Fix race condition during initial admin account creation (#26036)

* Update README.md

* Update README.md

* Update README.md

* Remove link to non-existent newsletter

Also format with `yarn lint:fix` as noted in the contributing guide.

* Update README.md

* Update good first issues query

* Fix race condition during initial admin account creation

* Update core/src/main/java/hudson/security/HudsonPrivateSecurityRealm.java

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Use synchronized method for initial admin account creation

---------

Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>
Co-authored-by: Kris Stern <krisstern@outlook.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 4 people

core/src/main/java/hudson/security/HudsonPrivateSecurityRealm.java

@@ -363,11 +363,14 @@ private String getErrorMessages(SignupInfo si) {
      * This can be run by anyone, but only to create the very first user account.
      */
     @RequirePOST
-    public void doCreateFirstAccount(StaplerRequest2 req, StaplerResponse2 rsp) throws IOException, ServletException {
+    public synchronized void doCreateFirstAccount(StaplerRequest2 req, StaplerResponse2 rsp)
+            throws IOException, ServletException {
+
         if (hasSomeUser()) {
             rsp.sendError(SC_UNAUTHORIZED, "First user was already created");
             return;
         }
+
         User u = createAccount(req, rsp, false, "firstUser.jelly");
         if (u != null) {
             tryToMakeAdmin(u);