fix for external paths...

fix_for_external_paths.patch

diff --git a/solr/test-framework/src/java/org/apache/solr/SolrTestCase.java b/solr/test-framework/src/java/org/apache/solr/SolrTestCase.java
index a29e3d0da43..7a321671c35 100644
--- a/solr/test-framework/src/java/org/apache/solr/SolrTestCase.java
+++ b/solr/test-framework/src/java/org/apache/solr/SolrTestCase.java
@@ -130,7 +130,7 @@ public class SolrTestCase extends LuceneTestCase {
       return;
     }
     final Path extPath = ExternalPaths.DEFAULT_CONFIGSET;
-    if (Files.isReadable(extPath /* implies exists() */) && Files.isDirectory(extPath)) {
+    if (extPath != null && Files.isReadable(extPath /* implies exists() */) && Files.isDirectory(extPath)) {
       log.info(
           "Setting '{}' system property to test-framework derived value of '{}'",
           SolrDispatchFilter.SOLR_CONFIGSET_DEFAULT_CONFDIR_ATTRIBUTE,
diff --git a/solr/test-framework/src/java/org/apache/solr/util/ExternalPaths.java b/solr/test-framework/src/java/org/apache/solr/util/ExternalPaths.java
index 940ec58b9e9..445d97bf848 100644
--- a/solr/test-framework/src/java/org/apache/solr/util/ExternalPaths.java
+++ b/solr/test-framework/src/java/org/apache/solr/util/ExternalPaths.java
@@ -16,19 +16,28 @@
  */
 package org.apache.solr.util;
 
+import java.lang.invoke.MethodHandles;
 import java.net.URL;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import org.apache.solr.common.SolrException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Some tests need to reach outside the classpath to get certain resources (e.g. the example
  * configuration). This class provides some paths to allow them to do this.
  *
+ * <p>When running in external plugin projects (outside the Solr source tree), {@link #SOURCE_HOME}
+ * will be null and the path getter methods will also return null. External projects can set the
+ * system property {@code solr.test.source.home} to provide an explicit source home directory.
+ *
  * @lucene.internal
  */
 public class ExternalPaths {
 
+  private static final Logger log = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
+
   /**
    * The main directory path for the solr source being built if it can be determined. If it can not
    * be determined -- possibly because the current context is a client code base using the test
@@ -39,29 +48,116 @@ public class ExternalPaths {
    */
   public static final Path SOURCE_HOME = determineSourceHome();
 
+  // Lazy-initialized paths to avoid NPE during class loading
+  private static volatile Path webappHome;
+  private static volatile Path defaultConfigSet;
+  private static volatile Path techproductsConfigSet;
+  private static volatile Path serverHome;
+
+  /**
+   * Returns the path to the webapp directory, or null if SOURCE_HOME is not available. This is
+   * typically used when running tests within the Solr source tree.
+   *
+   * @return Path to webapp/web directory, or null if not running in Solr source tree
+   */
+  public static Path getWebappHome() {
+    if (webappHome == null && SOURCE_HOME != null) {
+      synchronized (ExternalPaths.class) {
+        if (webappHome == null) {
+          webappHome = SOURCE_HOME.resolve("webapp/web").toAbsolutePath();
+        }
+      }
+    }
+    return webappHome;
+  }
+
+  /**
+   * Returns the path to the default configset, or null if SOURCE_HOME is not available.
+   *
+   * @return Path to default configset, or null if not running in Solr source tree
+   */
+  public static Path getDefaultConfigSet() {
+    if (defaultConfigSet == null && SOURCE_HOME != null) {
+      synchronized (ExternalPaths.class) {
+        if (defaultConfigSet == null) {
+          defaultConfigSet =
+              SOURCE_HOME.resolve("server/solr/configsets/_default/conf").toAbsolutePath();
+        }
+      }
+    }
+    return defaultConfigSet;
+  }
+
+  /**
+   * Returns the path to the techproducts configset, or null if SOURCE_HOME is not available.
+   *
+   * @return Path to techproducts configset, or null if not running in Solr source tree
+   */
+  public static Path getTechproductsConfigSet() {
+    if (techproductsConfigSet == null && SOURCE_HOME != null) {
+      synchronized (ExternalPaths.class) {
+        if (techproductsConfigSet == null) {
+          techproductsConfigSet =
+              SOURCE_HOME
+                  .resolve("server/solr/configsets/sample_techproducts_configs/conf")
+                  .toAbsolutePath();
+        }
+      }
+    }
+    return techproductsConfigSet;
+  }
+
   /**
-   * @see #SOURCE_HOME
+   * Returns the path to the server home directory, or null if SOURCE_HOME is not available.
+   *
+   * @return Path to server/solr directory, or null if not running in Solr source tree
    */
-  public static Path WEBAPP_HOME = SOURCE_HOME.resolve("webapp/web").toAbsolutePath();
+  public static Path getServerHome() {
+    if (serverHome == null && SOURCE_HOME != null) {
+      synchronized (ExternalPaths.class) {
+        if (serverHome == null) {
+          serverHome = SOURCE_HOME.resolve("server/solr").toAbsolutePath();
+        }
+      }
+    }
+    return serverHome;
+  }
 
