fs: expose copy-on-write flags for fs.copyFile()

doc/api/fs.md

@@ -1175,8 +1175,18 @@ operation. If an error occurs after the destination file has been opened for
 writing, Node.js will attempt to remove the destination.
 
 `flags` is an optional integer that specifies the behavior
-of the copy operation. The only supported flag is `fs.constants.COPYFILE_EXCL`,
-which causes the copy operation to fail if `dest` already exists.
+of the copy operation. It is possible to create a mask consisting of the bitwise
+OR of two or more values (e.g.
+`fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE`).
+
+* `fs.constants.COPYFILE_EXCL` - The copy operation will fail if `dest` already
+exists.
+* `fs.constants.COPYFILE_FICLONE` - The copy operation will attempt to create a
+copy-on-write reflink. If the platform does not support copy-on-write, then a
+fallback copy mechanism is used.
+* `fs.constants.COPYFILE_FICLONE_FORCE` - The copy operation will attempt to
+create a copy-on-write reflink. If the platform does not support copy-on-write,
+then the operation will fail.
 
 Example:
 
@@ -1216,8 +1226,18 @@ atomicity of the copy operation. If an error occurs after the destination file
 has been opened for writing, Node.js will attempt to remove the destination.
 
 `flags` is an optional integer that specifies the behavior
-of the copy operation. The only supported flag is `fs.constants.COPYFILE_EXCL`,
-which causes the copy operation to fail if `dest` already exists.
+of the copy operation. It is possible to create a mask consisting of the bitwise
+OR of two or more values (e.g.
+`fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE`).
+
+* `fs.constants.COPYFILE_EXCL` - The copy operation will fail if `dest` already
+exists.
+* `fs.constants.COPYFILE_FICLONE` - The copy operation will attempt to create a
+copy-on-write reflink. If the platform does not support copy-on-write, then a
+fallback copy mechanism is used.
+* `fs.constants.COPYFILE_FICLONE_FORCE` - The copy operation will attempt to
+create a copy-on-write reflink. If the platform does not support copy-on-write,
+then the operation will fail.
 
 Example:
 
@@ -3814,8 +3834,18 @@ error occurs after the destination file has been opened for writing, Node.js
 will attempt to remove the destination.
 
 `flags` is an optional integer that specifies the behavior
-of the copy operation. The only supported flag is `fs.constants.COPYFILE_EXCL`,
-which causes the copy operation to fail if `dest` already exists.
+of the copy operation. It is possible to create a mask consisting of the bitwise
+OR of two or more values (e.g.
+`fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE`).
+
+* `fs.constants.COPYFILE_EXCL` - The copy operation will fail if `dest` already
+exists.
+* `fs.constants.COPYFILE_FICLONE` - The copy operation will attempt to create a
+copy-on-write reflink. If the platform does not support copy-on-write, then a
+fallback copy mechanism is used.
+* `fs.constants.COPYFILE_FICLONE_FORCE` - The copy operation will attempt to
+create a copy-on-write reflink. If the platform does not support copy-on-write,
+then the operation will fail.
 
 Example:
 
@@ -4449,6 +4479,34 @@ The following constants are meant for use with [`fs.access()`][].
   </tr>
 </table>
 
+### File Copy Constants
+
+The following constants are meant for use with [`fs.copyFile()`][].
+
+<table>
+  <tr>
+    <th>Constant</th>
+    <th>Description</th>
+  </tr>
+  <tr>
+    <td><code>COPYFILE_EXCL</code></td>
+    <td>If present, the copy operation will fail with an error if the
+    destination path already exists.</td>
+  </tr>
+  <tr>
+    <td><code>COPYFILE_FICLONE</code></td>
+    <td>If present, the copy operation will attempt to create a
+    copy-on-write reflink. If the underlying platform does not support
+    copy-on-write, then a fallback copy mechanism is used.</td>
+  </tr>
+  <tr>
+    <td><code>COPYFILE_FICLONE_FORCE</code></td>
+    <td>If present, the copy operation will attempt to create a
+    copy-on-write reflink. If the underlying platform does not support
+    copy-on-write, then the operation will fail with an error.</td>
+  </tr>
+</table>
+
 ### File Open Constants
 
 The following constants are meant for use with `fs.open()`.

