Include the git blob id of the dir-index bundle in the ETag

While the content of raw files retrieved via the gateway should never
change, the look and feel of the directory index can and will change
between versions of go-ipfs.

Incorporate the hash of assets/bindata.go into the ETag when appropriate

assets/assets.go

@@ -1,6 +1,8 @@
 //go:generate git submodule update --init ./dir-index-html
 //go:generate go run github.com/go-bindata/go-bindata/go-bindata -pkg=assets init-doc dir-index-html/dir-index.html dir-index-html/knownIcons.txt
 //go:generate gofmt -w bindata.go
+//go:generate sh -c "sed -i \"s/.*BindataVersionHash.*/BindataVersionHash=\\\"$(git hash-object bindata.go)\\\"/\" bindata_version_hash.go"
+//go:generate gofmt -w bindata_version_hash.go
 package assets
 
 import (

assets/bindata_version_hash.go

@@ -0,0 +1,5 @@
+package assets
+
+const (
+	BindataVersionHash = "c1aa0601ac3eac2c50b296cf618a6747eeba8579"
+)

core/corehttp/gateway_handler.go

@@ -19,6 +19,7 @@ import (
 	"github.com/gabriel-vasile/mimetype"
 	"github.com/ipfs/go-cid"
 	files "github.com/ipfs/go-ipfs-files"
+	assets "github.com/ipfs/go-ipfs/assets"
 	dag "github.com/ipfs/go-merkledag"
 	mfs "github.com/ipfs/go-mfs"
 	path "github.com/ipfs/go-path"
@@ -222,16 +223,26 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 
 	defer dr.Close()
 
-	// Check etag send back to us
-	etag := "\"" + resolvedPath.Cid().String() + "\""
-	if r.Header.Get("If-None-Match") == etag || r.Header.Get("If-None-Match") == "W/"+etag {
+	var responseEtag string
+
+	// we need to figure out whether this is a directory before doing most of the heavy lifting below
+	_, ok := dr.(files.Directory)
+
+	if ok && assets.BindataVersionHash != "" {
+		responseEtag = `"DirIndex-` + assets.BindataVersionHash + `_CID-` + resolvedPath.Cid().String() + `"`
+	} else {
+		responseEtag = `"` + resolvedPath.Cid().String() + `"`
+	}
+
+	// Check etag sent back to us
+	if r.Header.Get("If-None-Match") == responseEtag || r.Header.Get("If-None-Match") == `W/`+responseEtag {
 		w.WriteHeader(http.StatusNotModified)
 		return
 	}
 
 	i.addUserHeaders(w) // ok, _now_ write user's headers.
 	w.Header().Set("X-IPFS-Path", urlPath)
-	w.Header().Set("Etag", etag)
+	w.Header().Set("Etag", responseEtag)
 
 	// set these headers _after_ the error, for we may just not have it
 	// and don't want the client to cache a 500 response...