Optimize file copies by using NIO methods

[fmeum]

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). InputStream.transferTo can use kernel buffers when both input streams are FileInputStreams.

[fmeum]

Stacked on #27458.

[tjgq]

Can you also replace ByteStreams.copy(in, out) with in.transferTo(out) below? It can still do in-kernel I/O when both sides are files, and falls back to essentially equivalent code otherwise.

[fmeum]

I did that and realized that the copy fallback in moveFile can also be optimized in this way.

[Copilot]

Pull Request Overview

This PR refactors exception handling in the VFS implementation to centralize and standardize error message formatting across different FileSystem implementations. The key changes introduce a new translateNioToIoException utility method that maps NIO exceptions to consistent java.io exceptions with Unix-style error messages.

• Centralized exception translation logic in FileSystem.translateNioToIoException()
• Refactored getIoFile() and getNioPath() methods to return null instead of throwing UnsupportedOperationException
• Optimized file copy operations to use NIO fast paths when available

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
FileSystem.java	Added centralized translateNioToIoException() method and error message constants; changed getIoFile()/getNioPath() to return null instead of throwing exceptions
JavaIoFileSystem.java	Simplified exception handling in createSymbolicLink(), readSymbolicLink(), and renameTo() to use the new translation method; removed duplicate error constants
WindowsFileSystem.java	Updated createSymbolicLink() to use centralized exception translation
PathTransformingDelegateFileSystem.java	Added @Nullable annotations to getIoFile() and getNioPath() methods
FileSystemUtils.java	Added fast path for file copying using NIO APIs; replaced custom buffer copying with transferTo(); removed copyLargeBuffer() utility method
AbstractFileSystem.java	Added null-check wrapper for getIoFile() call; removed duplicate ERR_PERMISSION_DENIED constant

Comments suppressed due to low confidence (1)

src/main/java/com/google/devtools/build/lib/vfs/AbstractFileSystem.java:99

• The getNioPath(path) method now returns @Nullable as per the changes, but this code doesn't handle the null case. If getNioPath() returns null, this will throw a NullPointerException when passed to Files.newByteChannel(). A null check should be added to handle file systems that don't support NIO paths.

      return Files.newByteChannel(getNioPath(path), READ_WRITE_BYTE_CHANNEL_OPEN_OPTIONS);

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[tjgq]

Can you also amend the testGetNioPath_* tests in FileSystemTest? They no longer need to account for the UnsupportedOperationException case.

[tjgq]

Importing this one myself.