refactor(tvix/nar-bridge): have Export return root node

… and nar size / sha256 digest.

Instead of producing sparse PathInfo messages when NARs are sent to
nar-bridge, the nar-bridge http server now keeps a lookup table
(narsha256) -> (rootNode, narSize)

This removes a whole bunch of noise, because we don't need to keep
sparse fields around.

A convenience function
`GenPathInfo(rootNode *castorev1pb.Node, narInfo *narinfo.NarInfo)` is
added, which is used to produce PathInfo messages, either when receiving
a NAR file over http and uploading it to a remote PathInfoService, or to
synthesize the PathInfoMessage to return to the client, if nar-bridge is
acting as a PathInfoService for a remove Nix HTTP Binary cache.

Change-Id: Ibba1ab6238a050816c4fab29cb21ae88877d8613
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9651
Tested-by: BuildkiteCI
Reviewed-by: Brian McGee <brian@bmcgee.ie>

tvix/nar-bridge/pkg/importer/roundtrip_test.go

@@ -30,7 +30,7 @@ func TestRoundtrip(t *testing.T) {
 	blobsMap := make(map[string][]byte, 0)
 	directoriesMap := make(map[string]*castorev1pb.Directory)
 
-	pathInfo, err := importer.Import(
+	rootNode, _, _, err := importer.Import(
 		context.Background(),
 		bytes.NewBuffer(narContents),
 		func(blobReader io.Reader) ([]byte, error) {
@@ -56,10 +56,10 @@ func TestRoundtrip(t *testing.T) {
 	require.NoError(t, err)
 
 	// done populating everything, now actually test the export :-)
-	var buf bytes.Buffer
+	var narBuf bytes.Buffer
 	err = storev1pb.Export(
-		&buf,
-		pathInfo.Node,
+		&narBuf,
+		rootNode,
 		func(directoryDgst []byte) (*castorev1pb.Directory, error) {
 			d, found := directoriesMap[base64.StdEncoding.EncodeToString(directoryDgst)]
 			if !found {
@@ -77,5 +77,5 @@ func TestRoundtrip(t *testing.T) {
 	)
 
 	require.NoError(t, err, "exporter shouldn't fail")
-	require.Equal(t, narContents, buf.Bytes())
+	require.Equal(t, narContents, narBuf.Bytes())
 }