lib/fs.js

@@ -1916,7 +1916,15 @@ fs.mkdtempSync = function(prefix, options) {
 
 // Define copyFile() flags.
 Object.defineProperties(fs.constants, {
-  COPYFILE_EXCL: { enumerable: true, value: constants.UV_FS_COPYFILE_EXCL }
+  COPYFILE_EXCL: { enumerable: true, value: constants.UV_FS_COPYFILE_EXCL },
+  COPYFILE_FICLONE: {
+    enumerable: true,
+    value: constants.UV_FS_COPYFILE_FICLONE
+  },
+  COPYFILE_FICLONE_FORCE: {
+    enumerable: true,
+    value: constants.UV_FS_COPYFILE_FICLONE_FORCE
+  }
 });
 
 

src/node_constants.cc

@@ -1314,6 +1314,8 @@ void DefineConstants(v8::Isolate* isolate, Local<Object> target) {
   // Define libuv constants.
   NODE_DEFINE_CONSTANT(os_constants, UV_UDP_REUSEADDR);
   NODE_DEFINE_CONSTANT(fs_constants, UV_FS_COPYFILE_EXCL);
+  NODE_DEFINE_CONSTANT(fs_constants, UV_FS_COPYFILE_FICLONE);
+  NODE_DEFINE_CONSTANT(fs_constants, UV_FS_COPYFILE_FICLONE_FORCE);
 
   os_constants->Set(OneByteString(isolate, "dlopen"), dlopen_constants);
   os_constants->Set(OneByteString(isolate, "errno"), err_constants);

test/parallel/test-fs-copyfile.js

@@ -8,7 +8,14 @@ const uv = process.binding('uv');
 const path = require('path');
 const src = fixtures.path('a.js');
 const dest = path.join(tmpdir.path, 'copyfile.out');
-const { COPYFILE_EXCL, UV_FS_COPYFILE_EXCL } = fs.constants;
+const {
+  COPYFILE_EXCL,
+  COPYFILE_FICLONE,
+  COPYFILE_FICLONE_FORCE,
+  UV_FS_COPYFILE_EXCL,
+  UV_FS_COPYFILE_FICLONE,
+  UV_FS_COPYFILE_FICLONE_FORCE
+} = fs.constants;
 
 function verify(src, dest) {
   const srcData = fs.readFileSync(src, 'utf8');
@@ -25,8 +32,14 @@ tmpdir.refresh();
 
 // Verify that flags are defined.
 assert.strictEqual(typeof COPYFILE_EXCL, 'number');
+assert.strictEqual(typeof COPYFILE_FICLONE, 'number');
+assert.strictEqual(typeof COPYFILE_FICLONE_FORCE, 'number');
 assert.strictEqual(typeof UV_FS_COPYFILE_EXCL, 'number');
+assert.strictEqual(typeof UV_FS_COPYFILE_FICLONE, 'number');
+assert.strictEqual(typeof UV_FS_COPYFILE_FICLONE_FORCE, 'number');
 assert.strictEqual(COPYFILE_EXCL, UV_FS_COPYFILE_EXCL);
+assert.strictEqual(COPYFILE_FICLONE, UV_FS_COPYFILE_FICLONE);
+assert.strictEqual(COPYFILE_FICLONE_FORCE, UV_FS_COPYFILE_FICLONE_FORCE);
 
 // Verify that files are overwritten when no flags are provided.
 fs.writeFileSync(dest, '', 'utf8');
@@ -38,9 +51,26 @@ verify(src, dest);
 fs.copyFileSync(src, dest, 0);
 verify(src, dest);
 
+// Verify that UV_FS_COPYFILE_FICLONE can be used.
+fs.unlinkSync(dest);
+fs.copyFileSync(src, dest, UV_FS_COPYFILE_FICLONE);
+verify(src, dest);
+
+// Verify that COPYFILE_FICLONE_FORCE can be used.
+try {
+  fs.unlinkSync(dest);
+  fs.copyFileSync(src, dest, COPYFILE_FICLONE_FORCE);
+  verify(src, dest);
+} catch (err) {
+  assert.strictEqual(err.syscall, 'copyfile');
+  assert(err.code === 'ENOTSUP' || err.code === 'ENOTTY' ||
+    err.code === 'ENOSYS');
+  assert.strictEqual(err.path, src);
+  assert.strictEqual(err.dest, dest);
+}
 
 // Copies asynchronously.
-fs.unlinkSync(dest);
+tmpdir.refresh(); // Don't use unlinkSync() since the last test may fail.
 fs.copyFile(src, dest, common.mustCall((err) => {
   assert.ifError(err);
   verify(src, dest);