avoid mutate conf in recipes

conans/model/conf.py

@@ -1,3 +1,4 @@
+import copy
 import re
 import os
 import fnmatch
@@ -153,7 +154,8 @@ def value(self):
         return self._value
 
     def copy(self):
-        return _ConfValue(self._name, self._value, self._path, self._update)
+        # Using copy for when self._value is a mutable list
+        return _ConfValue(self._name, copy.copy(self._value), self._path, self._update)
 
     def dumps(self):
         if self._value is None:
@@ -291,6 +293,7 @@ def get(self, conf_name, default=None, check_type=None, choices=None):
         :param default: Default value in case of conf does not have the conf_name key.
         :param check_type: Check the conf type(value) is the same as the given by this param.
                            There are two default smart conversions for bool and str types.
+        :param choices: list of possible values this conf can have, if value not in it, errors.
         """
         # Skipping this check only the user.* configurations
         self._check_conf_name(conf_name)
@@ -335,7 +338,7 @@ def show(self, fnpattern, pattern=""):
 
     def copy(self):
         c = Conf()
-        c._values = self._values.copy()
+        c._values = OrderedDict((k, v.copy()) for k, v in self._values.items())
         return c
 
     def dumps(self):

conans/test/integration/configuration/conf/test_conf.py

@@ -7,7 +7,6 @@
 
 from conan import conan_version
 from conan.internal.api import detect_api
-from conans.errors import ConanException
 from conans.test.assets.genconanfile import GenConanfile
 from conans.util.files import save, load
 from conans.test.utils.tools import TestClient
@@ -343,3 +342,34 @@ def generate(self):
     c.run('install pkg --build=*')
     assert "dep/0.1: SKIP-TEST: True" in c.out
     assert "conanfile.py (pkg/0.1): SKIP-TEST: False" in c.out
+
+
+def test_conf_should_be_immutable():
+    c = TestClient()
+    dep = textwrap.dedent("""
+        from conan import ConanFile
+        class Pkg(ConanFile):
+            name = "dep"
+            version = "0.1"
+            def generate(self):
+                self.conf.append("user.myteam:myconf", "value1")
+                self.output.info(f'user.myteam:myconf: {self.conf.get("user.myteam:myconf")}')
+        """)
+    pkg = textwrap.dedent("""
+        from conan import ConanFile
+        class Pkg(ConanFile):
+            name = "pkg"
+            version = "0.1"
+            requires = "dep/0.1"
+            def generate(self):
+                self.output.info(f'user.myteam:myconf: {self.conf.get("user.myteam:myconf")}')
+        """)
+    save(c.cache.new_config_path, 'user.myteam:myconf=["root_value"]')
+    c.save({"dep/conanfile.py": dep,
+            "pkg/conanfile.py": pkg})
+    c.run("create dep")
+    assert "dep/0.1: user.myteam:myconf: ['root_value', 'value1']" in c.out
+    c.run('create pkg --build=*')
+    assert "dep/0.1: user.myteam:myconf: ['root_value', 'value1']" in c.out
+    # The pkg/0.1 output should be non-modified
+    assert "pkg/0.1: user.myteam:myconf: ['root_value']" in c.out