feat(tvix/nar-bridge): add ToNixNarInfo()

Convenience function, moves all code converting from a PathInfo struct
to to go-nix's NarInfo.

Change-Id: Idf0dcc38675674563f2dfd3286a4a55fa2a24a82
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9593
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Brian McGee <brian@bmcgee.ie>

tvix/nar-bridge/pkg/http/narinfo.go

@@ -0,0 +1,49 @@
+package http
+
+import (
+	"fmt"
+
+	storev1pb "code.tvl.fyi/tvix/store/protos"
+	nixhash "github.com/nix-community/go-nix/pkg/hash"
+	"github.com/nix-community/go-nix/pkg/narinfo"
+	"github.com/nix-community/go-nix/pkg/narinfo/signature"
+	"github.com/nix-community/go-nix/pkg/nixbase32"
+)
+
+// ToNixNarInfo converts the PathInfo to a narinfo.NarInfo.
+func ToNixNarInfo(p *storev1pb.PathInfo) (*narinfo.NarInfo, error) {
+	// ensure the PathInfo is valid, and extract the StorePath from the node in
+	// there.
+	storePath, err := p.Validate()
+	if err != nil {
+		return nil, fmt.Errorf("failed to validate PathInfo: %w", err)
+	}
+
+	// convert the signatures from storev1pb signatures to narinfo signatures
+	narinfoSignatures := make([]signature.Signature, len(p.GetNarinfo().GetSignatures()))
+	for i, pathInfoSignature := range p.GetNarinfo().GetSignatures() {
+		narinfoSignatures[i] = signature.Signature{
+			Name: pathInfoSignature.GetName(),
+			Data: pathInfoSignature.GetData(),
+		}
+	}
+
+	// produce nixhash for the narsha256.
+	narHash, err := nixhash.FromHashTypeAndDigest(
+		0x12, // SHA2_256
+		p.GetNarinfo().GetNarSha256(),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("invalid narsha256: %w", err)
+	}
+
+	return &narinfo.NarInfo{
+		StorePath:   storePath.Absolute(),
+		URL:         "nar/" + nixbase32.EncodeToString(narHash.Digest()) + ".nar",
+		Compression: "none",
+		NarHash:     narHash,
+		NarSize:     uint64(p.GetNarinfo().GetNarSize()),
+		References:  p.GetNarinfo().GetReferenceNames(),
+		Signatures:  narinfoSignatures,
+	}, nil
+}