Download a zip file using io.Pipe() read/write golang - go

I am trying to stream out bytes of a zip file using io.Pipe() function in golang. I am using pipe reader to read the bytes of each file in the zip and then stream those out and use the pipe writer to write the bytes in the response object.
func main() {
r, w := io.Pipe()
// go routine to make the write/read non-blocking
go func() {
defer w.Close()
bytes, err := ReadBytesforEachFileFromTheZip()
err := json.NewEncoder(w).Encode(bytes)
handleErr(err)
}()
This is not a working implementation but a structure of what I am trying to achieve. I don't want to use ioutil.ReadAll since the file is going to be very large and Pipe() will help me avoid bringing all the data into memory. Can someone help with a working implementation using io.Pipe() ?

I made it work using golang io.Pipe().The Pipewriter writes byte to the pipe in chunks and the pipeReader reader from the other end. The reason for using a go-routine is to have a non-blocking write operation while simultaneous reads happen form the pipe.
Note: It's important to close the pipe writer (w.Close()) to send EOF on the stream otherwise it will not close the stream.
func DownloadZip() ([]byte, error) {
r, w := io.Pipe()
defer r.Close()
defer w.Close()
zip, err := os.Stat("temp.zip")
if err != nil{
return nil, err
}
go func(){
f, err := os.Open(zip.Name())
if err != nil {
return
}
buf := make([]byte, 1024)
for {
chunk, err := f.Read(buf)
if err != nil && err != io.EOF {
panic(err)
}
if chunk == 0 {
break
}
if _, err := w.Write(buf[:chunk]); err != nil{
return
}
}
w.Close()
}()
body, err := ioutil.ReadAll(r)
if err != nil {
return nil, err
}
return body, nil
}
Please let me know if someone has another way of doing it.

Related

Processing data in chunks with io.ReadFull results in corrupted file?

I'm trying to download and decrypt HLS streams by using io.ReadFull to process the data in chunks to conserve memory:
Irrelevant parts of code has been left out for simplicity.
func main() {
f, _ := os.Create(out.ts)
for _, v := range mediaPlaylist {
resp, _ := http.Get(v.URI)
for {
r, err := decryptHLS(key, iv, resp.Body)
if err != nil && err == io.EOF {
break
else if err != nil && err != io.ErrUnexpectedEOF {
panic(err)
}
io.Copy(f, r)
}
}
}
func decryptHLS(key []byte, iv []byte, r io.Reader) (io.Reader, error) {
block, _ := aes.NewCipher(key)
buf := make([]byte, 8192)
mode := cipher.NewCBCDecrypter(block, iv)
n, err := io.ReadFull(r, buf)
if err != nil && err != io.ErrUnexpectedEOF {
return nil, err
}
mode.CryptBlocks(buf, buf)
return bytes.NewReader(buf[:n]), err
}
At first this seems to work as file size is correct and no errors during download,
but the video is corrupted. Not completely as the file is still recognized as a video, but image and sound is distorted.
If I change the code to use ioutil.ReadAll instead, the final video files will no longer be corrupted:
func main() {
f, _ := os.Create(out.ts)
for _, v := range mediaPlaylist {
resp, _ := http.Get(v.URI)
segment, _ := ioutil.ReadAll(resp.Body)
r, _ := decryptHLS(key, iv, &segment)
io.Copy(f, r)
}
}
func decryptHLS(key []byte, iv []byte, s *[]byte) io.Reader {
block, _ := aes.NewCipher(key)
mode := cipher.NewCBCDecrypter(block, iv)
mode.CryptBlocks(*s, *s)
return bytes.NewReader(*s)
}
Any ideas why it works correctly when reading the entire segment into memory, and not when using io.ReadFull and processing it in chunks?
Internally, CBCDecrypter makes a copy of your iv, so subsequent blocks start with the initial IV rather than the one that's been mutated by previous decryptions.
Create the decrypter once, and you should be able to keep re-using it to decrypt block by block (assuming the block size is a multiple of the block size expected by this crypto algorithm).

Reading from a named pipe won't give any output and blocks the code indefinitely

I wrote a piece of code with a IPC purpose. The expected behaviour is that the code reads the content from the named-pipe and prints the string (with the Send("log", buff.String())). First I open the named-pipe 'reader' inside the goroutine, while the reader is open I send a signal that the data can be written to the named-pipe (with the Send("datarequest", "")). Here is the code:
var wg sync.WaitGroup
wg.Add(1)
go func() {
//reader part
file, err := os.OpenFile("tmp/"+os.Args[1], os.O_RDONLY, os.ModeNamedPipe)
if err != nil {
Send("error", err.Error())
}
var buff bytes.Buffer
_, err = io.Copy(&buff, file)
Send("log", buff.String())
if err != nil {
Send("error", err.Error())
}
wg.Done()
}()
Send("datarequest", "")
wg.Wait()
And here is the code which executes when the signal is send:
//writer part
file, err := os.OpenFile("tmp/" + execID, os.O_WRONLY, 0777)
if err != nil {
c <- "[error] error opening file: " + err.Error()
}
bytedata, _ := json.Marshal(moduleParameters)
file.Write(bytedata)
So the behaviour I get it that the code blocks indefinitely when I try to copy it. I really don't know why this happens. When I test it with cat in the terminal I do get the intended result so my question is how do I get the same result with code?
Edit
The execID is the same as os.Args[1]
The writer should close the file after it's done sending using file.Close(). Note that file.Close() may return error.

Copying stdin to a buffer

I want to copy a os.Stdin string to a buffer, to check for a user inputted text (e.g. "hibye") and put an if statement against it.
My current code just handles simple stdin stdiout copy operations (no buffer):
func interact(c net.Conn) {
// Read from Reader and write to Writer until EOF()
copy := func(r io.ReadCloser, w io.WriteCloser) {
defer func() {
r.Close()
w.Close()
}()
n, err := io.Copy(w, r)
if err != nil {
log.Printf("[%s]: ERROR: %s\n", c.RemoteAddr(), err)
log.Println(n)
}
}
go copy(c, os.Stdout)
go copy(os.Stdin, c)
}
Question: What is the most efficient way to implement a use of a buffer to have control over the passed strings?
bad example (failed attempt):
buf := make([]byte, 1024)
go copy (os.Stdin, []byte(buf))
if buf == "hibye" {
do stuff
}

Go defer - open and copy files in a loop

If I'm opening a file inside a for loop and will be finished with it at the end of that iteration, should I call Close immediately or trick Defer using a closure?
I have a series of filenames being read in from a chan string which have data to be copied into a zipfile. This is all being processed in a go func.
go func(fnames <-chan string, zipfilename string) {
f, _ := os.Create(zipfilename) // ignore error handling for this example
defer f.Close()
zf := zip.NewWriter(f)
defer zf.Close()
for fname := range fnames {
r, _ := os.Open(fname)
w, _ := zf.Create(r.Name())
io.Copy(w, r)
w.Close()
r.Close()
}(files, "some name.zip")
Inside my for loop, would it be more idiomatic Go to write:
for fname := range fnames {
func(){
r, _ := os.Open(fname)
defer r.Close()
w, _ := zf.Create(r.Name())
defer w.Close()
io.Copy(w, r)
}()
}
or should I continue with my code as-written?
You should be checking your errors. I know this is meant to just be an example, but in this case it is important. If all you do is defer Close(), you can't actually check if there was an error during defer.
The way I would write this is to create a helper function:
func copyFileToZip(zf *zip.Writer, filename string) error {
r, err := os.Open(filename)
if err != nil {
return err
}
defer r.Close()
w, err := zf.Create(r.Name())
if err != nil {
return err
}
defer w.Close()
_, err = io.Copy(w, r)
if err != nil {
return err
}
return w.Close()
}
Once you add in all that error handling, the function is big enough to make it a named function. It also has the added benefit of checking the error when closing the writer. Checking the reader's error is unnecessary since that won't affect if the data was written.

using io.Pipes() for sending and receiving message

I am using os.Pipes() in my program, but for some reason it gives a bad file descriptor error each time i try to write or read data from it.
Is there some thing I am doing wrong?
Below is the code
package main
import (
"fmt"
"os"
)
func main() {
writer, reader, err := os.Pipe()
if err != nil {
fmt.Println(err)
}
_,err= writer.Write([]byte("hello"))
if err != nil {
fmt.Println(err)
}
var data []byte
_, err = reader.Read(data)
if err != nil {
fmt.Println(err)
}
fmt.Println(string(data))
}
output :
write |0: Invalid argument
read |1: Invalid argument
You are using an os.Pipe, which returns a pair of FIFO connected files from the os. This is different than an io.Pipe which is implemented in Go.
The invalid argument errors are because you are reading and writing to the wrong files. The signature of os.Pipe is
func Pipe() (r *File, w *File, err error)
which shows that the returns values are in the order "reader, writer, error".
and io.Pipe:
func Pipe() (*PipeReader, *PipeWriter)
Also returning in the order "reader, writer"
When you check the error from the os.Pipe function, you are only printing the value. If there was an error, the files are invalid. You need to return or exit on that error.
Pipes are also blocking (though an os.Pipe has a small, hard coded buffer), so you need to read and write asynchronously. If you swapped this for an io.Pipe it would deadlock immediately. Dispatch the Read method inside a goroutine and wait for it to complete.
Finally, you are reading into a nil slice, which will read nothing. You need to allocate space to read into, and you need to record the number of bytes read to know how much of the buffer is used.
A more correct version of your example would look like:
reader, writer, err := os.Pipe()
if err != nil {
log.Fatal(err)
}
var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.Done()
data := make([]byte, 1024)
n, err = reader.Read(data)
if n > 0 {
fmt.Println(string(data[:n]))
}
if err != nil && err != io.EOF {
fmt.Println(err)
}
}()
_, err = writer.Write([]byte("hello"))
if err != nil {
fmt.Println(err)
}
wg.Wait()

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