ipn/ipnlocal: allow retrieval of serve config ETags from local API

This change adds API to ipn.LocalBackend to retrieve the ETag when
querying for the current serve config. This allows consumers of
ipn.LocalBackend.SetServeConfig to utilize the concurrency control
offered by ETags. Previous to this change, utilizing serve config ETags
required copying the local backend's internal ETag calcuation.

The local API server was previously copying the local backend's ETag
calculation as described above. With this change, the local API server
now uses the new ETag retrieval function instead. Serve config ETags are
therefore now opaque to clients, in line with best practices.

Fixes tailscale/corp#35857
Signed-off-by: Harry Harpham <harry@tailscale.com>

ipn/localapi/serve.go

@@ -6,8 +6,6 @@
 package localapi
 
 import (
-	"crypto/sha256"
-	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -31,14 +29,16 @@ func (h *Handler) serveServeConfig(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "serve config denied", http.StatusForbidden)
 			return
 		}
-		config := h.b.ServeConfig()
+		config, etag, err := h.b.ServeConfigETag()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
 		bts, err := json.Marshal(config)
 		if err != nil {
 			http.Error(w, "error encoding config: "+err.Error(), http.StatusInternalServerError)
 			return
 		}
-		sum := sha256.Sum256(bts)
-		etag := hex.EncodeToString(sum[:])
 		w.Header().Set("Etag", etag)
 		w.Header().Set("Content-Type", "application/json")
 		w.Write(bts)