feat: Add Role::fixed_replica_count function

Author: sbernauer

crates/stackable-operator/CHANGELOG.md

@@ -8,8 +8,10 @@ All notable changes to this project will be documented in this file.
 
 - Support the annotation `secrets.stackable.tech/backend.autotls.cert.domain-components-in-subject-dn`
   in the `SecretOperatorVolumeSourceBuilder` ([#1209]).
+- Add `Role::fixed_replica_count` function that returns (optionally) the number of fixed replicas ([#XXXX]).
 
 [#1209]: https://github.com/stackabletech/operator-rs/pull/1209
+[#XXXX]: https://github.com/stackabletech/operator-rs/pull/XXXX
 
 ## [0.113.0] - 2026-06-22
 

crates/stackable-operator/src/role_utils.rs

@@ -403,6 +403,39 @@ where
     }
 }
 
+impl<Config, ConfigOverrides, RoleConfig, CommonConfig>
+    Role<Config, ConfigOverrides, RoleConfig, CommonConfig>
+where
+    RoleConfig: Default + JsonSchema + Serialize,
+    CommonConfig: Default + JsonSchema + Serialize,
+    ConfigOverrides: Default + JsonSchema + Serialize,
+{
+    /// Returns [`Some<u32>`] in case the number of nodes is hard-coded to a certain value.
+    ///
+    /// This is the case when all `replicas` are set to [`Some<u16>`], in which case they are simply
+    /// summed.
+    ///
+    /// The argument `treat_zero_as_none` is a safety mechanism, which allows the caller to decide
+    /// if an explicit replica count of `0` should be treated as [`None`]. It also means that
+    /// [`None`] is returned in case no roleGroups are configured at all.
+    pub fn fixed_replica_count(&self, treat_zero_as_none: bool) -> Option<u32> {
+        // An empty role has no fixed replica count when zeros are treated as None.
+        if treat_zero_as_none && self.role_groups.is_empty() {
+            return None;
+        }
+
+        self.role_groups
+            .values()
+            .map(|rg| match rg.replicas {
+                None => None,
+                Some(0) if treat_zero_as_none => None,
+                // The individual replicas are [`u16`]s, so a [`u32`] sum has plenty of space.
+                Some(replicas) => Some(u32::from(replicas)),
+            })
+            .sum()
+    }
+}
+
 impl<Config, ConfigOverrides, RoleConfig>
     Role<Config, ConfigOverrides, RoleConfig, JavaCommonConfig>
 where
@@ -654,4 +687,69 @@ mod tests {
             ]
         );
     }
+
+    #[test]
+    fn fixed_replica_count_sums_all_set_replicas() {
+        let role = construct_role_with_replicas([Some(3), Some(2), Some(5)]);
+
+        assert_eq!(role.fixed_replica_count(false), Some(10));
+        assert_eq!(role.fixed_replica_count(true), Some(10));
+    }
+
+    #[test]
+    fn fixed_replica_count_is_none_if_any_replica_is_unset() {
+        let role = construct_role_with_replicas([Some(3), None, Some(2)]);
+
+        assert_eq!(role.fixed_replica_count(false), None);
+        assert_eq!(role.fixed_replica_count(true), None);
+    }
+
+    #[test]
+    fn fixed_replica_count_treats_zero_according_to_flag() {
+        let role = construct_role_with_replicas([Some(3), Some(0)]);
+
+        assert_eq!(role.fixed_replica_count(false), Some(3));
+        // With treat_zero_as_none the zero turns the whole count into None.
+        assert_eq!(role.fixed_replica_count(true), None);
+    }
+
+    #[test]
+    fn fixed_replica_count_of_single_zero_role_group() {
+        let role = construct_role_with_replicas([Some(0)]);
+
+        assert_eq!(role.fixed_replica_count(false), Some(0));
+        assert_eq!(role.fixed_replica_count(true), None);
+    }
+
+    #[test]
+    fn fixed_replica_count_of_role_without_role_groups_is_zero() {
+        let role = construct_role_with_replicas(vec![]);
+
+        assert_eq!(role.fixed_replica_count(false), Some(0));
+        assert_eq!(role.fixed_replica_count(true), None);
+    }
+
+    /// Builds a [`Role`] with one role group per passed `replicas` entry, so tests only need to
+    /// care about the replica counts that [`Role::fixed_replica_count`] operates on.
+    fn construct_role_with_replicas(
+        replicas: impl IntoIterator<Item = Option<u16>>,
+    ) -> Role<(), EmptyConfigOverrides, GenericRoleConfig, GenericCommonConfig> {
+        Role {
+            config: CommonConfiguration::default(),
+            role_config: GenericRoleConfig::default(),
+            role_groups: replicas
+                .into_iter()
+                .enumerate()
+                .map(|(index, replicas)| {
+                    (
+                        format!("role-group-{index}"),
+                        RoleGroup {
+                            config: CommonConfiguration::default(),
+                            replicas,
+                        },
+                    )
+                })
+                .collect(),
+        }
+    }
 }