WriterSeeker

WriterSeeker is the in-memory io.WriteSeeker implementation missing in the standard lib :-)

Use-case

In serverless / PaaS environments there is usually no file system access - you cannot read or write files to the container you are running in. This means that if you are using a function or library in Go that expects a File type (which implements the io.WriteSeeker interface), you are pretty much screwed. WriterSeeker solves this by letting you write and seek inside an in-memory buffer.

Usage Example

Let's say that you are using a library to generate PDF files. The library usually expects a File type to perform the writing to. You would create a File by using os.Open and then feed this to the Write function like so:

fWrite, err := os.Create(outputPath)
if err != nil {
    return err
}

defer fWrite.Close()

err = pdfWriter.Write(fWrite)

With WriterSeeker, you do not need the file, just work in-memory:

writerSeeker := &writerseeker.WriterSeeker{}
err = pdfWriter.Write(writerSeeker)

Now you can get a an io.Reader from the writerSeeker instance and boogie! for example, copy it's buffer to an io.Writer.

r := writerSeeker.Reader()
w := getWriter()
if _, err := io.Copy(w, r); err != nil {
 ...
 ...
 ...
}

License

The code is MIT licensed. It uses code from this post on StackOverflow, and according to the official docs this code can be safely used with MIT license.