Add Suborigin header to gateway responses (ipfs#3209)

This existed before but was disabled in 912a972 because the Suborigin spec
changed and it became incompatible.

This commit updates the generated Suborigin header to be conformant with the
latest spec.

License: MIT
Signed-off-by: James Stanley <james@incoherency.co.uk>

core/corehttp/gateway_handler.go

@@ -27,6 +27,7 @@ import (
 	cid "gx/ipfs/QmYhQaCYEcaPPjxJX7YcPcVKkQfRy6sJ7B3XmGFk82XYdQ/go-cid"
 	routing "gx/ipfs/QmafuecpeZp3k3sHJ5mUARHd4795revuadECQMkmHB8LfW/go-libp2p-routing"
 	node "gx/ipfs/Qmb3Hm9QDFmfYuET4pu7Kyg8JV78jFa1nvZx5vnCZsK4ck/go-ipld-format"
+	multibase "gx/ipfs/QmcxkxTVuURV2Ptse8TvkqH5BQDwV62X1x19JqqvbBzwUM/go-multibase"
 )
 
 const (
@@ -210,6 +211,33 @@ func (i *gatewayHandler) getOrHeadHandler(ctx context.Context, w http.ResponseWr
 	// expose those headers
 	w.Header().Set("Access-Control-Expose-Headers", "X-Stream-Output, X-Chunked-Output")
 
+	// Suborigin header, sandboxes apps from each other in the browser (even
+	// though they are served from the same gateway domain).
+	//
+	// Omitted if the path was treated by IPNSHostnameOption(), for example
+	// a request for http://example.net/ would be changed to /ipns/example.net/,
+	// which would turn into an incorrect Suborigin: example.net header.
+	// In this case the correct thing to do is omit the header because it is already
+	// handled correctly without a Suborigin.
+	//
+	// NOTE: This is not yet widely supported by browsers.
+	if !ipnsHostname {
+		// e.g.: 0="ipfs.io", 1="ipfs", 2="QmYuNaKwY...", ...
+		pathComponents := strings.SplitN(urlPath, "/", 4)
+		suboriginRaw := []byte(strings.ToLower(pathComponents[2]))
+		cidDecoded, err := cid.Decode(pathComponents[2])
+		if err == nil {
+			suboriginRaw = cidDecoded.Bytes()
+		}
+		base32Encoded, err := multibase.Encode(multibase.Base32, suboriginRaw)
+		if err != nil {
+			internalWebError(w, err)
+			return
+		}
+		suborigin := pathComponents[1] + "000" + strings.ToLower(base32Encoded)
+		w.Header().Set("Suborigin", suborigin)
+	}
+
 	// set these headers _after_ the error, for we may just not have it
 	// and dont want the client to cache a 500 response...
 	// and only if it's /ipfs!