[7.x] Allow simulating existing composable index template (#59733) (#59798)

Backports the following commits to 7.x:

    Allow simulating existing composable index template (#59733)

rest-api-spec/src/main/resources/rest-api-spec/test/indices.simulate_template/10_basic.yml

@@ -1,8 +1,8 @@
 ---
 "Simulate template without a template in the body":
   - skip:
-      version: " - 7.99.99"
-      reason: "not yet backported"
+      version: " - 7.8.99"
+      reason: "only available in 7.9+"
       features: ["default_shards"]
 
   - do:
@@ -31,8 +31,8 @@
 ---
 "Simulate index template specifying a new template":
   - skip:
-      version: " - 7.99.99"
-      reason: "not yet backported"
+      version: " - 7.8.99"
+      reason: "only available in 7.9+"
       features: ["default_shards"]
 
   - do:
@@ -85,8 +85,8 @@
 ---
 "Simulate template matches overlapping legacy and composable templates":
   - skip:
-      version: " - 7.99.99"
-      reason: "not yet backported"
+      version: " - 7.8.99"
+      reason: "only available in 7.9+"
       features: ["allowed_warnings", "default_shards"]
 
   - do:
@@ -144,3 +144,57 @@
   - match: {overlapping.0.index_patterns: ["t*", "t1*"]}
   - match: {overlapping.1.name: v2_template}
   - match: {overlapping.1.index_patterns: ["te*"]}
+
+---
+"Simulate replacing a template with a newer version":
+  - skip:
+      version: " - 7.99.99"
+      reason: "not yet backported"
+      features: ["allowed_warnings", "default_shards"]
+
+  - do:
+      indices.put_index_template:
+        name: v2_template
+        body:
+          index_patterns: te*
+          priority: 10
+          template:
+            settings:
+              number_of_shards:   10
+              number_of_replicas: 2
+            mappings:
+              properties:
+                field:
+                  type: text
+
+  - do:
+      cluster.put_component_template:
+        name: ct
+        body:
+          template:
+            settings:
+              index.number_of_replicas: 3
+            mappings:
+              properties:
+                ct_field:
+                  type: keyword
+
+  - do:
+      indices.simulate_template:
+        name: v2_template
+        body:
+          index_patterns: te*
+          priority: 10
+          template:
+            settings:
+              index.blocks.write: true
+            aliases:
+              test_alias: {}
+          composed_of: ["ct"]
+
+  - match: {template.settings.index.blocks.write: "true"}
+  - match: {template.settings.index.number_of_replicas: "3"}
+  - match: {template.mappings.properties.ct_field.type: "keyword"}
+  - length: {template.aliases: 1}
+  - is_true: template.aliases.test_alias
+  - length: {overlapping: 0}

server/src/main/java/org/elasticsearch/action/admin/indices/template/post/TransportSimulateTemplateAction.java

@@ -94,8 +94,10 @@ public class TransportSimulateTemplateAction
         // First, if a template body was requested, we need to "fake add" that template to the
         // cluster state, so it can be used when we resolved settings/etc
         if (request.getIndexTemplateRequest() != null) {
-            // we'll "locally" add the template defined by the user in the cluster state (as if it existed in the system)
-            simulateTemplateToAdd = "simulate_template_" + uuid;
+            // we'll "locally" add the template defined by the user in the cluster state (as if it
+            // existed in the system), either with a temporary name, or with the given name if
+            // specified, to simulate replacing the existing template
+            simulateTemplateToAdd = request.getTemplateName() == null ? "simulate_template_" + uuid : request.getTemplateName();
             // Perform validation for things like typos in component template names
             MetadataIndexTemplateService.validateV2TemplateRequest(state.metadata(), simulateTemplateToAdd,
                 request.getIndexTemplateRequest().indexTemplate());