SOLR-7177: include the target URL when a ConcurrentUpdateSolrClient update fails to connect

changelog/unreleased/SOLR-7177-cusc-connection-error-url.yml

@@ -0,0 +1,11 @@
+# See https://github.com/apache/solr/blob/main/dev-docs/changelog.adoc
+
+title: >
+  ConcurrentUpdateSolrClient now identifies the target server when an update fails to connect,
+  instead of reporting a bare connection error with no URL.
+type: fixed
+authors:
+  - name: Serhiy Bzhezytskyy
+links:
+  - name: SOLR-7177
+    url: https://issues.apache.org/jira/browse/SOLR-7177

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

@@ -340,8 +340,14 @@ void sendUpdateStream() throws Exception {
 
           } catch (OutOfMemoryError | InterruptedException e) {
             throw e;
-          } catch (Throwable e) {
+          } catch (RemoteSolrException e) {
             handleError(e, docIds, collection);
+          } catch (Throwable e) {
+            handleError(
+                new SolrServerException(
+                    "Error from server at " + basePath + ": " + e.getMessage(), e),
+                docIds,
+                collection);
           } finally {
             try {
               consumeFully(rspBody);

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

@@ -399,6 +399,90 @@ public void onError(Throwable ex) {
         "the per-id callback should not fire for a non-document error", 0, richCalls.get());
   }
 
+  /**
+   * A connection failure surfaces the target URL so the failing server is identifiable, rather than
+   * a bare socket error with no context.
+   */
+  @Test
+  public void testConnectionErrorIncludesUrl() throws Exception {
+    String unreachable = "http://127.0.0.1:1/solr"; // nothing listening -> connect failure
+    List<Throwable> errors = new CopyOnWriteArrayList<>();
+    ConcurrentUpdateBaseSolrClient.UpdateErrorHandler handler =
+        new ConcurrentUpdateBaseSolrClient.UpdateErrorHandler() {
+          @Override
+          public void onError(Throwable ex, List<String> ids, String collection) {
+            errors.add(ex);
+          }
+
+          @Override
+          public void onError(Throwable ex) {
+            errors.add(ex);
+          }
+        };
+
+    try (var httpClient = solrClient(null);
+        var concurrentClient =
+            errorHandlerConcurrentClient(unreachable, 10, 2, httpClient, handler)) {
+      SolrInputDocument doc = new SolrInputDocument();
+      doc.addField("id", "doc-1");
+      concurrentClient.add("collection1", doc);
+      concurrentClient.blockUntilFinished();
+    }
+
+    assertFalse("expected a connection error to be reported", errors.isEmpty());
+    boolean anyMentionsUrl =
+        errors.stream()
+            .anyMatch(
+                e -> {
+                  for (Throwable t = e; t != null; t = t.getCause()) {
+                    if (t.getMessage() != null && t.getMessage().contains("127.0.0.1:1")) {
+                      return true;
+                    }
+                  }
+                  return false;
+                });
+    assertTrue(
+        "connection error should identify the target server; got: " + errors, anyMentionsUrl);
+  }
+
+  /**
+   * A connection error is reported as a SolrServerException that preserves the original cause, so
+   * SolrCmdDistributor's checkRetry can unwrap it via getRootCause to decide whether to retry.
+   */
+  @Test
+  public void testConnectionErrorIsSolrServerExceptionPreservingCause() throws Exception {
+    String unreachable = "http://127.0.0.1:1/solr";
+    List<Throwable> errors = new CopyOnWriteArrayList<>();
+    ConcurrentUpdateBaseSolrClient.UpdateErrorHandler handler =
+        new ConcurrentUpdateBaseSolrClient.UpdateErrorHandler() {
+          @Override
+          public void onError(Throwable ex, List<String> ids, String collection) {
+            errors.add(ex);
+          }
+
+          @Override
+          public void onError(Throwable ex) {
+            errors.add(ex);
+          }
+        };
+
+    try (var httpClient = solrClient(null);
+        var concurrentClient =
+            errorHandlerConcurrentClient(unreachable, 10, 2, httpClient, handler)) {
+      SolrInputDocument doc = new SolrInputDocument();
+      doc.addField("id", "doc-1");
+      concurrentClient.add("collection1", doc);
+      concurrentClient.blockUntilFinished();
+    }
+
+    assertFalse("expected a connection error to be reported", errors.isEmpty());
+    Throwable reported = errors.get(0);
+    assertTrue(
+        "connection error should be wrapped as SolrServerException; got: " + reported,
+        reported instanceof SolrServerException);
+    assertNotNull("the original cause must be preserved", reported.getCause());
+  }
+
   /**
    * A naive lambda handler only registers for per-id failures; an error not tied to identifiable
    * documents is logged by the default general callback and does not stop the client.