Implement FileSystemUtils.copyFile via Files.copy if possible

`Files.copy` uses various optimizations such as kernel buffers (`sendfile` on Unix) or copy-on-write (`clonefile` on macOS, `copy_file_range` on Linux with a supported file system).

Author: fmeum

src/main/java/com/google/devtools/build/lib/vfs/AbstractFileSystem.java

@@ -14,6 +14,7 @@
 //
 package com.google.devtools.build.lib.vfs;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import static java.nio.file.StandardOpenOption.CREATE;
 import static java.nio.file.StandardOpenOption.READ;
 import static java.nio.file.StandardOpenOption.TRUNCATE_EXISTING;
@@ -62,7 +63,7 @@ public InputStream getInputStream(PathFragment path) throws IOException {
 
   /** Allows the mapping of PathFragment to InputStream to be overridden in subclasses. */
   protected InputStream createFileInputStream(PathFragment path) throws IOException {
-    return new FileInputStream(getIoFile(path));
+    return new FileInputStream(checkNotNull(getIoFile(path)));
   }
 
   /** Returns either normal or profiled FileInputStream. */

src/main/java/com/google/devtools/build/lib/vfs/FileSystem.java

@@ -797,21 +797,21 @@ public abstract void createFSDependentHardLink(PathFragment linkPath, PathFragme
   public void prefetchPackageAsync(PathFragment path, int maxDirs) {}
 
   /**
-   * Returns a {@link File} object for the given path. This method is only supported by file system
-   * implementations that are backed by the local file system.
+   * Returns a {@link File} object for the given path or null if this file system implementation is
+   * not backed by the local file system.
    */
+  @Nullable
   public File getIoFile(PathFragment path) {
-    throw new UnsupportedOperationException(
-        "getIoFile() not supported for " + getClass().getName());
+    return null;
   }
 
   /**
-   * Returns a {@link java.nio.file.Path} object for the given path. This method is only supported
-   * by file system implementations that are backed by the local file system.
+   * Returns a {@link java.nio.file.Path} object for the given path or null if this file system
+   * implementation is not backed by the local file system.
    */
+  @Nullable
   public java.nio.file.Path getNioPath(PathFragment path) {
-    throw new UnsupportedOperationException(
-        "getNioPath() not supported for " + getClass().getName());
+    return null;
   }
 
   // Mapping from FileSystemException reason strings on various platforms to the corresponding Unix

src/main/java/com/google/devtools/build/lib/vfs/FileSystemUtils.java

@@ -13,7 +13,10 @@
 // limitations under the License.
 package com.google.devtools.build.lib.vfs;
 
+import static com.google.devtools.build.lib.vfs.FileSystem.translateNioToIoException;
 import static java.nio.charset.StandardCharsets.ISO_8859_1;
+import static java.nio.file.StandardCopyOption.COPY_ATTRIBUTES;
+import static java.nio.file.StandardCopyOption.REPLACE_EXISTING;
 
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
@@ -374,6 +377,18 @@ public static ByteSink asByteSink(final Path path) {
    */
   @ThreadSafe  // but not atomic
   public static void copyFile(Path from, Path to) throws IOException {
+    var fromNio = from.getFileSystem().getNioPath(from.asFragment());
+    var toNio = to.getFileSystem().getNioPath(to.asFragment());
+    if (fromNio != null && toNio != null) {
+      // Fast path: Files.copy uses various optimizations such as kernel buffers (sendfile on Unix)
+      // or copy-on-write (clonefile on macOS, copy_file_range on Linux with a supported file system).
+      try {
+        java.nio.file.Files.copy(fromNio, toNio, REPLACE_EXISTING, COPY_ATTRIBUTES);
+      } catch (IOException e) {
+        throw translateNioToIoException(from.asFragment(), e);
+      }
+      return;
+    }
     try {
       to.delete();
     } catch (IOException e) {

src/main/java/com/google/devtools/build/lib/vfs/PathTransformingDelegateFileSystem.java

@@ -24,6 +24,7 @@
 import java.io.OutputStream;
 import java.nio.channels.SeekableByteChannel;
 import java.util.Collection;
+import javax.annotation.Nullable;
 
 /**
  * FileSystem implementation which delegates all operations to a provided instance with a
@@ -305,11 +306,13 @@ public void prefetchPackageAsync(PathFragment path, int maxDirs) {
     delegateFs.prefetchPackageAsync(toDelegatePath(path), maxDirs);
   }
 
+  @Nullable
   @Override
   public File getIoFile(PathFragment path) {
     return delegateFs.getIoFile(toDelegatePath(path));
   }
 
+  @Nullable
   @Override
   public java.nio.file.Path getNioPath(PathFragment path) {
     return delegateFs.getNioPath(toDelegatePath(path));