Tidy up action cache asset store

The Action Cache Asset Store is a bit hard to work with at the moment.  Lots of
logic is included in the `Get` and `Put` methods that can be extracted and
shared, while also clarifying what's really going on in these methods.

I've also modified some of the utility functions to be more generic which has
removed a load of boilerplate.

pkg/fetch/caching_fetcher_test.go

@@ -35,7 +35,7 @@ func TestFetchBlobCaching(t *testing.T) {
 		Uris:         []string{uri},
 	}
 	blobDigest := &remoteexecution.Digest{Hash: "d0d829c4c0ce64787cb1c998a9c29a109f8ed005633132fda4f29982487b04db", SizeBytes: 123}
-	refDigest, err := storage.AssetReferenceToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
+	refDigest, err := storage.ProtoToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
 	require.NoError(t, err)
 
 	t.Logf("Ref digest was %v", refDigest)
@@ -93,7 +93,7 @@ func TestFetchDirectoryCaching(t *testing.T) {
 		Uris:         []string{uri},
 	}
 	dirDigest := &remoteexecution.Digest{Hash: "d0d829c4c0ce64787cb1c998a9c29a109f8ed005633132fda4f29982487b04db", SizeBytes: 123}
-	refDigest, err := storage.AssetReferenceToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
+	refDigest, err := storage.ProtoToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
 	require.NoError(t, err)
 
 	backend := mock.NewMockBlobAccess(ctrl)
@@ -148,7 +148,7 @@ func TestCachingFetcherExpiry(t *testing.T) {
 		InstanceName: "foo",
 		Uris:         []string{uri},
 	}
-	refDigest, err := storage.AssetReferenceToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
+	refDigest, err := storage.ProtoToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
 	require.NoError(t, err)
 
 	backend := mock.NewMockBlobAccess(ctrl)
@@ -188,7 +188,7 @@ func TestCachingFetcherOldestContentAccepted(t *testing.T) {
 		Uris:                  []string{uri},
 		OldestContentAccepted: timestamppb.Now(),
 	}
-	refDigest, err := storage.AssetReferenceToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
+	refDigest, err := storage.ProtoToDigest(storage.NewAssetReference([]string{uri}, []*remoteasset.Qualifier{}), instanceName)
 	require.NoError(t, err)
 
 	backend := mock.NewMockBlobAccess(ctrl)

pkg/fetch/remote_execution_fetcher.go

@@ -46,7 +46,7 @@ func (rf *remoteExecutionFetcher) fetchCommon(ctx context.Context, req *remoteas
 	}
 	for _, uri := range req.Uris {
 		command := commandGenerator(uri)
-		commandDigest, err := storage.ProtoToDigest(command)
+		_, commandDigest, err := storage.ProtoSerialise(command)
 		if err != nil {
 			return nil, "", "", err
 		}
@@ -59,7 +59,7 @@ func (rf *remoteExecutionFetcher) fetchCommon(ctx context.Context, req *remoteas
 			CommandDigest:   commandDigest,
 			InputRootDigest: storage.EmptyDigest,
 		}
-		actionDigest, err := storage.ProtoToDigest(action)
+		_, actionDigest, err := storage.ProtoSerialise(action)
 		if err != nil {
 			return nil, "", "", err
 		}
@@ -208,7 +208,7 @@ func (rf *remoteExecutionFetcher) FetchDirectory(ctx context.Context, req *remot
 		return nil, err
 	}
 	root := tree.(*remoteexecution.Tree).Root
-	rootDigest, err := storage.ProtoToDigest(root)
+	_, rootDigest, err := storage.ProtoSerialise(root)
 	if err != nil {
 		return nil, err
 	}
@@ -221,7 +221,7 @@ func (rf *remoteExecutionFetcher) FetchDirectory(ctx context.Context, req *remot
 		return nil, err
 	}
 	for _, child := range tree.(*remoteexecution.Tree).Children {
-		childDigest, err := storage.ProtoToDigest(child)
+		_, childDigest, err := storage.ProtoSerialise(child)
 		if err != nil {
 			return nil, err
 		}

pkg/push/push_server_test.go

@@ -42,7 +42,7 @@ func TestPushServerPushBlobSuccess(t *testing.T) {
 		BlobDigest:   blobDigest,
 		Qualifiers:   qualifiers,
 	}
-	refDigest, err := storage.AssetReferenceToDigest(storage.NewAssetReference([]string{uri}, qualifiers), instanceName)
+	refDigest, err := storage.ProtoToDigest(storage.NewAssetReference([]string{uri}, qualifiers), instanceName)
 	require.NoError(t, err)
 
 	backend := mock.NewMockBlobAccess(ctrl)
@@ -82,7 +82,7 @@ func TestPushServerPushDirectorySuccess(t *testing.T) {
 		RootDirectoryDigest: rootDirectoryDigest,
 		Qualifiers:          qualifiers,
 	}
-	refDigest, err := storage.AssetReferenceToDigest(storage.NewAssetReference([]string{uri}, qualifiers), instanceName)
+	refDigest, err := storage.ProtoToDigest(storage.NewAssetReference([]string{uri}, qualifiers), instanceName)
 	require.NoError(t, err)
 
 	backend := mock.NewMockBlobAccess(ctrl)