   /**
-   * @see #SOURCE_HOME
+   * Backward compatibility - deprecated in favor of getWebappHome()
+   *
+   * @deprecated Use {@link #getWebappHome()} instead. This field may be null when running in
+   *     external plugin projects.
    */
-  public static Path DEFAULT_CONFIGSET =
-      SOURCE_HOME.resolve("server/solr/configsets/_default/conf").toAbsolutePath();
+  @Deprecated
+  public static final Path WEBAPP_HOME = getWebappHome();
 
   /**
-   * @see #SOURCE_HOME
+   * Backward compatibility - deprecated in favor of getDefaultConfigSet()
+   *
+   * @deprecated Use {@link #getDefaultConfigSet()} instead. This field may be null when running in
+   *     external plugin projects.
    */
-  public static Path TECHPRODUCTS_CONFIGSET =
-      SOURCE_HOME
-          .resolve("server/solr/configsets/sample_techproducts_configs/conf")
-          .toAbsolutePath();
+  @Deprecated
+  public static final Path DEFAULT_CONFIGSET = getDefaultConfigSet();
 
   /**
-   * @see #SOURCE_HOME
+   * Backward compatibility - deprecated in favor of getTechproductsConfigSet()
+   *
+   * @deprecated Use {@link #getTechproductsConfigSet()} instead. This field may be null when
+   *     running in external plugin projects.
    */
-  public static Path SERVER_HOME = SOURCE_HOME.resolve("server/solr").toAbsolutePath();
+  @Deprecated
+  public static final Path TECHPRODUCTS_CONFIGSET = getTechproductsConfigSet();
+
+  /**
+   * Backward compatibility - deprecated in favor of getServerHome()
+   *
+   * @deprecated Use {@link #getServerHome()} instead. This field may be null when running in
+   *     external plugin projects.
+   */
+  @Deprecated
+  public static final Path SERVER_HOME = getServerHome();
 
   /**
    * Ugly, ugly hack to determine the example home without depending on the CWD this is needed for
@@ -69,9 +165,24 @@ public class ExternalPaths {
    * determined, this method returns null.
    */
   static Path determineSourceHome() {
+    // Allow explicit override for external plugin projects via system property
+    String explicitSourceHome = System.getProperty("solr.test.source.home");
+    if (explicitSourceHome != null && !explicitSourceHome.isEmpty()) {
+      Path explicitPath = Path.of(explicitSourceHome);
+      if (Files.isDirectory(explicitPath)) {
+        log.info("Using explicit Solr test source home: {}", explicitPath);
+        return explicitPath.toAbsolutePath();
+      } else {
+        log.warn("Explicit solr.test.source.home is not a directory: {}", explicitPath);
+      }
+    }
+
     try {
+      // Try to find solr/conf in the filesystem first
       Path file = Path.of("solr/conf");
+
       if (!Files.exists(file)) {
+        // Try to find it in the classpath
         URL resourceUrl = ExternalPaths.class.getClassLoader().getResource("solr/conf");
         if (resourceUrl != null) {
           file = Path.of(resourceUrl.toURI());
@@ -82,15 +193,30 @@ public class ExternalPaths {
         }
       }
 
+      // Walk up the directory tree looking for the Solr source marker file
       Path base = file.toAbsolutePath();
-      while (!Files.exists(base.resolve("solr/test-framework/build.gradle")) && null != base) {
+      while (base != null && !Files.exists(base.resolve("solr/test-framework/build.gradle"))) {
         base = base.getParent();
       }
-      return (null == base) ? null : base.resolve("solr/").toAbsolutePath();
+
+      if (base == null) {
+        log.info(
+            "Could not determine Solr source home - not running in Solr source tree. "
+                + "This is expected for external plugin projects. "
+                + "Set -Dsolr.test.source.home=/path/to/solr if you need ExternalPaths.");
+        return null;
+      }
+
+      Path sourceHome = base.resolve("solr/").toAbsolutePath();
+      log.debug("Determined Solr source home: {}", sourceHome);
+      return sourceHome;
     } catch (Exception e) {
-      // all bets are off
-      throw new SolrException(
-          SolrException.ErrorCode.SERVER_ERROR, "Failed to determine source home", e);
+      // Gracefully return null for external plugin projects instead of throwing
+      log.warn(
+          "Failed to determine Solr source home - returning null. "
+              + "This is expected for external plugin projects: {}",
+          e.toString());
+      return null;
     }
   }
 }