IsGzipped consumes the io.Reader it's reading

[imjasonh]

func TestIsGzipped(t *testing.T) {
  var buf bytes.Buffer
  w := gzip.NewWriter(&buf)
  w.Write([]byte("hello"))

  if is, _ := IsGzipped(&buf); !is {
    t.Errorf("IsGzipped identified contents as not gzipped")
  }

  if is, _ := IsGzipped(&buf); !is {
    t.Errorf("IsGzipped identified contents as not gzipped on retry") // <-- boom
  }
}

IsGzipped determines whether the given reader represents gzipped data by consuming the first two bytes of the reader and looking for a magic header. This consumes the data, so subsequent IsGzipped checks will not identify the contents as gzipped.

This isn't a problem in practice while Readers are only checked once while reading tarballs, but this could become a problem in the future, or if a client decides to call IsGzipped themselves on a Reader before passing it to tarball.Image (naively wrapped in an Opener).

[jonjohnsonjr]

I think we just need something that implements Peek, like bufio.Reader:
https://golang.org/pkg/bufio/#Reader.Peek

Or something that implements ReadAt, like os.File:
https://golang.org/pkg/os/#File.ReadAt

For most things we're already passing around a File, but I think using a buffered Reader might be easier to bolt onto existing code.

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[jonjohnsonjr]

I'm pretty sure this is the reason I can't do this:

$ crane append -f <(tar -c vendor/ -f -) -t gcr.io/jonjohnson-test/some-vendor-directory 
2020/11/29 11:46:07 base unspecified, using empty image
2020/11/29 11:46:12 pushed blob: sha256:1b7c025f16477c734ce6b60bfbaa5d8398f9c66b96d53bdeea129445c3718de0
2020/11/29 11:46:13 pushing image gcr.io/jonjohnson-test/some-vendor-directory: PUT https://gcr.io/v2/jonjohnson-test/some-vendor-directory/blobs/uploads/AJAD5v2UpUS6IPDnYZRCKZWltOKRsrztLTaErFajji4ZaAAAOEBhi3jLCFwptDQ4Qha5CgcT1CZaIC1LLhXvQiLf3DbxOCz-K-emChYIfkSkqlrNEQXWbunn-wm8Ts--FeWsYedBb0bYDMSwblKhCbd4HQXd3kt7akPeSjYmu2MoQYrjkeyAocPT9QD_?digest=sha256%3A2378ec17076387a1b5a3663b6bad06e1cbd2e43716ceb4ca713d1db15b479d6b: DIGEST_INVALID: 'digest' parameter 'sha256:2378ec17076387a1b5a3663b6bad06e1cbd2e43716ceb4ca713d1db15b479d6b' does not match computed digest 'sha256:81da0491c5af5635831f6a3febb5d9bfd66987ba3ecc42e58dc3d80938c25705'.