Efficient and Simple File Copying in Go

Copying a file efficiently in Go, without resorting to complex libraries or operating system-dependent code, can be achieved using a robust implementation that handles various edge cases.

Using Hard Links for Fast Duplication

A fast way to duplicate an existing file is to use the os.Link() function. This creates a hard link between the original file and the destination file, essentially making them the same file rather than creating a copy. This approach saves time and disk space, especially for large files.

Handling Different System Restrictions

However, hard links come with restrictions depending on the operating system and file system configurations. For example, some systems do not allow hard links between files on different file systems or partitions.

Robust and Generic Copying Function

To create a generic and robust copy function, take the following steps:

1. Check for permissions, directory existence, and other potential issues before attempting a copy.
2. Check if both files exist and are identical using os.SameFile. If they are, return success.
3. Attempt a hard link with os.Link. Return if successful.
4. If all else fails, copy the file byte by byte.

Optimization and Simplicity

To improve performance, the byte copy can be performed in a separate go routine to avoid blocking the calling thread. However, this adds complexity to the caller's responsibility of handling the success or failure.

For easier implementation, one can create two separate copy functions: CopyFile() for synchronous copying and CopyFileAsync() for asynchronous copying, which returns a signaling channel to the caller.

Sample Code

The provided code demonstrates a robust CopyFile function that attempts a hard link before performing a byte-copy fallback:

package main

import (
    "fmt"
    "io"
    "os"
)

func CopyFile(src, dst string) (err error) {
    // Validate input
    // ...

    if err = os.Link(src, dst); err == nil {
        return
    }
    err = copyFileContents(src, dst)
    return
}

func copyFileContents(src, dst string) (err error) {
    // Copy file contents
    // ...
    return
}

func main() {
    // ...
    err := CopyFile(os.Args[1], os.Args[2])
    if err != nil {
        // ...
    }
}