Add a GetManifests function.

Intended as an implementation detail of parallel `getPages`.

src/backend.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"slices"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -21,6 +22,10 @@ func splitBlobName(name string) []string {
 	}
 }
 
+type GetManifestOptions struct {
+	BypassCache bool
+}
+
 type Backend interface {
 	// Retrieve a blob. Returns `reader, size, mtime, err`.
 	GetBlob(ctx context.Context, name string) (reader io.ReadSeeker, size uint64, mtime time.Time, err error)
@@ -52,8 +57,40 @@ type Backend interface {
 	CheckDomain(ctx context.Context, domain string) (found bool, err error)
 }
 
-type GetManifestOptions struct {
-	BypassCache bool
+// Retrieve several manifests. This operation succeeds if all requested manifests could be
+// retrieved, and fails otherwise. The returned error is the first error that occurs.
+func GetManifests(
+	backend Backend, ctx context.Context, names []string, opts GetManifestOptions,
+) (
+	manifests map[string]*Manifest, err error,
+) {
+	type Result struct {
+		name     string
+		manifest *Manifest
+		err      error
+	}
+
+	wg := sync.WaitGroup{}
+	ch := make(chan Result, len(names))
+	for _, name := range names {
+		wg.Go(func() {
+			manifest, err := backend.GetManifest(ctx, name, opts)
+			ch <- Result{name, manifest, err}
+		})
+	}
+	wg.Wait()
+	close(ch)
+
+	manifests = make(map[string]*Manifest)
+	for result := range ch {
+		if result.err == nil {
+			manifests[result.name] = result.manifest
+		} else {
+			err = result.err
+			break
+		}
+	}
+	return
 }
 
 var backend Backend