fix(sns): Improve validate_new_target_version error messages (#2868)

The previous error messages could be misleading since they talked about
there being "no upgrade steps remaining" (when really it is possible
that the cache is just out of date), or overly expose implementation
details. This PR enhances them slightly to be more informative.

rs/sns/governance/src/governance.rs

@@ -477,16 +477,19 @@ impl GovernanceProto {
     {
         let deployed_version = self.deployed_version_or_err()?;
 
-        let upgrade_steps = {
-            let cached_upgrade_steps = self.cached_upgrade_steps_or_err()?;
-            cached_upgrade_steps.take_from(&deployed_version)?
-        };
+        let cached_upgrade_steps = self.cached_upgrade_steps_or_err()?;
 
-        if upgrade_steps.is_empty() {
-            return Err(
-                "Cannot advance SNS target version: there are no pending upgrades.".to_string(),
-            );
-        }
+        let upgrade_steps = cached_upgrade_steps.take_from(&deployed_version);
+        let upgrade_steps = match upgrade_steps {
+            Ok(upgrade_steps) if !upgrade_steps.is_empty() => upgrade_steps,
+            _ => {
+                return Err(format!(
+                    "The currently deployed SNS version is not in the cached_upgrade_steps. You may need to wait for the upgrade steps to be refreshed. \
+                    This shouldn't take more than {} seconds.", 
+                    UPGRADE_STEPS_INTERVAL_REFRESH_BACKOFF_SECONDS
+                ));
+            }
+        };
 
         let new_target = if let Some(new_target) = new_target {
             let new_target = Version::try_from(new_target).map_err(|err| err.to_string())?;

rs/sns/governance/src/proposal/advance_sns_target_version.rs

@@ -239,7 +239,68 @@ fn test_no_pending_upgrades() {
         // Inspect the observed results.
         assert_eq!(
             err,
-            "Cannot advance SNS target version: there are no pending upgrades."
+            "The currently deployed SNS version is not in the cached_upgrade_steps. You may need to wait for the upgrade steps to be refreshed. \
+            This shouldn't take more than 3600 seconds."
+        );
+    }
+}
+
+#[test]
+fn test_deployed_version_not_in_cached_upgrade_steps() {
+    // Prepare the world.
+    let pre_deployed_version = sns_version_for_tests();
+    let deployed_version = Version {
+        root_wasm_hash: vec![
+            67, 28, 179, 51, 254, 179, 247, 98, 247, 66, 176, 222, 165, 135, 69, 99, 58, 42, 44,
+            164, 16, 117, 233, 147, 49, 131, 216, 80, 180, 221, 178, 89,
+        ],
+        ..pre_deployed_version.clone()
+    };
+    let non_existent_version = Version {
+        root_wasm_hash: vec![
+            1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+            25, 26, 27, 28, 29, 30,
+        ],
+        ..deployed_version.clone()
+    };
+
+    // Smoke check: Make sure all versions are different
+    let versions = vec![pre_deployed_version.clone(), non_existent_version.clone()];
+    assert!(
+        versions.iter().collect::<HashSet::<_>>().len() == versions.len(),
+        "Duplicates!"
+    );
+
+    let mut governance_proto = standard_governance_proto_for_tests(Some(deployed_version.clone()));
+    governance_proto.cached_upgrade_steps = Some(CachedUpgradeStepsPb {
+        upgrade_steps: Some(Versions { versions }),
+        ..Default::default()
+    });
+    let env = NativeEnvironment::new(Some(canister_test_id(501)));
+
+    // Run code under test.
+    for action in [
+        Action::AdvanceSnsTargetVersion(AdvanceSnsTargetVersion { new_target: None }),
+        Action::AdvanceSnsTargetVersion(AdvanceSnsTargetVersion {
+            new_target: Some(SnsVersion::from(deployed_version)),
+        }),
+        Action::AdvanceSnsTargetVersion(AdvanceSnsTargetVersion {
+            new_target: Some(SnsVersion::from(pre_deployed_version)),
+        }),
+        Action::AdvanceSnsTargetVersion(AdvanceSnsTargetVersion {
+            new_target: Some(SnsVersion::from(non_existent_version)),
+        }),
+    ] {
+        let err = validate_and_render_action(&Some(action), &env, &governance_proto, vec![])
+            .now_or_never()
+            .unwrap()
+            .unwrap_err();
+
+        // Inspect the observed results.
+        assert_eq!(
+            err,
+            "The currently deployed SNS version is not in the cached_upgrade_steps. You may need to wait for the upgrade steps to be refreshed. \
+            This shouldn't take more than 3600 seconds."
         );
     }
 }