How to Effectively Mock the Filesystem for Unit Testing in Go?

Mock Testing the Filesystem in Go

Introduction

Unit testing code that interacts with the filesystem requires mocking the filesystem to isolate testing from the actual file system operations. This article provides an example of using an interface and mocked types to effectively test filesystem-interacting functions in Go.

Mocked Filesystem Interface

The provided solution utilizes an interface, fileSystem, to represent filesystem operations. The concrete implementation, osFS, handles actual file operations in production. To test code that relies on osFS, we need to create a mocked version, mockedFS, that inherits fileSystem but controls the behavior of file operations during the test.

Mocking os.FileInfo

To mock the os.FileInfo interface returned by fs.Stat(), we create a mockedFileInfo type that embeds os.FileInfo and overrides the required methods. This allows us to control the size reported by Stat().

Setting Up the Mocked Filesystem

To test code using the mocked filesystem, we temporarily replace the global fs variable with an instance of mockedFS. This effectively intercepts any filesystem operations performed by the tested function.

Test Example

The provided example function, getSize(), returns the size of a file or an error if Stat() fails. To test this function fully, we use mockedFS to control the behavior of Stat():

• Error case: Set mockedFS.reportErr to true to simulate a failure in Stat().
• Valid size case: Set mockedFS.reportErr to false and specify a size for mockedFS.reportSize.

The test cases verify that the function correctly handles both the error and success scenarios.

Conclusion

By using an interface and mocked types, we can easily mock the filesystem during unit testing in Go. This allows us to isolate the tested code from external dependencies and ensures reliable testing results.

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