Watcher: Slack message empty text (#31596)

Slack accepts an empty text or attachments, but not both. This commit
ensures that both are not empty when creating a watch.

Closes #30071

Replacing old pull request: #31288

Author: albendz

x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/slack/message/SlackMessage.java

@@ -12,6 +12,7 @@
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.xpack.watcher.common.text.TextTemplate;
 import org.elasticsearch.xpack.watcher.common.text.TextTemplateEngine;
+import org.elasticsearch.common.Nullable;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -29,7 +30,11 @@ public class SlackMessage implements MessageElement {
     final String text;
     final Attachment[] attachments;
 
-    public SlackMessage(String from, String[] to, String icon, String text, Attachment[] attachments) {
+    public SlackMessage(String from, String[] to, String icon, @Nullable String text, @Nullable Attachment[] attachments) {
+        if(text == null && attachments == null) {
+            throw new IllegalArgumentException("Both text and attachments cannot be null.");
+        }
+
         this.from = from;
         this.to = to;
         this.icon = icon;

x-pack/plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/notification/slack/message/SlackMessageTests.java

@@ -49,7 +49,7 @@ public void testToXContent() throws Exception {
         }
         String icon = randomBoolean() ? null : randomAlphaOfLength(10);
         String text = randomBoolean() ? null : randomAlphaOfLength(50);
-        Attachment[] attachments = randomBoolean() ? null : new Attachment[randomIntBetween(0, 2)];
+        Attachment[] attachments = (text != null && randomBoolean()) ? null : new Attachment[randomIntBetween(0, 2)];
         if (attachments != null) {
             for (int i = 0; i < attachments.length; i++) {
                 String fallback = randomBoolean() ? null : randomAlphaOfLength(10);
@@ -600,6 +600,22 @@ public void testUrlPathIsFiltered() throws Exception {
         }
     }
 
+    public void testCanHaveNullText()  throws Exception {
+        SlackMessage slackMessage = new SlackMessage("from", new String[] {"to"}, "icon", null, new Attachment[1]);
+        assertNull(slackMessage.getText());
+        assertNotNull(slackMessage.getAttachments());
+    }
+
+    public void testCanHaveNullAttachments()  throws Exception {
+        SlackMessage slackMessage = new SlackMessage("from", new String[] {"to"}, "icon", "text", null);
+        assertNotNull(slackMessage.getText());
+        assertNull(slackMessage.getAttachments());
+    }
+
+    public void testCannotHaveNullAttachmentsAndNullText() throws Exception {
+        expectThrows(IllegalArgumentException.class, () -> new SlackMessage("from", new String[]{"to"}, "icon", null, null));
+    }
+
     private static void writeFieldIfNotNull(XContentBuilder builder, String field, Object value) throws IOException {
         if (value != null) {
             builder.field(field, value);