How to pass io.Reader instance to a function in Golang?

I've been dealing with an issue where I have to put an io.Reader instance as a parameter to a function provided as an end-point by an api. The task I need to do is to upload local folder to company's cloud storage.

func (s *server) uploadFileToPutIo(w http.ResponseWriter, r *http.Request) {
    tokenSource := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: token})
    oauthClient := oauth2.NewClient(context.TODO(), tokenSource)
    client := putio.NewClient(oauthClient)

    var testIO io.Reader // ? 

    upload, err := client.Files.Upload(context.TODO(), testIO, "test", 0)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(upload.File)
    sendResponse(w, []byte("successful"), http.StatusOK)
}

When I make a request to this end-point /upload under POST method. I get the following error.

2021/12/01 18:28:47 http: panic serving 127.0.0.1:61057: runtime error: invalid memory address or nil pointer dereference
goroutine 8 [running]:
net/http.(*conn).serve.func1(0xc000108d20)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:1804 +0x153
panic(0x1390ae0, 0x164fdd0)
        /usr/local/Cellar/go/1.16.6/libexec/src/runtime/panic.go:971 +0x499
io.copyBuffer(0x1462700, 0xc000026360, 0x0, 0x0, 0xc000170000, 0x8000, 0x8000, 0x0, 0x0, 0x13d5e01)
        /usr/local/Cellar/go/1.16.6/libexec/src/io/io.go:423 +0x10b
io.Copy(...)
        /usr/local/Cellar/go/1.16.6/libexec/src/io/io.go:382
github.com/putdotio/go-putio/putio.(*FilesService).Upload(0xc000010088, 0x1468390, 0xc00001c088, 0x0, 0x0, 0x13ef46f, 0x6, 0x0, 0x170d108, 0x90, ...)
        /Users/barisertas/go/pkg/mod/github.com/putdotio/go-putio/putio@v0.0.0-20200123120452-16d982cac2b8/files.go:235 +0x187
main.(*server).uploadFileToPutIo(0xc000010028, 0x1467d60, 0xc00014a2a0, 0xc000154500)
        /Users/barisertas/workspace/mini-project/api/handler.go:79 +0xe5
net/http.HandlerFunc.ServeHTTP(0xc000012db0, 0x1467d60, 0xc00014a2a0, 0xc000154500)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:2049 +0x44
github.com/gorilla/mux.(*Router).ServeHTTP(0xc000144000, 0x1467d60, 0xc00014a2a0, 0xc000154300)
        /Users/barisertas/go/pkg/mod/github.com/gorilla/mux@v1.8.0/mux.go:210 +0xd3
net/http.serverHandler.ServeHTTP(0xc00014a000, 0x1467d60, 0xc00014a2a0, 0xc000154300)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:2867 +0xa3
net/http.(*conn).serve(0xc000108d20, 0x1468400, 0xc00005e300)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:1932 +0x8cd
created by net/http.(*Server).Serve
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:2993 +0x39b
2021/12/01 18:28:47 http: panic serving 127.0.0.1:61059: runtime error: invalid memory address or nil pointer dereference
goroutine 11 [running]:
net/http.(*conn).serve.func1(0xc000108dc0)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:1804 +0x153
panic(0x1390ae0, 0x164fdd0)
        /usr/local/Cellar/go/1.16.6/libexec/src/runtime/panic.go:971 +0x499
io.copyBuffer(0x1462700, 0xc000026400, 0x0, 0x0, 0xc000198000, 0x8000, 0x8000, 0x0, 0x0, 0x13d5e01)
        /usr/local/Cellar/go/1.16.6/libexec/src/io/io.go:423 +0x10b
io.Copy(...)
        /usr/local/Cellar/go/1.16.6/libexec/src/io/io.go:382
github.com/putdotio/go-putio/putio.(*FilesService).Upload(0xc0000100c8, 0x1468390, 0xc00001c088, 0x0, 0x0, 0x13ef46f, 0x6, 0x0, 0x170d108, 0x90, ...)
        /Users/barisertas/go/pkg/mod/github.com/putdotio/go-putio/putio@v0.0.0-20200123120452-16d982cac2b8/files.go:235 +0x187
main.(*server).uploadFileToPutIo(0xc000010028, 0x1467d60, 0xc00014a380, 0xc000154800)
        /Users/barisertas/workspace/mini-project/api/handler.go:79 +0xe5
net/http.HandlerFunc.ServeHTTP(0xc000012db0, 0x1467d60, 0xc00014a380, 0xc000154800)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:2049 +0x44
github.com/gorilla/mux.(*Router).ServeHTTP(0xc000144000, 0x1467d60, 0xc00014a380, 0xc000154600)
        /Users/barisertas/go/pkg/mod/github.com/gorilla/mux@v1.8.0/mux.go:210 +0xd3
net/http.serverHandler.ServeHTTP(0xc00014a000, 0x1467d60, 0xc00014a380, 0xc000154600)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:2867 +0xa3
net/http.(*conn).serve(0xc000108dc0, 0x1468400, 0xc00005e580)
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:1932 +0x8cd
created by net/http.(*Server).Serve
        /usr/local/Cellar/go/1.16.6/libexec/src/net/http/server.go:2993 +0x39b

