feat(tvix/nar-bridge): init

This provides a Nix HTTP Binary Cache interface in front of a tvix-store
that's reachable via gRPC.

TODOs:

 - remove import command, move serve up to toplevel. We have nix-copy-
   closure and tvix-store commands.
 - loop into CI. We should be able to fetch the protos as a third-party
   dependency.
 - Check if we can test nar-bridge slightly easier in an integration
   test.
 - Ensure we support connecting to unix sockets and grpc+http at least,
   using the same syntax as tvix-store.
 - Don't buffer the entire blob when rendering NAR

Co-Authored-By: Connor Brewster <cbrewster@hey.com>
Co-Authored-By: Márton Boros <martonboros@gmail.com>
Co-Authored-By: Vo Minh Thu <noteed@gmail.com>

Change-Id: I6064474e49dfe78cea67676957462d9f28658d4a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9339
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/nar-bridge/pkg/server/blob_upload.go

@@ -0,0 +1,48 @@
+package server
+
+import (
+	storev1pb "code.tvl.fyi/tvix/store/protos"
+	"context"
+	"encoding/base64"
+	"fmt"
+	log "github.com/sirupsen/logrus"
+	"io"
+)
+
+// this returns a callback function that can be used as fileCb
+// for the reader.Import function call
+func genBlobServiceWriteCb(ctx context.Context, blobServiceClient storev1pb.BlobServiceClient) func(io.Reader) error {
+	return func(fileReader io.Reader) error {
+		// Read from fileReader into a buffer.
+		// We currently buffer all contents and send them to blobServiceClient at once,
+		// but that's about to change.
+		contents, err := io.ReadAll(fileReader)
+		if err != nil {
+			return fmt.Errorf("unable to read all contents from file reader: %w", err)
+		}
+
+		log := log.WithField("blob_size", len(contents))
+
+		log.Infof("about to upload blob")
+
+		putter, err := blobServiceClient.Put(ctx)
+		if err != nil {
+			// return error to the importer
+			return fmt.Errorf("error from blob service: %w", err)
+		}
+		err = putter.Send(&storev1pb.BlobChunk{
+			Data: contents,
+		})
+		if err != nil {
+			return fmt.Errorf("putting blob chunk: %w", err)
+		}
+		resp, err := putter.CloseAndRecv()
+		if err != nil {
+			return fmt.Errorf("close blob putter: %w", err)
+		}
+
+		log.WithField("digest", base64.StdEncoding.EncodeToString(resp.GetDigest())).Info("uploaded blob")
+
+		return nil
+	}
+}