Add granular error list to alias action response (#106514)

When an alias action list is posted with must_exist==false, and succeeds only partially, a list of results for each action are now returned. The results contain information about the requested action, indices, and aliases. If must_exist==true, or all actions fail, the call will return a 400 status along with the associated exception.

docs/reference/alias.asciidoc

@@ -121,6 +121,77 @@ POST _aliases
 // TEST[s/^/PUT _data_stream\/logs-nginx.access-prod\nPUT _data_stream\/logs-my_app-default\n/]
 // end::alias-multiple-actions-example[]
 
+[discrete]
+[[multiple-action-results]]
+=== Multiple action results
+
+When using multiple actions, if some succeed and some fail, a list of per-action results will be returned.
+
+Consider a similar action list to the previous example, but now with an alias `log-non-existing`, which does not yet exist.
+In this case, the `remove` action will fail, but the `add` action will succeed.
+The response will contain the list `action_results`, with a result for every requested action.
+
+[source,console]
+----
+POST _aliases
+{
+  "actions": [
+    {
+      "remove": {
+        "index": "index1",
+        "alias": "logs-non-existing"
+      }
+    },
+    {
+      "add": {
+        "index": "index2",
+        "alias": "logs-non-existing"
+      }
+    }
+  ]
+}
+----
+// TEST[s/^/PUT \/index1\nPUT \/index2\n/]
+
+The API returns the following result:
+
+[source,console-result]
+--------------------------------------------------
+{
+  "acknowledged": true,
+  "errors": true,
+  "action_results": [
+    {
+      "action": {
+        "type": "remove",
+        "indices": [ "index1" ],
+        "aliases": [ "logs-non-existing" ],
+      },
+      "status": 404,
+      "error": {
+        "type": "aliases_not_found_exception",
+        "reason": "aliases [logs-non-existing] missing",
+        "resource.type": "aliases",
+        "resource.id": "logs-non-existing"
+      }
+    },
+    {
+      "action": {
+        "type": "add",
+        "indices": [ "index2" ],
+        "aliases": [ "logs-non-existing" ],
+      },
+      "status": 200
+    }
+  ]
+}
+--------------------------------------------------
+
+Allowing the action list to succeed partially may not provide the desired result.
+It may be more appropriate to set `must_exist` to `true`, which will cause the entire action
+list to fail if a single action fails.
+
+
 [discrete]
 [[add-alias-at-creation]]
 === Add an alias at index creation

docs/reference/indices/aliases.asciidoc

@@ -145,10 +145,16 @@ the alias points to one data stream.
 +
 Only the `add` action supports this parameter.
 
+// tag::alias-options[]
 `must_exist`::
 (Optional, Boolean)
-If `true`, the alias must exist to perform the action. Defaults to `false`. Only
-the `remove` action supports this parameter.
+Affects the behavior when attempting to remove an alias which does not exist.
+If `true`, removing an alias which does not exist will cause all actions to fail.
+If `false`, removing an alias which does not exist will only cause that removal to fail.
+Defaults to `false`.
+// end::alias-options[]
++
+Only the `remove` action supports this parameter.
 
 // tag::alias-options[]
 `routing`::
@@ -168,3 +174,51 @@ stream aliases don't support this parameter.
 Only the `add` action supports this parameter.
 =====
 ====
+
+
+
+[role="child_attributes"]
+[[indices-aliases-api-response-body]]
+==== {api-response-body-title}
+
+`acknowledged`::
+(Boolean)
+If `true`, the request received a response from the master node within the
+`timeout` period.
+
+`errors`::
+(Boolean)
+If `true`, at least one of the requested actions failed.
+
+`action_results`::
+(Optional, array of objects) Results for each requested action.
++
+.Properties of `action_results` objects
+[%collapsible%open]
+====
+
+`action`::
+(object)
+Description of the associated action request.
++
+.Properties of `action` object
+[%collapsible%open]
+=====
+`type`::
+(string) The type of the associated action, one of `add`, `remove`, or `remove_index`.
+
+`indices`::
+(array of strings) List of indices in the associated action.
+
+`aliases`::
+(array of strings) List of aliases in the associated action.
+=====
+
+`status`::
+(integer) HTTP status code returned for the action.
+
+`error`::
+(Optional, object) Contains additional information about the failed action.
++
+Only present if the action failed.
+====