How Can I Mock or Abstract the Filesystem in Go for Testing and Flexibility?

Mocking/Abstracting Filesystem in Go

Requirement:
The goal is to monitor all file system reads and writes in a Go application and potentially replace the physical filesystem with an in-memory alternative.

Solution:
Andrew Gerrand, the creator of Go, provides a simple yet effective solution:

var fs fileSystem = osFS{}

type fileSystem interface {
    Open(name string) (file, error)
    Stat(name string) (os.FileInfo, error)
}

type file interface {
    io.Closer
    io.Reader
    io.ReaderAt
    io.Seeker
    Stat() (os.FileInfo, error)
}

// osFS implements fileSystem using the local disk.
type osFS struct{}

func (osFS) Open(name string) (file, error)        { return os.Open(name) }
func (osFS) Stat(name string) (os.FileInfo, error) { return os.Stat(name) }

To use this solution:

• Define an interface fileSystem that defines the basic file system operations.
• Implement fileSystem for the local disk (osFS) or any custom in-memory filesystem.
• Assign the chosen fileSystem implementation to the global fs variable.

Your application code should use the fs variable as the file system interface instead of directly using the os package. This allows you to easily mock or replace the filesystem as needed.

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