delete system message via rest api

do a post to `<jenkins_url>/customizable-header/deleteSystemMessage?id=<id>`

Author: mawinter69

README.md

@@ -51,19 +51,24 @@ One or more system messages can be shown between the header and the breadcrumb b
 things related to the instance, e.g. a planned update of Jenkins, a downtime due to hardware replacement or an ongoing 
 incident. You can include html (sanitized with owasp) in the message to apply some simple styling or include a link
 with more details.<br/>
-System messages can have an expiration time to automatically remove them. They can be dismissed on a per-user basis.
+System messages can have an expiration time to automatically remove them. They can be dismissed on a per-user basis. 
 
-### Create System Message via REST api
+![System Message](/docs/pics/system-message.png)<br/>
+
+### Create/Delete System Messages via REST api
 You can create system messages by doing a post request to `<jenkins_url>/customizable-header/addSystemMessage`.
+To be able to later delete a system message, pass an id parameter in the call. In case you omit the id, an id will be generated and returned in the response body.
+
 Parameters:
 
-|Parameter|required| description                                                  |
-|---------|--------|--------------------------------------------------------------|
-| message | true   | The message, can contain html                                |
-| level | true   | Message level, one of `info`, `success`, `warning`, `danger` |
-| expireDate | false | Expiration date for the message, format: `yyyy-M-d H:m`  |
+| Parameter  |required| description                                                  |
+|------------|--------|--------------------------------------------------------------|
+| message    | true   | The message, can contain html                                |
+| level      | true   | Message level, one of `info`, `success`, `warning`, `danger` |
+| expireDate | false | Expiration date for the message, format: `yyyy-M-d H:m`      |
+| uid        | false | An optional unique id                                        |
 
-![System Message](/docs/pics/system-message.png)<br/>
+To delete a system message do a post request to `<jenkins_url>/customizable-header/deleteSystemMessage?id=<id>`
 
 ## The weather symbols
 To demonstrate the custom weather symbols download the svgs from `docs/svgs` to `userContent/svgs` in your

src/main/java/io/jenkins/plugins/customizable_header/CustomHeaderConfiguration.java

@@ -172,6 +172,13 @@ public void addSystemMessage(SystemMessage message) {
     save();
   }
 
+  public void deleteSystemMessage(String id) {
+    synchronized (systemMessages) {
+      systemMessages.removeIf(sm -> sm.getUid().equals(id));
+    }
+    save();
+  }
+
   public List<AppNavLink> getLinks() {
     return links;
   }

src/main/java/io/jenkins/plugins/customizable_header/HeaderRootAction.java

@@ -64,14 +64,14 @@ public boolean hasLinks() {
 
   @POST
   public HttpResponse doAddSystemMessage(@QueryParameter(fixEmpty = true) String message, @QueryParameter(fixEmpty = true) String level,
-                                @QueryParameter String expireDate) throws IOException {
+                                @QueryParameter String expireDate, @QueryParameter(fixEmpty = true) String id) throws IOException {
     Jenkins.get().checkPermission(Jenkins.ADMINISTER);
     if (message == null || level == null) {
       throw HttpResponses.error(HttpServletResponse.SC_BAD_REQUEST, "Missing parameters: message and level are mandatory");
     }
     try {
       SystemMessage.SystemMessageColor lvl = SystemMessage.SystemMessageColor.valueOf(level);
-      SystemMessage msg = new SystemMessage(message, lvl, null);
+      SystemMessage msg = new SystemMessage(message, lvl, id);
       msg.setExpireDate(expireDate);
       CustomHeaderConfiguration config = CustomHeaderConfiguration.get();
       config.addSystemMessage(msg);
@@ -83,6 +83,17 @@ public HttpResponse doAddSystemMessage(@QueryParameter(fixEmpty = true) String m
     }
   }
 
+  @POST
+  public void doDeleteSystemMessage(@QueryParameter(fixEmpty = true) String id) throws IOException {
+    Jenkins.get().checkPermission(Jenkins.ADMINISTER);
+    if (id == null) {
+      throw HttpResponses.error(HttpServletResponse.SC_BAD_REQUEST, "Missing parameters: id is mandatory");
+    }
+    CustomHeaderConfiguration config = CustomHeaderConfiguration.get();
+    config.deleteSystemMessage(id);
+  }
+
+
   @POST
   public void doDismissMessage(@QueryParameter String uid) throws IOException {
     User user = User.current();