
+        result[name] = value.group(1) if kind and kind.group(1) == 'boolean' and value else None
+    return result
+
+
+def meson_definitions(meson):
+    """Return {option: (definition, add_if_on)} from meson.build."""
+    return {m.group(2): (m.group(3), m.group(1) is None)
+            for m in re.finditer(r"if (not )?get_option\('(\w+)'\)\s*\n\s*json_defines \+= '(\w+=\w+)'", meson)}
+
+
+def describe(definition):
+    name, add_if_on = definition
+    return f'{name} if {"enabled" if add_if_on else "disabled"}'
+
+
+def compare(errors, where, expected, actual):
+    for option, definition in expected.items():
+        if option not in actual:
+            errors.append(f'{where}: no definition for option {option} (expected {describe(definition)})')
+        elif actual[option] != definition:
+            errors.append(f'{where}: option {option} adds {describe(actual[option])}, expected {describe(definition)}')
+    for option in actual.keys() - expected.keys():
+        errors.append(f'{where}: definition for option {option}, which the CMake target does not have')
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__.split('\n')[0])
+    parser.add_argument('root', nargs='?', default=REPO_ROOT, help='repository root (default: %(default)s)')
+    root = parser.parse_args().root
+
+    cmake = read(root, 'CMakeLists.txt')
+    reference = cmake_target_definitions(cmake)
+    defaults = cmake_defaults(cmake)
+    errors = []
+
+    compare(errors, 'CMakeLists.txt (pkg-config)', reference, cmake_pkgconfig_definitions(cmake))
+    compare(errors, 'meson.build', reference, meson_definitions(read(root, 'meson.build')))
+
+    options = meson_options(read(root, 'meson_options.txt'))
+    for option in reference:
+        if option not in defaults:
+            errors.append(f'CMakeLists.txt: no option(JSON_{option} ... ON|OFF)')
+        elif option not in options:
+            errors.append(f'meson_options.txt: option {option} missing')
+        elif options[option] != defaults[option]:
+            errors.append(f'meson_options.txt: option {option} must be boolean with value {defaults[option]} as in CMake')
+    for option, default in MESON_ONLY.items():
+        if options.get(option) != default:
+            errors.append(f'meson_options.txt: option {option} must be boolean with value {default}')
+    for option in options.keys() - reference.keys() - MESON_ONLY.keys():
+        errors.append(f'meson_options.txt: option {option} has no counterpart in the CMake target')
+
+    docs = read(root, DOCS)
+    for option in sorted(set(options) & (reference.keys() | MESON_ONLY.keys())):
+        if f'`{option}`' not in docs:
+            errors.append(f'{DOCS}: Meson option {option} not listed')
+
+    for error in errors:
+        print(error, file=sys.stderr)
+    if errors:
+        print('The Meson build and the pkg-config files must offer the options of the CMake target; see '
+              'tools/check_build_options/README.md.', file=sys.stderr)
+        return 1
+    print(f'OK: {len(reference)} options with compile definitions agree between CMake, pkg-config, and Meson.')
+    return 0
+
+
+if __name__ == '__main__':
+    sys.exit(main())