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/gen_pathinfo.go

@@ -0,0 +1,62 @@
+package importer
+
+import (
+	castorev1pb "code.tvl.fyi/tvix/castore/protos"
+	storev1pb "code.tvl.fyi/tvix/store/protos"
+	"fmt"
+	"github.com/nix-community/go-nix/pkg/narinfo"
+	"github.com/nix-community/go-nix/pkg/storepath"
+)
+
+// GenPathInfo takes a rootNode and narInfo and assembles a PathInfo.
+// The rootNode is renamed to match the StorePath in the narInfo.
+func GenPathInfo(rootNode *castorev1pb.Node, narInfo *narinfo.NarInfo) (*storev1pb.PathInfo, error) {
+	// parse the storePath from the .narinfo
+	storePath, err := storepath.FromAbsolutePath(narInfo.StorePath)
+	if err != nil {
+		return nil, fmt.Errorf("unable to parse StorePath: %w", err)
+	}
+
+	// construct the references, by parsing ReferenceNames and extracting the digest
+	references := make([][]byte, len(narInfo.References))
+	for i, referenceStr := range narInfo.References {
+		// parse reference as store path
+		referenceStorePath, err := storepath.FromString(referenceStr)
+		if err != nil {
+			return nil, fmt.Errorf("unable to parse reference %s as storepath: %w", referenceStr, err)
+		}
+		references[i] = referenceStorePath.Digest
+	}
+
+	// construct the narInfo.Signatures[*] from pathInfo.Narinfo.Signatures[*]
+	narinfoSignatures := make([]*storev1pb.NARInfo_Signature, len(narInfo.Signatures))
+	for i, narinfoSig := range narInfo.Signatures {
+		narinfoSignatures[i] = &storev1pb.NARInfo_Signature{
+			Name: narinfoSig.Name,
+			Data: narinfoSig.Data,
+		}
+	}
+
+	// assemble the PathInfo.
+	pathInfo := &storev1pb.PathInfo{
+		// embed a new root node with the name set to the store path basename.
+		Node:       castorev1pb.RenamedNode(rootNode, storePath.String()),
+		References: references,
+		Narinfo: &storev1pb.NARInfo{
+			NarSize:        narInfo.NarSize,
+			NarSha256:      narInfo.FileHash.Digest(),
+			Signatures:     narinfoSignatures,
+			ReferenceNames: narInfo.References,
+		},
+	}
+
+	// run Validate on the PathInfo, more as an additional sanity check our code is sound,
+	// to make sure we populated everything properly, before returning it.
+	// Fail hard if we fail validation, this is a code error.
+	if _, err = pathInfo.Validate(); err != nil {
+		panic(fmt.Sprintf("PathInfo failed validation: %v", err))
+	}
+
+	return pathInfo, nil
+
+}