rewriter: Bugfix concerning `UnknownValue`

Without this commit, the static introspection tool crashes when
introspecting systemd since certain values are `UnknownValue`
which was unexpected.

(I tested sytemd's commit hash fefcb935cd.)

3 files changed, 12 insertions, 5 deletions

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index 73f99cb66..8049f160d 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -33,7 +33,7 @@ from .compilers import (
     is_header, is_object, is_source, clink_langs, sort_clink, all_languages,
     is_known_suffix, detect_static_linker
 )
-from .interpreterbase import FeatureNew, FeatureDeprecated
+from .interpreterbase import FeatureNew, FeatureDeprecated, UnknownValue
 
 if T.TYPE_CHECKING:
     from typing_extensions import Literal, TypedDict
@@ -648,7 +648,7 @@ class Target(HoldableObject, metaclass=abc.ABCMeta):
     def process_kwargs_base(self, kwargs: T.Dict[str, T.Any]) -> None:
         if 'build_by_default' in kwargs:
             self.build_by_default = kwargs['build_by_default']
-            if not isinstance(self.build_by_default, bool):
+            if not isinstance(self.build_by_default, (bool, UnknownValue)):
                 raise InvalidArguments('build_by_default must be a boolean value.')
 
         if not self.build_by_default and kwargs.get('install', False):
diff --git a/mesonbuild/mintro.py b/mesonbuild/mintro.py
index db62dc06c..6986186bd 100644
--- a/mesonbuild/mintro.py
+++ b/mesonbuild/mintro.py
@@ -31,6 +31,12 @@ if T.TYPE_CHECKING:
 
     from .interpreter import Interpreter
 
+class IntrospectionEncoder(json.JSONEncoder):
+    def default(self, obj: T.Any) -> T.Any:
+        if isinstance(obj, UnknownValue):
+            return 'unknown'
+        return json.JSONEncoder.default(self, obj)
+
 def get_meson_info_file(info_dir: str) -> str:
     return os.path.join(info_dir, 'meson-info.json')
 
@@ -492,12 +498,12 @@ def print_results(options: argparse.Namespace, results: T.Sequence[T.Tuple[str,
         return 1
     elif len(results) == 1 and not options.force_dict:
         # Make to keep the existing output format for a single option
-        print(json.dumps(results[0][1], indent=indent))
+        print(json.dumps(results[0][1], indent=indent, cls=IntrospectionEncoder))
     else:
         out = {}
         for i in results:
             out[i[0]] = i[1]
-        print(json.dumps(out, indent=indent))
+        print(json.dumps(out, indent=indent, cls=IntrospectionEncoder))
     return 0
 
 def get_infodir(builddir: T.Optional[str] = None) -> str:
diff --git a/mesonbuild/rewriter.py b/mesonbuild/rewriter.py
index 9023033b9..4c2fb11bc 100644
--- a/mesonbuild/rewriter.py
+++ b/mesonbuild/rewriter.py
@@ -17,6 +17,7 @@ from mesonbuild.mesonlib import MesonException, setup_vsenv, relpath
 from . import mlog, environment
 from functools import wraps
 from .mparser import Token, ArrayNode, ArgumentNode, ArithmeticNode, AssignmentNode, BaseNode, StringNode, BooleanNode, ElementaryNode, IdNode, FunctionNode, PlusAssignmentNode
+from .mintro import IntrospectionEncoder
 import json, os, re, sys, codecs
 import typing as T
 from pathlib import Path
@@ -399,7 +400,7 @@ class Rewriter:
     def print_info(self) -> None:
         if self.info_dump is None:
             return
-        sys.stdout.write(json.dumps(self.info_dump, indent=2))
+        sys.stdout.write(json.dumps(self.info_dump, indent=2, cls=IntrospectionEncoder))
 
     def on_error(self) -> T.Tuple[AnsiDecorator, AnsiDecorator]:
         if self.skip_errors: