Add resumable download support via range requests

.gitignore

@@ -10,3 +10,6 @@ cmd/gcrane/gcrane
 cmd/krane/krane
 
 .DS_Store
+coverage.txt
+examples/*/main
+examples/resumable-download/resumable-download

examples/resumable-download/README.md

@@ -0,0 +1,33 @@
+# Resumable Download Example
+
+This example demonstrates how to use the resumable download feature to fetch specific byte ranges from container registry layers using HTTP range requests.
+
+## Usage
+
+```bash
+go run main.go <digest-ref> <start-byte> <end-byte>
+```
+
+## Example
+
+```bash
+# Fetch the first 1024 bytes from a layer
+go run main.go gcr.io/my-repo/my-image@sha256:abc123... 0 1023
+
+# Resume a download starting from byte 1024
+go run main.go gcr.io/my-repo/my-image@sha256:abc123... 1024 2047
+```
+
+## Use Cases
+
+- **Resumable Downloads**: If a download is interrupted, you can resume from where it left off
+- **Partial Content Access**: Access only the portion of a layer you need
+- **Progressive Loading**: Load content incrementally for better user experience
+- **Bandwidth Optimization**: Download only the required portions of large layers
+
+## Notes
+
+- Range requests require a digest reference (not a tag)
+- The byte offsets are inclusive (start and end bytes are both included)
+- Hash verification is not performed on partial content
+- Not all registries support range requests (though most modern ones do)

examples/resumable-download/main.go

@@ -0,0 +1,66 @@
+// Copyright 2024 Google LLC All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This example demonstrates how to use resumable downloads to fetch
+// specific byte ranges from container registry layers.
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
+)
+
+func main() {
+	if len(os.Args) < 4 {
+		fmt.Fprintf(os.Stderr, "Usage: %s <digest-ref> <start-byte> <end-byte>\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "Example: %s gcr.io/my-repo/my-image@sha256:abc123... 0 1023\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	// Parse the digest reference
+	ref, err := name.NewDigest(os.Args[1])
+	if err != nil {
+		log.Fatalf("Failed to parse digest: %v", err)
+	}
+
+	// Parse start and end byte offsets
+	var start, end int64
+	if _, err := fmt.Sscanf(os.Args[2], "%d", &start); err != nil {
+		log.Fatalf("Failed to parse start byte: %v", err)
+	}
+	if _, err := fmt.Sscanf(os.Args[3], "%d", &end); err != nil {
+		log.Fatalf("Failed to parse end byte: %v", err)
+	}
+
+	// Fetch the byte range
+	log.Printf("Fetching bytes %d-%d from %s...", start, end, ref.Name())
+	rc, err := remote.LayerRange(ref, start, end)
+	if err != nil {
+		log.Fatalf("Failed to fetch byte range: %v", err)
+	}
+	defer rc.Close()
+
+	// Copy the range to stdout
+	n, err := io.Copy(os.Stdout, rc)
+	if err != nil {
+		log.Fatalf("Failed to read bytes: %v", err)
+	}
+
+	log.Printf("\nSuccessfully read %d bytes", n)
+}

