SOLR-17589 First use of PUT/POST request with HttpJdkSolrClient generates error log entry on solr server due to initial HEAD request

solr/CHANGES.txt

@@ -185,6 +185,8 @@ Other Changes
 
 * SOLR-16503: Most remaining usages of Apache HttpClient in Solr switched to Jetty HttpClient (HTTP 2). (Sanjay Dutt, David Smiley)
 
+* SOLR-17589: Prevent error log entry on solr server due to initial HEAD request from HttpJdkSolrClient. (Paul Blanchaert via James Dyer)
+
 ==================  9.8.0 ==================
 New Features
 ---------------------

solr/solrj/src/java/org/apache/solr/client/solrj/impl/HttpJdkSolrClient.java

@@ -392,7 +392,8 @@ private synchronized boolean maybeTryHeadRequestSync(String url) {
     }
     HttpRequest.Builder headReqB =
         HttpRequest.newBuilder(uriNoQueryParams)
-            .method("HEAD", HttpRequest.BodyPublishers.noBody());
+            .method("HEAD", HttpRequest.BodyPublishers.noBody())
+            .header("Content-Type", ClientUtils.TEXT_JSON);
     decorateRequest(headReqB, new QueryRequest());
     try {
       httpClient.send(headReqB.build(), HttpResponse.BodyHandlers.discarding());

solr/solrj/src/test/org/apache/solr/client/solrj/impl/HttpJdkSolrClientTest.java

@@ -498,6 +498,21 @@ public void testPing() throws Exception {
     }
   }
 
+  @Test
+  public void testMaybeTryHeadRequestHasContentType() throws Exception {
+    DebugServlet.clear();
+    String url = getBaseUrl() + DEBUG_SERVLET_PATH;
+    try (HttpJdkSolrClient client = builder(url).build()) {
+      assertTrue(client.maybeTryHeadRequest(url));
+
+      // if https, the client won't attempt a HEAD request
+      if (client.headRequested) {
+        assertEquals("head", DebugServlet.lastMethod);
+        assertTrue(DebugServlet.headers.containsKey("content-type"));
+      }
+    }
+  }
+
   /**
    * This is not required for any test, but there appears to be a bug in the JDK client where it
    * does not release all threads if the client has not performed any queries, even after a forced