Adding a test for data stream write indices

Author: masseyke

qa/smoke-test-ingest-with-all-dependencies/src/yamlRestTest/resources/rest-api-spec/test/ingest/80_ingest_simulate.yml

@@ -1800,3 +1800,89 @@ setup:
   - match: { docs.0.doc._source.num: true }
   - match: { docs.0.doc._source.is_valid: true }
   - match: { docs.0.doc.error.type: "document_parsing_exception" }
+
+---
+"Test ingest simulate with mapping addition for data streams when write index has different mapping":
+  # In this test, we make sure that when the index template is a data stream template, simulate ingest works the same whether the data
+  # stream has been created or not -- either way, we expect it to use the template rather than the data stream / index mappings and settings.
+
+  - skip:
+      features:
+        - headers
+        - allowed_warnings
+
+  - do:
+      cluster.put_component_template:
+        name: mappings_template
+        body:
+          template:
+            mappings:
+              dynamic: strict
+              properties:
+                foo:
+                  type: boolean
+                bar:
+                  type: boolean
+
+  - do:
+      allowed_warnings:
+        - "index template [my-template-1] has index patterns [simple-data-stream1] matching patterns from existing older templates [global] with patterns (global => [*]); this template [my-template-1] will take precedence during new index creation"
+      indices.put_index_template:
+        name: my-template-1
+        body:
+          index_patterns: [simple-data-stream1]
+          composed_of:
+            - mappings_template
+          data_stream: {}
+
+  - do:
+      indices.create_data_stream:
+        name: simple-data-stream1
+  - is_true: acknowledged
+
+  - do:
+      cluster.health:
+        wait_for_status: yellow
+
+  # Now that the data stream exists, we change the template to remove the mapping for bar. The write index still has the
+  # old mapping.
+  - do:
+      cluster.put_component_template:
+        name: mappings_template
+        body:
+          template:
+            mappings:
+              properties:
+                foo:
+                  type: boolean
+
+  # We expect the mapping_addition to be added to the mapping of the write index, which has a boolean bar field. So this
+  # simulate ingest ought to fail.
+  - do:
+      headers:
+        Content-Type: application/json
+      simulate.ingest:
+        index: simple-data-stream1
+        body: >
+          {
+            "docs": [
+              {
+                "_id": "asdf",
+                "_source": {
+                  "@timestamp": 1234,
+                  "bar": "baz"
+                }
+              }
+            ],
+            "mapping_addition": {
+              "properties": {
+                "baz": {
+                  "type": "keyword"
+                }
+              }
+            }
+          }
+  - length: { docs: 1 }
+  - match: { docs.0.doc._index: "simple-data-stream1" }
+  - match: { docs.0.doc._source.bar: "baz" }
+  - match: { docs.0.doc.error.type: "document_parsing_exception" }