return 503 when instance is wayyyyy gone

nexus/src/app/instance.rs

@@ -125,8 +125,14 @@ impl From<SledAgentInstanceError> for dropshot::HttpError {
                 HttpError::for_internal_error(s)
             }
             // Error responses from sled-agent that indicate the instance is
-            // unhealthy should be mapped to a 500 error.
-            e if e.vmm_gone() => HttpError::for_internal_error(e.to_string()),
+            // unhealthy should be mapped to a 503 error.
+            e if e.vmm_gone() => {
+                let mut error = HttpError::for_unavail(None, e.to_string());
+                error.external_message = "The instance was running but is no \
+                    longer reachable. It is being moved to the Failed state."
+                    .to_string();
+                error
+            }
             // Other client errors can be handled by the normal
             // `external::Error` to `HttpError` conversions.
             SledAgentInstanceError(e) => HttpError::from(Error::from(e)),

nexus/tests/integration_tests/instances.rs

@@ -1118,11 +1118,11 @@ async fn test_instance_failed_after_sled_agent_forgets_vmm_can_be_restarted(
     let instance_id = make_forgotten_instance(&cptestctx, instance_name).await;
 
     // Attempting to reboot the forgotten instance will result in a 404
-    // NO_SUCH_INSTANCE from the sled-agent, which Nexus turns into a 500.
+    // NO_SUCH_INSTANCE from the sled-agent, which Nexus turns into a 503.
     expect_instance_reboot_fail(
         client,
         instance_name,
-        http::StatusCode::INTERNAL_SERVER_ERROR,
+        http::StatusCode::SERVICE_UNAVAILABLE,
     )
     .await;
 
@@ -1146,11 +1146,11 @@ async fn test_instance_failed_after_sled_agent_forgets_vmm_can_be_deleted(
     let instance_id = make_forgotten_instance(&cptestctx, instance_name).await;
 
     // Attempting to reboot the forgotten instance will result in a 404
-    // NO_SUCH_INSTANCE from the sled-agent, which Nexus turns into a 500.
+    // NO_SUCH_INSTANCE from the sled-agent, which Nexus turns into a 503.
     expect_instance_reboot_fail(
         client,
         instance_name,
-        http::StatusCode::INTERNAL_SERVER_ERROR,
+        http::StatusCode::SERVICE_UNAVAILABLE,
     )
     .await;