And this is the documentation of the function I am trying to use:

// Upload reads from given io.Reader and uploads the file contents to Put.io
// servers under directory given by parent. If parent is negative, user's
// preferred folder is used.
//
// If the uploaded file is a torrent file, Put.io will interpret it as a
// transfer and Transfer field will be present to represent the status of the
// tranfer. Likewise, if the uploaded file is a regular file, Transfer field
// would be nil and the uploaded file will be represented by the File field.
//
// This method reads the file contents into the memory, so it should be used for
// <150MB files.
func (f *FilesService) Upload(ctx context.Context, r io.Reader, filename string, parent int64) (Upload, error) {
    if filename == "" {
        return Upload{}, fmt.Errorf("filename cannot be empty")
    }

    var buf bytes.Buffer
    mw := multipart.NewWriter(&buf)

    // negative parent means use user's preferred download folder.
    if parent >= 0 {
        err := mw.WriteField("parent_id", itoa(parent))
        if err != nil {
            return Upload{}, err
        }
    }

    formfile, err := mw.CreateFormFile("file", filename)
    if err != nil {
        return Upload{}, err
    }

    _, err = io.Copy(formfile, r)
    if err != nil {
        return Upload{}, err
    }

    err = mw.Close()
    if err != nil {
        return Upload{}, err
    }

    req, err := f.client.NewRequest(ctx, "POST", "/v2/files/upload", &buf)
    if err != nil {
        return Upload{}, err
    }
    req.Header.Set("Content-Type", mw.FormDataContentType())

    var response struct {
        Upload
    }
    _, err = f.client.Do(req, &response)
    if err != nil {
        return Upload{}, err
    }
    return response.Upload, nil
}

I am confused about putting the io.Reader instance to a function. How can I put the io.Reader function properly under this case. Just an instance after creating as follows: var testIO io.Reader or should I do some extra operations?

Solution

This:

var testIO io.Reader

is equivalent to this:

testIO := io.Reader(nil)

so this is why you are getting a panic of a nil-pointer reference:

2021/12/01 18:28:47 http: panic serving 127.0.0.1:61059: runtime error: invalid memory address or nil pointer dereference
goroutine 11 [running]:

An io.Reader is an interface which allows one to pass generic values, provided they implement the interface (i.e. implement the Read method).

Since you are uploading a file, your byte stream should come from an OS file. os.File implements the correct Read method - so is compatible io.Reader.

So try:

f, err := os.Open(uploadFilePath)
if err != nil { /* ... */ }

upload, err := client.Files.Upload(context.TODO(), f, "test", 0)