pkg/v1/remote/README.md

@@ -34,6 +34,46 @@ func main() {
 }
 ```
 
+### Resumable Downloads
+
+The `remote` package supports resumable downloads via HTTP range requests. This is useful for downloading large layers or for resuming interrupted downloads:
+
+```go
+package main
+
+import (
+	"io"
+	"os"
+
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
+)
+
+func main() {
+	// Parse a blob reference (digest)
+	ref, err := name.NewDigest("gcr.io/my-repo/my-image@sha256:abcd...")
+	if err != nil {
+		panic(err)
+	}
+
+	// Download a specific byte range (bytes 1000-9999)
+	// This is useful for resuming downloads or fetching specific portions
+	rc, err := remote.LayerRange(ref, 1000, 9999)
+	if err != nil {
+		panic(err)
+	}
+	defer rc.Close()
+
+	// Copy the range to a file or process it
+	_, err = io.Copy(os.Stdout, rc)
+	if err != nil {
+		panic(err)
+	}
+}
+```
+
+Note: When using range requests, hash verification is not performed on the partial content. To verify the integrity of the full blob, download it completely using `remote.Layer()` instead.
+
 ## Structure
 
 <p align="center">

pkg/v1/remote/doc.go

@@ -14,4 +14,8 @@
 
 // Package remote provides facilities for reading/writing v1.Images from/to
 // a remote image registry.
+//
+// This package supports resumable downloads via HTTP range requests. Use
+// LayerRange to download specific byte ranges of layer blobs, which is useful
+// for resuming interrupted downloads or implementing progressive loading.
 package remote

pkg/v1/remote/fetcher.go

@@ -246,18 +246,41 @@ func (f *fetcher) headManifest(ctx context.Context, ref name.Reference, acceptab
 }
 
 func (f *fetcher) fetchBlob(ctx context.Context, size int64, h v1.Hash) (io.ReadCloser, error) {
+	return f.fetchBlobRange(ctx, size, h, nil)
+}
+
+// ByteRange represents a byte range for partial blob downloads.
+type ByteRange struct {
+	Start int64 // Starting byte offset (inclusive)
+	End   int64 // Ending byte offset (inclusive)
+}
+
+// fetchBlobRange fetches a blob or a byte range of a blob.
+// If byteRange is nil, fetches the entire blob.
+func (f *fetcher) fetchBlobRange(ctx context.Context, size int64, h v1.Hash, byteRange *ByteRange) (io.ReadCloser, error) {
 	u := f.url("blobs", h.String())
 	req, err := http.NewRequest(http.MethodGet, u.String(), nil)
 	if err != nil {
 		return nil, err
 	}
 
+	// Add Range header if byte range is specified
+	if byteRange != nil {
+		req.Header.Set("Range", fmt.Sprintf("bytes=%d-%d", byteRange.Start, byteRange.End))
+	}
+
 	resp, err := f.client.Do(req.WithContext(ctx))
 	if err != nil {
 		return nil, redact.Error(err)
 	}
 
-	if err := transport.CheckError(resp, http.StatusOK); err != nil {
+	// For range requests, we expect either 200 (full content) or 206 (partial content)
+	expectedStatus := http.StatusOK
+	if byteRange != nil {
+		expectedStatus = http.StatusPartialContent
+	}
+
+	if err := transport.CheckError(resp, http.StatusOK, expectedStatus); err != nil {
 		resp.Body.Close()
 		return nil, err
 	}
@@ -266,13 +289,27 @@ func (f *fetcher) fetchBlob(ctx context.Context, size int64, h v1.Hash) (io.Read
 	// If we have an expected size and Content-Length doesn't match, return an error.
 	// If we don't have an expected size and we do have a Content-Length, use Content-Length.
 	if hsize := resp.ContentLength; hsize != -1 {
-		if size == verify.SizeUnknown {
-			size = hsize
-		} else if hsize != size {
-			return nil, fmt.Errorf("GET %s: Content-Length header %d does not match expected size %d", u.String(), hsize, size)
+		// For range requests, Content-Length is the size of the range, not the full blob
+		if byteRange != nil {
+			expectedRangeSize := byteRange.End - byteRange.Start + 1
+			if hsize != expectedRangeSize {
+				return nil, fmt.Errorf("GET %s: Content-Length header %d does not match expected range size %d", u.String(), hsize, expectedRangeSize)
+			}
+		} else {
+			if size == verify.SizeUnknown {
+				size = hsize
+			} else if hsize != size {
+				return nil, fmt.Errorf("GET %s: Content-Length header %d does not match expected size %d", u.String(), hsize, size)
+			}
 		}
 	}
 
+	// For range requests, we cannot verify the hash of partial content
+	if byteRange != nil {
+		// Just return the response body without hash verification
+		return resp.Body, nil
+	}
+
 	return verify.ReadCloser(resp.Body, size, h)
 }
 

pkg/v1/remote/layer.go

@@ -27,16 +27,17 @@ import (
 
 // remoteImagelayer implements partial.CompressedLayer
 type remoteLayer struct {
-	ctx     context.Context
-	fetcher fetcher
-	digest  v1.Hash
+	ctx       context.Context
+	fetcher   fetcher
+	digest    v1.Hash
+	byteRange *ByteRange
 }
 
 // Compressed implements partial.CompressedLayer
 func (rl *remoteLayer) Compressed() (io.ReadCloser, error) {
 	// We don't want to log binary layers -- this can break terminals.
 	ctx := redact.NewContext(rl.ctx, "omitting binary blobs from logs")
-	return rl.fetcher.fetchBlob(ctx, verify.SizeUnknown, rl.digest)
+	return rl.fetcher.fetchBlobRange(ctx, verify.SizeUnknown, rl.digest, rl.byteRange)
 }
 
 // Compressed implements partial.CompressedLayer
@@ -75,3 +76,30 @@ func Layer(ref name.Digest, options ...Option) (v1.Layer, error) {
 	}
 	return newPuller(o).Layer(o.context, ref)
 }
+
+// LayerRange reads a byte range of the given blob reference from a registry as an io.ReadCloser.
+// A blob reference here is just a punned name.Digest where the digest portion is the
+// digest of the blob to be read and the repository portion is the repo where that blob lives.
+//
+// The byte range is specified with start and end offsets (both inclusive).
+// This is useful for resumable downloads where you want to download a specific portion of a layer.
+//
+// Note: Since this returns partial content, hash verification is not performed on the returned data.
+func LayerRange(ref name.Digest, start, end int64, options ...Option) (io.ReadCloser, error) {
+	o, err := makeOptions(options...)
+	if err != nil {
+		return nil, err
+	}
+
+	f, err := makeFetcher(o.context, ref.Context(), o)
+	if err != nil {
+		return nil, err
+	}
+
+	h, err := v1.NewHash(ref.Identifier())
+	if err != nil {
+		return nil, err
+	}
+
+	return f.fetchBlobRange(o.context, verify.SizeUnknown, h, &ByteRange{Start: start, End: end})
+}