From 009a847273a48633e735c8934c6c994f0e9877f3 Mon Sep 17 00:00:00 2001
From: Lee Hinman <dakrone@users.noreply.github.com>
Date: Thu, 15 Feb 2024 11:23:19 -0700
Subject: [PATCH] Always show `composed_of` field for composable index
 templates (#105315)

* Always show `composed_of` field for composable index templates

Prior to e786cfa7061b427cf6185ad907069838dd679574 we inadvertently always added composable index
templates with `composed_of: []` beacuse
https://github.com/elastic/elasticsearch/commit/e786cfa7061b427cf6185ad907069838dd679574#diff-5081302eb39033199deb1977d544d1cd7867212a92b8d77e0aa0ded361272b11L618-L630
created a new `ComposableIndexTemplate` from an existing one, and the `.composedOf()` field returned
an empty list of no component templates were provided:

https://github.com/elastic/elasticsearch/blob/89e714ee5dc60db8b4979ab6372ff767e108e9da/server/src/main/java/org/elasticsearch/cluster/metadata/ComposableIndexTemplate.java#L172-L177

This meant that before 8.12.0 we would always show `composed_of: []` for composable index templates.
This commit recreates this behavior, and always displays the empty list even if no component
templates are used by a composable index template.

Resolves #104627
---
 docs/changelog/105315.yaml                          |  6 ++++++
 .../test/indices.get_index_template/10_basic.yml    | 13 +++++++++++++
 .../cluster/metadata/ComposableIndexTemplate.java   |  2 +-
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 docs/changelog/105315.yaml

diff --git a/docs/changelog/105315.yaml b/docs/changelog/105315.yaml
new file mode 100644
index 0000000000000..207e72467a689
--- /dev/null
+++ b/docs/changelog/105315.yaml
@@ -0,0 +1,6 @@
+pr: 105315
+summary: Always show `composed_of` field for composable index templates
+area: Indices APIs
+type: bug
+issues:
+ - 104627
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.get_index_template/10_basic.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.get_index_template/10_basic.yml
index 41e5506c412cd..fcf4a75af2227 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.get_index_template/10_basic.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.get_index_template/10_basic.yml
@@ -188,3 +188,16 @@ setup:
                   type: keyword
             lifecycle:
               data_retention: "30d"
+
+---
+"Get index template always shows composed_of":
+  - skip:
+      version: " - 8.12.99"
+      reason: "A bug was fixed in 8.13.0 to make `composed_of` always returned"
+
+  - do:
+      indices.get_index_template:
+        name: test
+
+  - match: {index_templates.0.name: test}
+  - match: {index_templates.0.index_template.composed_of: []}
diff --git a/server/src/main/java/org/elasticsearch/cluster/metadata/ComposableIndexTemplate.java b/server/src/main/java/org/elasticsearch/cluster/metadata/ComposableIndexTemplate.java
index dfd4431eb073c..7702ec0ac0b5c 100644
--- a/server/src/main/java/org/elasticsearch/cluster/metadata/ComposableIndexTemplate.java
+++ b/server/src/main/java/org/elasticsearch/cluster/metadata/ComposableIndexTemplate.java
@@ -125,7 +125,7 @@ private ComposableIndexTemplate(
     ) {
         this.indexPatterns = indexPatterns;
         this.template = template;
-        this.componentTemplates = componentTemplates;
+        this.componentTemplates = componentTemplates == null ? List.of() : componentTemplates;
         this.priority = priority;
         this.version = version;
         this.metadata = metadata;