fix: change CloudStorageFileSystemProvider to throw a FileAlreadyExis…

…tsException if copy receives a 412 (googleapis#815)

google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/CloudStorageFileSystemProvider.java

@@ -468,7 +468,7 @@ private SeekableByteChannel newWriteChannel(Path path, Set<? extends OpenOption>
               infoBuilder.build(),
               writeOptions.toArray(new Storage.BlobWriteOption[writeOptions.size()])));
     } catch (StorageException oops) {
-      throw asIoException(oops);
+      throw asIoException(oops, false);
     }
   }
 
@@ -688,7 +688,7 @@ public void copy(Path source, Path target, CopyOption... options) throws IOExcep
           retryHandler.handleStorageException(oops);
           // we're being aggressive by retrying even on scenarios where we'd normally reopen.
         } catch (StorageException retriesExhaustedException) {
-          throw asIoException(retriesExhaustedException);
+          throw asIoException(retriesExhaustedException, true);
         }
       }
     }
@@ -1013,12 +1013,18 @@ Page<Bucket> listBuckets(Storage.BucketListOption... options) {
     return storage.list(options);
   }
 
-  private IOException asIoException(StorageException oops) {
+  private IOException asIoException(StorageException oops, boolean operationWasCopy) {
     // RPC API can only throw StorageException, but CloudStorageFileSystemProvider
     // can only throw IOException. Square peg, round hole.
     // TODO(#810): Research if other codes should be translated similarly.
     if (oops.getCode() == 404) {
       return new NoSuchFileException(oops.getReason());
+    } else if (operationWasCopy && oops.getCode() == 412) {
+      return new FileAlreadyExistsException(
+          String.format(
+              "Copy failed for reason '%s'. This most likely means the destination already exists"
+                  + " and java.nio.file.StandardCopyOption.REPLACE_EXISTING was not specified.",
+              oops.getReason()));
     }
 
     Throwable cause = oops.getCause();

google-cloud-nio/src/test/java/com/google/cloud/storage/contrib/nio/it/ITGcsNio.java

@@ -53,6 +53,7 @@
 import java.nio.channels.FileChannel;
 import java.nio.channels.ReadableByteChannel;
 import java.nio.channels.SeekableByteChannel;
+import java.nio.file.FileAlreadyExistsException;
 import java.nio.file.FileSystem;
 import java.nio.file.FileVisitResult;
 import java.nio.file.Files;
@@ -1108,6 +1109,63 @@ public void testListObject() throws IOException {
     assertThat(objects.size()).isEqualTo(1);
   }
 
+  @Test(expected = FileAlreadyExistsException.class)
+  public void testCopy_replaceFile_withoutOption() throws IOException {
+    CloudStorageFileSystem fs = getTestBucket();
+    String uuid = UUID.randomUUID().toString();
+
+    CloudStoragePath foo = fs.getPath(uuid, "foo.txt");
+    CloudStoragePath bar = fs.getPath(uuid, "bar.txt");
+
+    try {
+      Files.createFile(foo);
+      Files.createFile(bar);
+
+      Files.copy(foo, bar);
+    } finally {
+      Files.deleteIfExists(foo);
+      Files.deleteIfExists(bar);
+    }
+  }
+
+  @Test
+  public void testCopy_replaceFile_withOption() throws IOException {
+    CloudStorageFileSystem fs = getTestBucket();
+    String uuid = UUID.randomUUID().toString();
+
+    CloudStoragePath foo = fs.getPath(uuid, "foo.txt");
+    CloudStoragePath bar = fs.getPath(uuid, "bar.txt");
+
+    try {
+      Files.createFile(foo);
+      Files.createFile(bar);
+
+      Files.copy(foo, bar, StandardCopyOption.REPLACE_EXISTING);
+    } finally {
+      Files.deleteIfExists(foo);
+      Files.deleteIfExists(bar);
+    }
+  }
+
+  @Test(expected = NoSuchFileException.class)
+  public void testCopy_replaceFile_withOption_srcDoesNotExist() throws IOException {
+    CloudStorageFileSystem fs = getTestBucket();
+    String uuid = UUID.randomUUID().toString();
+
+    CloudStoragePath foo = fs.getPath(uuid, "foo.txt");
+    CloudStoragePath bar = fs.getPath(uuid, "bar.txt");
+
+    try {
+      // explicitly do not create foo
+      Files.createFile(bar);
+
+      Files.copy(foo, bar);
+    } finally {
+      Files.deleteIfExists(foo);
+      Files.deleteIfExists(bar);
+    }
+  }
+
   private CloudStorageFileSystem getTestBucket() throws IOException {
     // in typical usage we use the single-argument version of forBucket
     // and rely on the user being logged into their project with the