Add global expanded APK cache

pkg/apk/implementation.go

@@ -30,6 +30,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/chainguard-dev/go-apk/pkg/expandapk"
@@ -48,6 +49,11 @@ import (
 	"github.com/hashicorp/go-retryablehttp"
 )
 
+// This is terrible but simpler than plumbing around a cache for now.
+// We just hold the expanded APK in memory rather than re-parsing it every time,
+// which is expensive. This also dedupes simultaneous fetches.
+var globalApkCache = &apkCache{}
+
 type APK struct {
 	arch              string
 	version           string
@@ -750,7 +756,53 @@ func (a *APK) cachedPackage(ctx context.Context, pkg *repository.RepositoryPacka
 	return &exp, nil
 }
 
+type apkResult struct {
+	exp *expandapk.APKExpanded
+	err error
+}
+
+type apkCache struct {
+	// url -> *sync.Once
+	onces sync.Map
+
+	// url -> apkResult
+	resps sync.Map
+}
+
+func (c *apkCache) get(ctx context.Context, a *APK, pkg *repository.RepositoryPackage) (*expandapk.APKExpanded, error) {
+	u := pkg.Url()
+	// Do all the expensive things inside the once.
+	once, _ := c.onces.LoadOrStore(u, &sync.Once{})
+	once.(*sync.Once).Do(func() {
+		exp, err := expandPackage(ctx, a, pkg)
+		c.resps.Store(u, apkResult{
+			exp: exp,
+			err: err,
+		})
+	})
+
+	v, ok := c.resps.Load(u)
+	if !ok {
+		panic(fmt.Errorf("did not see apk %q after writing it", u))
+	}
+
+	result := v.(apkResult)
+	return result.exp, result.err
+}
+
 func (a *APK) expandPackage(ctx context.Context, pkg *repository.RepositoryPackage) (*expandapk.APKExpanded, error) {
+	if a.cache == nil {
+		// If we don't have a cache configured, don't use the global cache.
+		// Calling APKExpanded.Close() will clean up a tempdir.
+		// This is fine when we have a cache because we move all the backing files into the cache.
+		// This is not fine when we don't have a cache because the tempdir contains all our state.
+		return expandPackage(ctx, a, pkg)
+	}
+
+	return globalApkCache.get(ctx, a, pkg)
+}
+
+func expandPackage(ctx context.Context, a *APK, pkg *repository.RepositoryPackage) (*expandapk.APKExpanded, error) {
 	ctx, span := otel.Tracer("go-apk").Start(ctx, "expandPackage", trace.WithAttributes(attribute.String("package", pkg.Name)))
 	defer span.End()