show an error if local storage is full

data/core.yaml

@@ -576,6 +576,7 @@ en:
       offline: The OpenStreetMap API is offline. Your edits are safe locally. Please come back later.
       readonly: The OpenStreetMap API is currently read-only. You can continue editing, but must wait to save your changes.
       rateLimit: The OpenStreetMap API is limiting anonymous connections. You can fix this by logging in.
+      local_storage_full: You have made too many edits to back up. Consider saving your changes now.
     retry: Retry
   commit:
     title: Upload to OpenStreetMap

modules/core/history.js

@@ -15,7 +15,7 @@ import {
 
 
 export function coreHistory(context) {
-    var dispatch = d3_dispatch('reset', 'change', 'merge', 'restore', 'undone', 'redone');
+    var dispatch = d3_dispatch('reset', 'change', 'merge', 'restore', 'undone', 'redone', 'storage_error');
     var lock = utilSessionMutex('lock');
 
     // restorable if iD not open in another window/tab and a saved history exists in localStorage
@@ -659,8 +659,9 @@ export function coreHistory(context) {
             if (lock.locked() &&
                 // don't overwrite existing, unresolved changes
                 !_hasUnresolvedRestorableChanges) {
+                const success = prefs(getKey('saved_history'), history.toJSON() || null);
 
-                prefs(getKey('saved_history'), history.toJSON() || null);
+                if (!success) dispatch.call('storage_error');
             }
             return history;
         },

modules/core/preferences.js

@@ -16,18 +16,25 @@ _storage = _storage || (() => {
 // corePreferences is an interface for persisting basic key-value strings
 // within and between iD sessions on the same site.
 //
+/**
+ * @param {string} k
+ * @param {string?} v
+ * @returns {boolean} true if the action succeeded
+ */
 function corePreferences(k, v) {
 
   try {
     if (arguments.length === 1) return _storage.getItem(k);
     else if (v === null) _storage.removeItem(k);
     else _storage.setItem(k, v);
+    return true;
   } catch (e) {
     /* eslint-disable no-console */
     if (typeof console !== 'undefined') {
       console.error('localStorage quota exceeded');
     }
     /* eslint-enable no-console */
+    return false;
   }
 
 }

modules/ui/status.js

@@ -69,6 +69,11 @@ export function uiStatus(context) {
 
         osm.on('apiStatusChange.uiStatus', update);
 
+        context.history().on('storage_error', () => {
+            selection.html(t.html('osm_api_status.message.local_storage_full'));
+            selection.attr('class', 'api-status error');
+        });
+
         // reload the status periodically regardless of other factors
         window.setInterval(function() {
             osm.reloadApiStatus();