


How to Effectively Mock Filesystem Interactions for Unit Testing in Go?
Dec 22, 2024 am 12:31 AMTesting Filesystem Interaction in Go with Mocks
Mocking the filesystem can be valuable when unit testing functions that interact with the filesystem. To demonstrate its usage, let's examine an interface-based approach, where we create a custom fileSystem interface that encapsulates the filesystem operations.
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) }
Mock Implementation:
To mock the filesystem, we can create a mocked version of the fileSystem interface, called mockedFS. This allows us to control the behavior of the filesystem operations during testing.
Example Function:
Let's consider a function getSize that returns the file size using the filesystem.Stat method:
func getSize(name string) (int64, error) { stat, err := fs.Stat(name) if err != nil { return 0, err } return stat.Size(), nil }
Setting Up the Mock:
Before testing getSize, we need to replace the global fs variable with our mocked instance:
func TestGetSize(t *testing.T) { // Save the original filesystem oldFs := fs // Replace with mocked filesystem mfs := &mockedFS{} fs = mfs // Restore original filesystem after test defer func() { fs = oldFs }() }
Test Execution:
The test code below verifies the getSize function's behavior under various error and file size conditions:
type mockedFS struct { reportErr bool reportSize int64 } func (mfs) Stat(name string) (os.FileInfo, error) { if mfs.reportErr { return nil, os.ErrNotExist } return &mockedFileInfo{size: mfs.reportSize}, nil } type mockedFileInfo struct { size int64 } func (mfi mockedFileInfo) Size() int64 { return mfi.size } // Test Error Case func TestGetSize(t *testing.T) { ... mfs.reportErr = true if _, err := getSize("hello.go"); err == nil { ... } // Test Non-Error Case func TestGetSize(t *testing.T) { ... mfs.reportSize = 123 if size, err := getSize("hello.go"); ... }
This example demonstrates how to create a custom filesystem interface, mock its implementation, and use it to test a function that interacts with the filesystem, ensuring a controlled testing environment in Go.
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