Cached Reader

Cached Reader is an implementation of a reader that could be re-read from the beginning. One of the scenarios is when you need to verify the file header or request data by examining content at the beginning of the stream.

Install

  go get github.com/testhub-io/cachedreader

Example 1. Minimal example

package main

import (
	"bytes"
	"fmt"
	"io"

	"github.com/testhub-io/cachedreader"
)

func main() {
	var str = []byte("the quick brown fox jumps over the lazy dog")
	r := cachedreader.NewCachedReader(bytes.NewReader(str)) // create cached reader
	the := make([]byte, 3)
	_, err := r.Read(the) // Read first 3 bytes
	if err != nil {
		panic(err)
	}
	fmt.Println(string(the)) // print "the"

	b, err := io.ReadAll(r)
	if err != nil {
		panic(err)
	}
	fmt.Println(string(b)) // print whole string
}

Example 2. Check if http request body is a valid tar.gz file and store it

// http handler 
func storeFile(w http.ResponseWriter, req *http.Request) {

	bufReader := cachedreader.NewCachedReader(req.Body)

	uncompressedStream, err := gzip.NewReader(bufReader) // uncompress the gzip stream
	if err != nil {
		w.Write([]byte(fmt.Sprintf("Failed to read gz stream. %v", err)))
		return
	}

	tr := tar.NewReader(uncompressedStream) // create tar reader
	_, err = tr.Next() // read the file header
	if err != nil { // if error then stream is no a valid tar file
		w.Write([]byte(fmt.Sprintf("Not a tar.gz stream. %v", err)))
		return
	}
	bufReader.Reset() // reset reader to the biggining

	out, err := os.Create("filename.tar.gz")
	if err != nil {
		w.Write([]byte(fmt.Sprintf("Error creating file. %v", err)))
		return
	}
	defer out.Close() 

	io.Copy(out, bufReader) // write content to a file form the beginning of the stream
}