fix(cc_apt_configure): avoid unneeded call to apt-install (#4519)

If no apt-sources config is given and no apt depencies are required to
be installed, the avoid unneeded calls to apt-update and apt-install.

The problem was introduced in 015543d.

cloudinit/config/cc_apt_configure.py

@@ -733,7 +733,8 @@ def _ensure_dependencies(cfg, aa_repo_match, cloud):
     for command in required_cmds:
         if not shutil.which(command):
             missing_packages.append(PACKAGE_DEPENDENCY_BY_COMMAND[command])
-    cloud.distro.install_packages(sorted(missing_packages))
+    if missing_packages:
+        cloud.distro.install_packages(sorted(missing_packages))
 
 
 def add_apt_key(ent, cloud, target=None, hardened=False, file_name=None):

tests/unittests/config/test_cc_apt_configure.py

@@ -6,12 +6,14 @@
 
 import pytest
 
+from cloudinit.config import cc_apt_configure
 from cloudinit.config.schema import (
     SchemaValidationError,
     get_schema,
     validate_cloudconfig_schema,
 )
 from tests.unittests.helpers import skipUnlessJsonSchema
+from tests.unittests.util import get_cloud
 
 
 class TestAPTConfigureSchema:
@@ -197,3 +199,70 @@ def test_schema_validation(self, config, error_msg):
                 error_msg = error_msg.replace("primary", "security")
                 with pytest.raises(SchemaValidationError, match=error_msg):
                     validate_cloudconfig_schema(config, schema, strict=True)
+
+
+class TestEnsureDependencies:
+    @pytest.mark.parametrize(
+        "cfg, already_installed, expected_install",
+        (
+            pytest.param({}, [], [], id="empty_cfg_no_pkg_installs"),
+            pytest.param(
+                {"sources": {"s1": {"keyid": "haveit"}}},
+                ["gpg"],
+                [],
+                id="cfg_needs_gpg_no_installs_when_gpg_present",
+            ),
+            pytest.param(
+                {"sources": {"s1": {"keyid": "haveit"}}},
+                [],
+                ["gnupg"],
+                id="cfg_needs_gpg_installs_gnupg_when_absent",
+            ),
+            pytest.param(
+                {"primary": [{"keyid": "haveit"}]},
+                [],
+                ["gnupg"],
+                id="cfg_primary_needs_gpg_installs_gnupg_when_absent",
+            ),
+            pytest.param(
+                {"security": [{"keyid": "haveit"}]},
+                [],
+                ["gnupg"],
+                id="cfg_security_needs_gpg_installs_gnupg_when_absent",
+            ),
+            pytest.param(
+                {"sources": {"s1": {"source": "ppa:yep"}}},
+                ["add-apt-repository"],
+                [],
+                id="cfg_needs_sw_prop_common_when_present",
+            ),
+            pytest.param(
+                {"sources": {"s1": {"source": "ppa:yep"}}},
+                [],
+                ["software-properties-common"],
+                id="cfg_needs_sw_prop_common_when_add_apt_repo_absent",
+            ),
+        ),
+    )
+    def test_only_install_needed_packages(
+        self, cfg, already_installed, expected_install, mocker
+    ):
+        """Only invoke install_packages when package installs are necessary"""
+        mycloud = get_cloud("debian")
+        install_packages = mocker.patch.object(
+            mycloud.distro, "install_packages"
+        )
+        matcher = re.compile(cc_apt_configure.ADD_APT_REPO_MATCH).search
+
+        def fake_which(cmd):
+            if cmd in already_installed:
+                return "foundit"
+            return None
+
+        which = mocker.patch.object(cc_apt_configure.shutil, "which")
+        which.side_effect = fake_which
+        cc_apt_configure._ensure_dependencies(cfg, matcher, mycloud)
+        if expected_install:
+            install_packages.assert_called_once_with(expected_install)
+        else:
+            install_packages.assert_not_called()