dns: Fix purging DNS config

When user desires:

```yml
---
dns-resolver:
  config:
    search: []
```

It means user want to remove all search but preserve servers and
options, current nmstate incorrectly treat this as purge also.

This patch only treat these two as purge.

```yml
dns-resolver:
  config: {}
```

and

```yml
dns-resolver:
  config:
    server: []
    search: []
    options: []
```

Unit and integration test case included.

Signed-off-by: Gris Ge <fge@redhat.com>

rust/src/lib/dns.rs

@@ -54,6 +54,12 @@ const SUPPORTED_DNS_OPTS_WITH_VALUE: [&str; 3] =
 ///      - trust-ad
 ///      - rotate
 /// ```
+/// To purge all static DNS configuration:
+/// ```yml
+/// ---
+/// dns-resolver:
+///   config: {}
+/// ```
 pub struct DnsState {
     #[serde(skip_serializing_if = "Option::is_none")]
     /// The running effective state. The DNS server might be from DHCP(IPv6
@@ -120,11 +126,15 @@ impl DnsClientState {
         self.server.is_none() && self.search.is_none() && self.options.is_none()
     }
 
-    // Whether user want to purge all DNS settings
+    // Any of these conditions means purge full DNS config:
+    //  * `server`, `search` and `options` are None. Equal to desire state
+    //    `config: {}`
+    //  * `server`, `search` and `options` are `Some<Vec::new()>`.
     pub(crate) fn is_purge(&self) -> bool {
-        self.server.as_deref().unwrap_or_default().is_empty()
-            && self.search.as_deref().unwrap_or_default().is_empty()
-            && self.options.as_deref().unwrap_or_default().is_empty()
+        self.server.is_none() && self.search.is_none() & self.options.is_none()
+            || self.server.as_deref() == Some(&[])
+                && self.search.as_deref() == Some(&[])
+                && self.options.as_deref() == Some(&[])
     }
 
     pub(crate) fn is_null(&self) -> bool {

rust/src/lib/unit_tests/dns.rs

@@ -83,3 +83,40 @@ fn test_invalid_dns_option_with_value() {
         assert_eq!(e.kind(), ErrorKind::InvalidArgument);
     }
 }
+
+#[test]
+fn test_is_purge_dns_empty_dict() {
+    let desired: DnsState = serde_yaml::from_str(
+        r"---
+        config: {}
+        ",
+    )
+    .unwrap();
+    assert!(desired.config.unwrap().is_purge());
+}
+
+#[test]
+fn test_is_purge_dns_full_empty_dict() {
+    let desired: DnsState = serde_yaml::from_str(
+        r"---
+        config:
+          server: []
+          search: []
+          options: []
+        ",
+    )
+    .unwrap();
+    assert!(desired.config.unwrap().is_purge());
+}
+
+#[test]
+fn test_not_purge() {
+    let desired: DnsState = serde_yaml::from_str(
+        r"---
+        config:
+          search: []
+        ",
+    )
+    .unwrap();
+    assert!(!desired.config.unwrap().is_purge());
+}

tests/integration/dns_test.py

@@ -147,9 +147,9 @@ def test_dns_edit_3_more_nameservers(dns_servers):
 @pytest.mark.tier1
 @pytest.mark.parametrize(
     "empty_dns_config",
-    [{DNS.SERVER: [], DNS.SEARCH: []}, {}],
+    [{DNS.SERVER: [], DNS.SEARCH: [], DNS.OPTIONS: []}, {}],
     ids=[
-        "empty_server_and_search",
+        "empty_server_search_and_opt",
         "empty_dict",
     ],
 )
@@ -573,3 +573,47 @@ def test_invalid_dns_option_with_value(static_dns):
     )
     with pytest.raises(NmstateValueError):
         libnmstate.apply(desired_state)
+
+
+@pytest.fixture
+def static_dns_with_options(static_dns):
+    desired_state = {
+        DNS.KEY: {
+            DNS.CONFIG: {
+                DNS.OPTIONS: ["debug", "rotate"],
+            }
+        },
+    }
+    libnmstate.apply(desired_state)
+
+
+def test_remove_all_dns_options(static_dns_with_options):
+    pre_apply_state = libnmstate.show()
+
+    desired_state = {
+        DNS.KEY: {
+            DNS.CONFIG: {
+                DNS.OPTIONS: [],
+            }
+        },
+    }
+    libnmstate.apply(desired_state)
+
+    current_state = libnmstate.show()
+    assert DNS.OPTIONS not in current_state[DNS.KEY][DNS.CONFIG]
+    assert (
+        current_state[DNS.KEY][DNS.CONFIG][DNS.SERVER]
+        == pre_apply_state[DNS.KEY][DNS.CONFIG][DNS.SERVER]
+    )
+    assert (
+        current_state[DNS.KEY][DNS.CONFIG][DNS.SEARCH]
+        == pre_apply_state[DNS.KEY][DNS.CONFIG][DNS.SEARCH]
+    )
+
+
+def test_purge_dns_full_config(static_dns_with_options):
+    desired_state = {DNS.KEY: {DNS.CONFIG: {}}}
+    libnmstate.apply(desired_state)
+
+    current_state = libnmstate.show()
+    assert not current_state[DNS.KEY][DNS.CONFIG]

tests/integration/nm/dns_test.py

@@ -171,11 +171,7 @@ def auto_eth1(eth1_up):
         }
     )
     yield
-    libnmstate.apply(
-        {
-            DNS.KEY: {DNS.CONFIG: {DNS.SERVER: [], DNS.SEARCH: []}},
-        }
-    )
+    libnmstate.apply({DNS.KEY: {DNS.CONFIG: {}}})
 
 
 def test_static_dns_search_with_auto_dns(auto_eth1):