fix: reporters receive copy of message

doc/whatsnew/fragments/7214.bugfix

@@ -0,0 +1,3 @@
+Message send to reporter are now copied so a reporter cannot modify the message sent to other reporters.
+
+Closes #7214

pylint/reporters/multi_reporter.py

@@ -6,6 +6,7 @@
 
 import os
 from collections.abc import Callable
+from copy import copy
 from typing import TYPE_CHECKING, TextIO
 
 from pylint.message import Message
@@ -77,7 +78,8 @@ def linter(self, value: PyLinter) -> None:
     def handle_message(self, msg: Message) -> None:
         """Handle a new message triggered on the current file."""
         for rep in self._sub_reporters:
-            rep.handle_message(msg)
+            # We provide a copy so reporters can't modify message for others.
+            rep.handle_message(copy(msg))
 
     def writeln(self, string: str = "") -> None:
         """Write a line in the output buffer."""

tests/reporters/unittest_reporting.py

@@ -18,10 +18,12 @@
 from _pytest.recwarn import WarningsRecorder
 
 from pylint import checkers
+from pylint.interfaces import HIGH
 from pylint.lint import PyLinter
+from pylint.message.message import Message
 from pylint.reporters import BaseReporter, MultiReporter
 from pylint.reporters.text import ParseableTextReporter, TextReporter
-from pylint.typing import FileItem
+from pylint.typing import FileItem, MessageLocationTuple
 
 if TYPE_CHECKING:
     from pylint.reporters.ureports.nodes import Section
@@ -334,6 +336,50 @@ def test_multi_format_output(tmp_path: Path) -> None:
     )
 
 
+def test_multi_reporter_independant_messages() -> None:
+    """Messages should not be modified by multiple reporters"""
+
+    check_message = "Not modified"
+
+    class ReporterModify(BaseReporter):
+        def handle_message(self, msg: Message) -> None:
+            msg.msg = "Modified message"
+
+        def writeln(self, string: str = "") -> None:
+            pass
+
+        def _display(self, layout: Section) -> None:
+            pass
+
+    class ReporterCheck(BaseReporter):
+        def handle_message(self, msg: Message) -> None:
+            assert (
+                msg.msg == check_message
+            ), "Message object should not be changed by other reporters."
+
+        def writeln(self, string: str = "") -> None:
+            pass
+
+        def _display(self, layout: Section) -> None:
+            pass
+
+    multi_reporter = MultiReporter([ReporterModify(), ReporterCheck()], lambda: None)
+
+    message = Message(
+        symbol="missing-docstring",
+        msg_id="C0123",
+        location=MessageLocationTuple("abspath", "path", "module", "obj", 1, 2, 1, 3),
+        msg=check_message,
+        confidence=HIGH,
+    )
+
+    multi_reporter.handle_message(message)
+
+    assert (
+        message.msg == check_message
+    ), "Message object should not be changed by reporters."
+
+
 def test_display_results_is_renamed() -> None:
     class CustomReporter(TextReporter):
         def _display(self, layout: Section) -> None: