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/util.go

@@ -0,0 +1,24 @@
+package server
+
+import (
+	"fmt"
+	nixhash "github.com/nix-community/go-nix/pkg/hash"
+)
+
+// parseNarHashFromUrl parses a nixbase32 string representing a sha256 NarHash
+// and returns a nixhash.Hash when it was able to parse, or an error.
+func parseNarHashFromUrl(narHashFromUrl string) (*nixhash.Hash, error) {
+	// peek at the length. If it's 52 characters, assume sha256,
+	// if it's something else, this is an error.
+	l := len(narHashFromUrl)
+	if l != 52 {
+		return nil, fmt.Errorf("invalid length of narHash: %v", l)
+	}
+
+	nixHash, err := nixhash.ParseNixBase32("sha256:" + narHashFromUrl)
+	if err != nil {
+		return nil, fmt.Errorf("unable to parse nixbase32 hash: %w", err)
+	}
+
+	return nixHash, nil
+}