模拟bufio.Scanner输入

I've had a lot of success defining interfaces and substituting mocks during testing, but I've hit a problem with mocking bufio.Scanner input:

file := &mockFile{
    ReadFunc: func(p []byte) (int, error) {
        reader := bufio.NewReader(bytes.NewReader([]byte(consulPropertiesFile)))
        return reader.Read(p)
    },
    CloseFunc: func() error {
        return nil
    },
}

fs := &mockFileSystem{
    OpenFunc: func(name string) (File, error) {
        return file, nil
    },
}

properties, err := readConsulPropertiesFile(fs)

While this seems to work, once the scanner has gotten to the end of my string, it just seems to return to the beginning, and reads too much (it seems to read more than a line this time around). It's like I need to manually return an EOF error at the appropriate time in my ReadFunc, but I'm not sure how to figure out when I should do that...

Scanner code (lifted from here):

f, err := file.Open("/run/secrets/consul-web4.properties")
if err != nil {
    return nil, err
}
defer f.Close()

properties := map[string]string{}

scanner := bufio.NewScanner(f)
for scanner.Scan() {
    line := scanner.Text()
    if equal := strings.Index(line, "="); equal >= 0 {
        if key := strings.TrimSpace(line[:equal]); len(key) > 0 {
            value := ""
            if len(line) > equal {
                value = strings.TrimSpace(line[equal+1:])
            }
            properties[key] = value
        }
    }
}

I've yet to refactor the above...

I've tried the following variations on my test string:

const input = `key=value
key=value
key=value
`

const input = "key=value
key=value
key=value
"

And I've tried bufio.Reader & io.Reader implementations.

Any help / insight appreciated!

Thanks to @Adrian:

I realised on my way home it was probably due to instantiating a new reader every time the method is called. Moving the reader instantiation out of my ReadFunc worked perfectly!

And thanks to @Thundercat for the strings.NewReader() tip, updated code:

reader := ioutil.NopCloser(strings.NewReader(consulPropertiesFile))

file := &mockFile{
    ReadFunc: func(p []byte) (int, error) {
        return reader.Read(p)
    },
    CloseFunc: func() error {
        return nil
    },
}

For anyone who comes across this post looking for information on how to mock file system etc:

An interface for opening and stat'ing files, plus concrete implementation using os package:

// FileSystem - interface to the filesystem
type FileSystem interface {
    Open(name string) (File, error)
    Stat(name string) (os.FileInfo, error)
}

// OsFS - filesystem backed by default os package
type OsFS struct {
}

// Open - open a file
func (o *OsFS) Open(name string) (File, error) {
    return os.Open(name)
}

// Stat - get file status
func (o *OsFS) Stat(name string) (os.FileInfo, error) {
    return os.Stat(name)
}

File interface mentioned above:

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

Mock the file you want your filesystem to return (implement the methods that will be used by your code):

type mockFile struct {
    CloseFunc  func() error
    ReadFunc   func(p []byte) (int, error)
    ReadAtFunc func(p []byte, off int64) (int, error)
    SeekFunc   func(offset int64, whence int) (int64, error)
    StatFunc   func() (os.FileInfo, error)
}

func (m *mockFile) Close() error {
    return m.CloseFunc()
}

func (m *mockFile) Read(p []byte) (int, error) {
    return m.ReadFunc(p)
}

func (m *mockFile) ReadAt(p []byte, off int64) (int, error) {
    return m.ReadAtFunc(p, off)
}

func (m *mockFile) Seek(offset int64, whence int) (int64, error) {
    return m.SeekFunc(offset, whence)
}

func (m *mockFile) Stat() (os.FileInfo, error) {
    return m.StatFunc()
}

---

reader := ioutil.NopCloser(strings.NewReader("some string, replace with bytes etc"))

file := &mockFile{
    ReadFunc: func(p []byte) (int, error) {
        return reader.Read(p)
    },
    CloseFunc: func() error {
        return nil
    },
}

And return your mockFile via mockFileSystem:

fs := &mockFileSystem{
    OpenFunc: func(name string) (File, error) {
        return file, nil
    },
}