fix solrResourceLoader for https://issues.apache.org/jira/browse/SOLR-6887. This patch is placed in the public domain.

gistfile1.diff

diff --git a/solr/core/src/java/org/apache/solr/core/SolrResourceLoader.java b/solr/core/src/java/org/apache/solr/core/SolrResourceLoader.java
index 9cbd048..86902c4 100644
--- a/solr/core/src/java/org/apache/solr/core/SolrResourceLoader.java
+++ b/solr/core/src/java/org/apache/solr/core/SolrResourceLoader.java
@@ -175,21 +175,41 @@ public class SolrResourceLoader implements ResourceLoader,Closeable
    */
   void addToClassLoader(final String baseDir, final FileFilter filter, boolean quiet) {
     File base = FileUtils.resolvePath(new File(getInstanceDir()), baseDir);
-    if (base != null && base.exists() && base.isDirectory()) {
-      File[] files = base.listFiles(filter);
-      if (files == null || files.length == 0) {
-        if (!quiet) {
-          log.warn("No files added to classloader from lib: "
-                   + baseDir + " (resolved as: " + base.getAbsolutePath() + ").");
-        }
-      } else {
-        this.classLoader = replaceClassLoader(classLoader, base, filter);
+    if (base == null) {
+      if (!quiet) {
+        log.warn("Can not add \"" + baseDir + "\"; resolvePath relative to \"" + getInstanceDir() + "\" returned null");
       }
-    } else {
+      return;
+    }
+    File canonical;
+    try {
+      canonical = base.getCanonicalFile();
+    }
+    catch(IOException ioe) {
+      log.warn("Can not add \"" + baseDir + "\"; cannot obtain canonical file");
+      return;
+    }
+    base = canonical;
+    if (!base.exists()) {
       if (!quiet) {
-        log.warn("Can't find (or read) directory to add to classloader: "
-            + baseDir + " (resolved as: " + base.getAbsolutePath() + ").");
+        log.warn("Can not add \"" + baseDir + "\": \"" + base.getAbsolutePath() + "\" does not exist");
       }
+      return;
+    }
+    if (!base.isDirectory()) {
+      if (!quiet) {
+        log.warn("Can not add \"" + baseDir + "\": \"" + base.getAbsolutePath() + "\" is not a directory");
+      }
+      return;
+    }
+    File[] files = base.listFiles(filter);
+    if (files == null || files.length == 0) {
+      if (!quiet) {
+        log.warn("No files added to classloader from lib: "
+                 + baseDir + " (resolved as: " + base.getAbsolutePath() + ").");
+      }
+    } else {
+      this.classLoader = replaceClassLoader(classLoader, base, filter);
     }
   }