Be more defensive if provided destPath is wrong

The path could already exist, *but* not be a directory.
Nice catch James.

Associated tests.

core/src/test/java/hudson/util/AtomicFileWriterTest.java

@@ -1,6 +1,6 @@
 package hudson.util;
 
-import org.junit.After;
+import org.hamcrest.core.StringContains;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -10,15 +10,22 @@ import java.io.File;
 import java.io.IOException;
 import java.nio.charset.Charset;
 import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 
-import static org.junit.Assert.*;
+import static org.hamcrest.core.StringContains.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThat;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
 
 public class AtomicFileWriterTest {
+    @Rule
+    public TemporaryFolder tmp = new TemporaryFolder();
     File af;
     AtomicFileWriter afw;
     String expectedContent = "hello world";
-    @Rule public TemporaryFolder tmp = new TemporaryFolder();
-
 
     @Before
     public void setUp() throws IOException {
@@ -26,6 +33,19 @@ public class AtomicFileWriterTest {
         afw = new AtomicFileWriter(af.toPath(), Charset.defaultCharset());
     }
 
+    @Test
+    public void symlinkToDirectory() throws Exception {
+        final File folder = tmp.newFolder();
+        final File containingSymlink = tmp.newFolder();
+        final Path zeSymlink = Files.createSymbolicLink(Paths.get(containingSymlink.getAbsolutePath(), "ze_symlink"),
+                                                         folder.toPath());
+
+
+        final Path childFileInSymlinkToDir = Paths.get(zeSymlink.toString(), "childFileInSymlinkToDir");
+
+        new AtomicFileWriter(childFileInSymlinkToDir, Charset.forName("UTF-8"));
+    }
+
     @Test
     public void createFile() throws Exception {
         // Verify the file we created exists
@@ -68,4 +88,21 @@ public class AtomicFileWriterTest {
         // Then
         assertTrue(Files.notExists(afw.getTemporaryPath()));
     }
+
+    @Test
+    public void badPath() throws Exception {
+        final File newFile = tmp.newFile();
+        File parentExistsAndIsAFile = new File(newFile, "badChild");
+
+        assertTrue(newFile.exists());
+        assertFalse(parentExistsAndIsAFile.exists());
+
+        try {
+            new AtomicFileWriter(parentExistsAndIsAFile.toPath(), Charset.forName("UTF-8"));
+            fail("Expected a failure");
+        } catch (IOException e) {
+            assertThat(e.getMessage(),
+                       containsString("exists and is neither a directory nor a symlink to a directory"));
+        }
+    }
 }