Fail hard on empty classpath elements.

This can happen easily, if somehow old 1.x shellscripts survive and try to launch 2.x code.
I have the feeling this happens maybe because of packaging upgrades or something.
Either way: we can just fail hard and clear in this situation, rather than the current situation
where CWD might be /, and we might traverse the entire filesystem until we hit an error...

Relates to #13864

core/src/main/java/org/elasticsearch/bootstrap/JarHell.java

@@ -88,17 +88,35 @@ public static void checkJarHell() throws Exception {
     }
 
     /**
-     * Parses the classpath into a set of URLs
+     * Parses the classpath into an array of URLs
+     * @return array of URLs
+     * @throws IllegalStateException if the classpath contains empty elements
      */
-    @SuppressForbidden(reason = "resolves against CWD because that is how classpaths work")
     public static URL[] parseClassPath()  {
-        String elements[] = System.getProperty("java.class.path").split(System.getProperty("path.separator"));
+        return parseClassPath(System.getProperty("java.class.path"));
+    }
+
+    /**
+     * Parses the classpath into a set of URLs. For testing.
+     * @param classPath classpath to parse (typically the system property {@code java.class.path})
+     * @return array of URLs
+     * @throws IllegalStateException if the classpath contains empty elements
+     */
+    @SuppressForbidden(reason = "resolves against CWD because that is how classpaths work")
+    static URL[] parseClassPath(String classPath) {
+        String elements[] = classPath.split(System.getProperty("path.separator"));
         URL urlElements[] = new URL[elements.length];
         for (int i = 0; i < elements.length; i++) {
             String element = elements[i];
-            // empty classpath element behaves like CWD.
+            // Technically empty classpath element behaves like CWD.
+            // So below is the "correct" code, however in practice with ES, this is usually just a misconfiguration,
+            // from old shell scripts left behind or something:
+            //   if (element.isEmpty()) {
+            //      element = System.getProperty("user.dir");
+            //   }
+            // Instead we just throw an exception, and keep it clean.
             if (element.isEmpty()) {
-                element = System.getProperty("user.dir");
+                throw new IllegalStateException("Classpath should not contain empty elements! (outdated shell script from a previous version?) classpath='" + classPath + "'");
             }
             try {
                 urlElements[i] = PathUtils.get(element).toUri().toURL();

core/src/test/java/org/elasticsearch/bootstrap/JarHellTests.java

@@ -275,4 +275,64 @@ public void testInvalidVersions() {
             }
         }
     }
+
+    // classpath testing is system specific, so we just write separate tests for *nix and windows cases
+
+    /**
+     * Parse a simple classpath with two elements on unix
+     */
+    public void testParseClassPathUnix() throws Exception {
+        assumeTrue("test is designed for unix-like systems only", ":".equals(System.getProperty("path.separator")));
+        assumeTrue("test is designed for unix-like systems only", "/".equals(System.getProperty("file.separator")));
+
+        Path element1 = createTempDir();
+        Path element2 = createTempDir();
+
+        URL expected[] = { element1.toUri().toURL(), element2.toUri().toURL() };
+        assertArrayEquals(expected, JarHell.parseClassPath(element1.toString() + ":" + element2.toString()));
+    }
+
+    /**
+     * Make sure an old unix classpath with an empty element (implicitly CWD: i'm looking at you 1.x ES scripts) fails
+     */
+    public void testEmptyClassPathUnix() throws Exception {
+        assumeTrue("test is designed for unix-like systems only", ":".equals(System.getProperty("path.separator")));
+        assumeTrue("test is designed for unix-like systems only", "/".equals(System.getProperty("file.separator")));
+
+        try {
+            JarHell.parseClassPath(":/element1:/element2");
+            fail("should have hit exception");
+        } catch (IllegalStateException expected) {
+            assertTrue(expected.getMessage().contains("should not contain empty elements"));
+        }
+    }
+
+    /**
+     * Parse a simple classpath with two elements on windows
+     */
+    public void testParseClassPathWindows() throws Exception {
+        assumeTrue("test is designed for windows-like systems only", ";".equals(System.getProperty("path.separator")));
+        assumeTrue("test is designed for windows-like systems only", "\\".equals(System.getProperty("file.separator")));
+
+        Path element1 = createTempDir();
+        Path element2 = createTempDir();
+
+        URL expected[] = { element1.toUri().toURL(), element2.toUri().toURL() };
+        assertArrayEquals(expected, JarHell.parseClassPath(element1.toString() + ";" + element2.toString()));
+    }
+
+    /**
+     * Make sure an old windows classpath with an empty element (implicitly CWD: i'm looking at you 1.x ES scripts) fails
+     */
+    public void testEmptyClassPathWindows() throws Exception {
+        assumeTrue("test is designed for windows-like systems only", ";".equals(System.getProperty("path.separator")));
+        assumeTrue("test is designed for windows-like systems only", "\\".equals(System.getProperty("file.separator")));
+
+        try {
+            JarHell.parseClassPath(";c:\\element1;c:\\element2");
+            fail("should have hit exception");
+        } catch (IllegalStateException expected) {
+            assertTrue(expected.getMessage().contains("should not contain empty elements"));
+        }
+    }
 }