Accept null device code interval

Section 3.2 of RFC 8628 states that the Device Authorization Response's
`interval` field is optional. Previously, this crate accepted responses
without an `interval` field, but rejected responses in which the
`interval` field was `null`. This change treats `null` values as if they
were omitted.

Fixes #278.

src/devicecode.rs

@@ -475,6 +475,37 @@ fn default_devicecode_interval() -> u64 {
     5
 }
 
+fn deserialize_devicecode_interval<'de, D>(deserializer: D) -> Result<u64, D::Error>
+where
+    D: serde::de::Deserializer<'de>,
+{
+    struct NumOrNull;
+
+    impl<'de> serde::de::Visitor<'de> for NumOrNull {
+        type Value = u64;
+
+        fn expecting(&self, formatter: &mut Formatter) -> std::fmt::Result {
+            formatter.write_str("non-negative integer or null")
+        }
+
+        fn visit_u64<E>(self, v: u64) -> Result<Self::Value, E>
+        where
+            E: Error,
+        {
+            Ok(v)
+        }
+
+        fn visit_unit<E>(self) -> Result<Self::Value, E>
+        where
+            E: serde::de::Error,
+        {
+            Ok(default_devicecode_interval())
+        }
+    }
+
+    deserializer.deserialize_any(NumOrNull)
+}
+
 /// Trait for adding extra fields to the `DeviceAuthorizationResponse`.
 pub trait ExtraDeviceAuthorizationFields: DeserializeOwned + Debug + Serialize {}
 
@@ -516,7 +547,10 @@ where
     /// The minimum amount of time in seconds that the client SHOULD wait
     /// between polling requests to the token endpoint.  If no value is
     /// provided, clients MUST use 5 as the default.
-    #[serde(default = "default_devicecode_interval")]
+    #[serde(
+        default = "default_devicecode_interval",
+        deserialize_with = "deserialize_devicecode_interval"
+    )]
     interval: u64,
 
     #[serde(bound = "EF: ExtraDeviceAuthorizationFields", flatten)]
@@ -674,10 +708,12 @@ where
 #[cfg(test)]
 mod tests {
     use crate::basic::BasicTokenType;
+    use crate::devicecode::default_devicecode_interval;
     use crate::tests::{mock_http_client, mock_http_client_success_fail, new_client};
     use crate::{
-        DeviceAuthorizationUrl, DeviceCodeErrorResponse, DeviceCodeErrorResponseType,
-        RequestTokenError, Scope, StandardDeviceAuthorizationResponse, TokenResponse,
+        DeviceAuthorizationResponse, DeviceAuthorizationUrl, DeviceCodeErrorResponse,
+        DeviceCodeErrorResponseType, EmptyExtraDeviceAuthorizationFields, RequestTokenError, Scope,
+        StandardDeviceAuthorizationResponse, TokenResponse,
     };
 
     use chrono::{DateTime, Utc};
@@ -1120,4 +1156,54 @@ mod tests {
             _ => unreachable!("Error should be ExpiredToken"),
         }
     }
+
+    #[test]
+    fn test_device_auth_response_default_interval() {
+        let response: DeviceAuthorizationResponse<EmptyExtraDeviceAuthorizationFields> =
+            serde_json::from_str(
+                r#"{
+                    "device_code": "12345",
+                    "verification_uri": "https://verify/here",
+                    "user_code": "abcde",
+                    "expires_in": 300
+                }"#,
+            )
+            .unwrap();
+
+        assert_eq!(response.interval, default_devicecode_interval());
+    }
+
+    #[test]
+    fn test_device_auth_response_non_default_interval() {
+        let response: DeviceAuthorizationResponse<EmptyExtraDeviceAuthorizationFields> =
+            serde_json::from_str(
+                r#"{
+                    "device_code": "12345",
+                    "verification_uri": "https://verify/here",
+                    "user_code": "abcde",
+                    "expires_in": 300,
+                    "interval": 10
+                }"#,
+            )
+            .unwrap();
+
+        assert_eq!(response.interval, 10);
+    }
+
+    #[test]
+    fn test_device_auth_response_null_interval() {
+        let response: DeviceAuthorizationResponse<EmptyExtraDeviceAuthorizationFields> =
+            serde_json::from_str(
+                r#"{
+                    "device_code": "12345",
+                    "verification_uri": "https://verify/here",
+                    "user_code": "abcde",
+                    "expires_in": 300,
+                    "interval": null
+                }"#,
+            )
+            .unwrap();
+
+        assert_eq!(response.interval, default_devicecode_interval());
+    }
 }