imagetools inspect: add --format flag

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

vendor/github.com/moby/buildkit/util/contentutil/fetcher.go

@@ -0,0 +1,73 @@
+package contentutil
+
+import (
+	"context"
+	"io"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/remotes"
+	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+)
+
+func FromFetcher(f remotes.Fetcher) content.Provider {
+	return &fetchedProvider{
+		f: f,
+	}
+}
+
+type fetchedProvider struct {
+	f remotes.Fetcher
+}
+
+func (p *fetchedProvider) ReaderAt(ctx context.Context, desc ocispecs.Descriptor) (content.ReaderAt, error) {
+	rc, err := p.f.Fetch(ctx, desc)
+	if err != nil {
+		return nil, err
+	}
+
+	return &readerAt{Reader: rc, Closer: rc, size: desc.Size}, nil
+}
+
+type readerAt struct {
+	io.Reader
+	io.Closer
+	size   int64
+	offset int64
+}
+
+func (r *readerAt) ReadAt(b []byte, off int64) (int, error) {
+	if ra, ok := r.Reader.(io.ReaderAt); ok {
+		return ra.ReadAt(b, off)
+	}
+
+	if r.offset != off {
+		if seeker, ok := r.Reader.(io.Seeker); ok {
+			if _, err := seeker.Seek(off, io.SeekStart); err != nil {
+				return 0, err
+			}
+			r.offset = off
+		} else {
+			return 0, errors.Errorf("unsupported offset")
+		}
+	}
+
+	var totalN int
+	for len(b) > 0 {
+		n, err := r.Reader.Read(b)
+		if err == io.EOF && n == len(b) {
+			err = nil
+		}
+		r.offset += int64(n)
+		totalN += n
+		b = b[n:]
+		if err != nil {
+			return totalN, err
+		}
+	}
+	return totalN, nil
+}
+
+func (r *readerAt) Size() int64 {
+	return r.size
+}