Support gs:// URI scheme in hammer (#314)

* Support `gs://` URI scheme in hammer

* Move `GSFetcher` to `internal/client/gcp` package

Author: roger2hk

internal/client/fetcher.go

@@ -81,7 +81,7 @@ func (h HTTPFetcher) fetch(ctx context.Context, p string) ([]byte, error) {
 		break
 	case http.StatusNotFound:
 		// Need to return ErrNotExist here, by contract.
-		return nil, fmt.Errorf("get(%q): %v", u.String(), os.ErrNotExist)
+		return nil, fmt.Errorf("get(%q): %w", u.String(), os.ErrNotExist)
 	default:
 		return nil, fmt.Errorf("get(%q): %v", u.String(), r.StatusCode)
 	}

internal/client/gcp/fetcher.go

@@ -0,0 +1,74 @@
+// Copyright 2025 The Tessera authors. 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.
+
+package gcp
+
+import (
+	"context"
+	"fmt"
+	"io"
+
+	gcs "cloud.google.com/go/storage"
+	"github.com/transparency-dev/tessera/api/layout"
+)
+
+// NewGSFetcher creates a new GSFetcher for the Google Cloud Storage bucket, using
+// the provided GCS client.
+//
+// bucket should not contain any slash.
+// c may be nil, in which case a new GCS client will be used.
+func NewGSFetcher(ctx context.Context, bucket string, c *gcs.Client) (*GSFetcher, error) {
+	if c == nil {
+		var err error
+		c, err = gcs.NewClient(ctx, gcs.WithJSONReads())
+		if err != nil {
+			return nil, err
+		}
+	}
+	return &GSFetcher{
+		bucket: bucket,
+		c:      c,
+	}, nil
+}
+
+// GSFetcher knows how to fetch log artifacts from a Google Cloud Storage bucket.
+type GSFetcher struct {
+	bucket string
+	c      *gcs.Client
+}
+
+func (f GSFetcher) fetch(ctx context.Context, p string) ([]byte, error) {
+	r, err := f.c.Bucket(f.bucket).Object(p).NewReader(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("getObject: failed to create reader for object %q in bucket %q: %w", p, f.bucket, err)
+	}
+
+	d, err := io.ReadAll(r)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read %q: %v", p, err)
+	}
+	return d, r.Close()
+}
+
+func (f GSFetcher) ReadCheckpoint(ctx context.Context) ([]byte, error) {
+	return f.fetch(ctx, layout.CheckpointPath)
+}
+
+func (f GSFetcher) ReadTile(ctx context.Context, l, i uint64, p uint8) ([]byte, error) {
+	return f.fetch(ctx, layout.TilePath(l, i, p))
+}
+
+func (f GSFetcher) ReadEntryBundle(ctx context.Context, i uint64, p uint8) ([]byte, error) {
+	return f.fetch(ctx, fmt.Sprintf("tile/data/%s", layout.NWithSuffix(0, i, p)))
+}

internal/hammer/hammer.go

@@ -112,7 +112,7 @@ func main() {
 		klog.Exitf("Failed to create verifier: %v", err)
 	}
 
-	f, w, err := loadtest.NewLogClients(logURL, writeLogURL, loadtest.ClientOpts{
+	f, w, err := loadtest.NewLogClients(ctx, logURL, writeLogURL, loadtest.ClientOpts{
 		Client:           hc,
 		BearerToken:      *bearerToken,
 		BearerTokenWrite: *bearerTokenWrite,

internal/hammer/loadtest/client.go

@@ -29,6 +29,7 @@ import (
 	"time"
 
 	"github.com/transparency-dev/tesseract/internal/client"
+	"github.com/transparency-dev/tesseract/internal/client/gcp"
 	"github.com/transparency-dev/tesseract/internal/types/rfc6962"
 	"github.com/transparency-dev/tesseract/internal/types/staticct"
 	"k8s.io/klog/v2"
@@ -51,7 +52,7 @@ type ClientOpts struct {
 
 // NewLogClients returns a fetcher and a writer that will read
 // and write leaves to all logs in the `log_url` flag set.
-func NewLogClients(readLogURLs, writeLogURLs []string, opts ClientOpts) (LogReader, LeafWriter, error) {
+func NewLogClients(ctx context.Context, readLogURLs, writeLogURLs []string, opts ClientOpts) (LogReader, LeafWriter, error) {
 	if len(readLogURLs) == 0 {
 		return nil, nil, fmt.Errorf("URL(s) for reading log must be provided")
 	}
@@ -75,7 +76,7 @@ func NewLogClients(readLogURLs, writeLogURLs []string, opts ClientOpts) (LogRead
 
 	fetchers := []fetcher{}
 	for _, s := range readLogURLs {
-		fetchers = append(fetchers, newFetcher(rootUrlOrDie(s), opts.BearerToken))
+		fetchers = append(fetchers, newFetcher(ctx, rootUrlOrDie(s), opts.BearerToken))
 	}
 	writers := []httpLeafWriter{}
 	for _, s := range writeLogURLs {
@@ -89,7 +90,7 @@ func NewLogClients(readLogURLs, writeLogURLs []string, opts ClientOpts) (LogRead
 }
 
 // newFetcher creates a Fetcher for the log at the given root location.
-func newFetcher(root *url.URL, bearerToken string) fetcher {
+func newFetcher(ctx context.Context, root *url.URL, bearerToken string) fetcher {
 	switch root.Scheme {
 	case "http", "https":
 		c, err := client.NewHTTPFetcher(root, nil)
@@ -102,6 +103,12 @@ func newFetcher(root *url.URL, bearerToken string) fetcher {
 		return c
 	case "file":
 		return client.FileFetcher{Root: root.Path}
+	case "gs":
+		c, err := gcp.NewGSFetcher(ctx, root.Host, nil)
+		if err != nil {
+			klog.Exitf("NewGSFetcher: %v", err)
+		}
+		return c
 	}
 	klog.Exitf("Unknown scheme on log URL: %q", root.Scheme)
 	return nil