feat(gw): set Last-Modified to value from dag-pb

(if present)

CHANGELOG.md

@@ -21,6 +21,7 @@ The following emojis are used to highlight certain changes:
 ### Changed
 
 - `chunker` refactored to reduce overall memory use by reducing heap fragmentation [#649](https://github.com/ipfs/boxo/pull/649)
+- `gateway` deserialized responses will have `Last-Modified` set to value from optional UnixFS 1.5 modification time field (if present in DAG)
 
 ### Removed
 

gateway/backend_blocks.go

@@ -156,6 +156,12 @@ func (bb *BlocksBackend) Get(ctx context.Context, path path.ImmutablePath, range
 		return md, nil, err
 	}
 
+	// Set modification time in ContentPathMetadata if found in dag-pb's optional mtime field (UnixFS 1.5)
+	mtime := f.ModTime()
+	if !mtime.IsZero() {
+		md.ModTime = mtime
+	}
+
 	if d, ok := f.(files.Directory); ok {
 		dir, err := uio.NewDirectoryFromNode(bb.dagService, nd)
 		if err != nil {
@@ -231,6 +237,12 @@ func (bb *BlocksBackend) Head(ctx context.Context, path path.ImmutablePath) (Con
 		return ContentPathMetadata{}, nil, err
 	}
 
+	// Set modification time in ContentPathMetadata if found in dag-pb's optional mtime field (UnixFS 1.5)
+	mtime := fileNode.ModTime()
+	if !mtime.IsZero() {
+		md.ModTime = mtime
+	}
+
 	sz, err := fileNode.Size()
 	if err != nil {
 		return ContentPathMetadata{}, nil, err

gateway/gateway.go

@@ -209,7 +209,8 @@ type ContentPathMetadata struct {
 	PathSegmentRoots     []cid.Cid
 	LastSegment          path.ImmutablePath
 	LastSegmentRemainder []string
-	ContentType          string // Only used for UnixFS requests
+	ContentType          string    // Only used for UnixFS requests
+	ModTime              time.Time // Optional, non-zero values may be present in UnixFS 1.5 DAGs
 }
 
 // ByteRange describes a range request within a UnixFS file. "From" and "To" mostly

gateway/handler.go

@@ -410,18 +410,25 @@ func addCacheControlHeaders(w http.ResponseWriter, r *http.Request, contentPath
 		}
 
 		if lastMod.IsZero() {
-			// Otherwise, we set Last-Modified to the current time to leverage caching heuristics
+			// If no lastMod, set Last-Modified to the current time to leverage caching heuristics
 			// built into modern browsers: https://github.com/ipfs/kubo/pull/8074#pullrequestreview-645196768
 			modtime = time.Now()
 		} else {
+			// set Last-Modified to a meaningful value e.g. one read from dag-pb (UnixFS 1.5, mtime field)
+			// or the last time DNSLink / IPNS Record was modified / resoved or cache
 			modtime = lastMod
 		}
+
 	} else {
 		w.Header().Set("Cache-Control", immutableCacheControl)
-		modtime = noModtime // disable Last-Modified
 
-		// TODO: consider setting Last-Modified if UnixFS V1.5 ever gets released
-		// with metadata: https://github.com/ipfs/kubo/issues/6920
+		if lastMod.IsZero() {
+			// (noop) skip Last-Modified on immutable response
+			modtime = noModtime
+		} else {
+			// set Last-Modified to value read from dag-pb (UnixFS 1.5, mtime field)
+			modtime = lastMod
+		}
 	}
 
 	return modtime

gateway/handler_defaults.go

@@ -94,6 +94,14 @@ func (i *handler) serveDefaults(ctx context.Context, w http.ResponseWriter, r *h
 
 	setIpfsRootsHeader(w, rq, &pathMetadata)
 
+	// On deserialized responses, we prefer Last-Modified from pathMetadata
+	// (mtime in UnixFS 1.5 DAG). This also applies to /ipns/, because value
+	// from dag-pb, if present, is more meaningful than lastMod inferred from
+	// namesys.
+	if !pathMetadata.ModTime.IsZero() {
+		rq.lastMod = pathMetadata.ModTime
+	}
+
 	resolvedPath := pathMetadata.LastSegment
 	switch mc.Code(resolvedPath.RootCid().Prefix().Codec) {
 	case mc.Json, mc.DagJson, mc.Cbor, mc.DagCbor: