cmd/tailscale, ipn/ipnlocal: add debug command to write to StateStore for dev

Not for end users (unless directed by support). Mostly for ease of
development for some upcoming webserver work.

Change-Id: I43acfed217514567acb3312367b24d620e739f88
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -3966,3 +3966,15 @@ func (b *LocalBackend) Doctor(ctx context.Context, logf logger.Logf) {
 
 	doctor.RunChecks(ctx, logf, checks...)
 }
+
+// SetDevStateStore updates the LocalBackend's state storage to the provided values.
+//
+// It's meant only for development.
+func (b *LocalBackend) SetDevStateStore(key, value string) error {
+	if b.store == nil {
+		return errors.New("no state store")
+	}
+	err := b.store.WriteState(ipn.StateKey(key), []byte(value))
+	b.logf("SetDevStateStore(%q, %q) = %v", key, value, err)
+	return err
+}

ipn/localapi/localapi.go

@@ -61,6 +61,7 @@ var handler = map[string]localAPIHandler{
 	"component-debug-logging": (*Handler).serveComponentDebugLogging,
 	"debug":                   (*Handler).serveDebug,
 	"derpmap":                 (*Handler).serveDERPMap,
+	"dev-set-state-store":     (*Handler).serveDevSetStateStore,
 	"dial":                    (*Handler).serveDial,
 	"file-targets":            (*Handler).serveFileTargets,
 	"goroutines":              (*Handler).serveGoroutines,
@@ -401,6 +402,23 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 	io.WriteString(w, "done\n")
 }
 
+func (h *Handler) serveDevSetStateStore(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitWrite {
+		http.Error(w, "debug access denied", http.StatusForbidden)
+		return
+	}
+	if r.Method != "POST" {
+		http.Error(w, "POST required", http.StatusMethodNotAllowed)
+		return
+	}
+	if err := h.b.SetDevStateStore(r.FormValue("key"), r.FormValue("value")); err != nil {
+		http.Error(w, err.Error(), 500)
+		return
+	}
+	w.Header().Set("Content-Type", "text/plain")
+	io.WriteString(w, "done\n")
+}
+
 func (h *Handler) serveComponentDebugLogging(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitWrite {
 		http.Error(w, "debug access denied", http.StatusForbidden)