check for l2cache existing is now cached and only checked when necessary

src/neuroglancer/datasource/graphene/frontend.ts

@@ -1547,6 +1547,7 @@ class GrapheneGraphServerInterface {
 class GrapheneGraphSource extends SegmentationGraphSource {
   private connections = new Set<GraphConnection>();
   public graphServer: GrapheneGraphServerInterface;
+  private l2CacheAvailable: boolean|undefined = undefined;
 
   constructor(
       public info: GrapheneMultiscaleVolumeInfo,
@@ -1573,6 +1574,22 @@ class GrapheneGraphSource extends SegmentationGraphSource {
         VisibleSegmentEquivalencePolicy.NONREPRESENTATIVE_EXCLUDED;
   }
 
+  async isL2CacheUrlAvailable() {
+    if (this.l2CacheAvailable !== undefined) {
+      return this.l2CacheAvailable;
+    }
+    try {
+      const {l2CacheUrl, table} = this.info.app;
+      const tableMapping = await cancellableFetchSpecialOk(
+          undefined, `${l2CacheUrl}/table_mapping`, {}, responseJson);
+      this.l2CacheAvailable = !!(tableMapping && tableMapping[table]);
+      return this.l2CacheAvailable;
+    } catch (e) {
+      console.error('e', e);
+      return false;
+    }
+  }
+
   getRoot(segment: Uint64) {
     return this.graphServer.getRoot(segment);
   }
@@ -1581,14 +1598,8 @@ class GrapheneGraphSource extends SegmentationGraphSource {
       first: SegmentSelection, second: SegmentSelection, precisionMode: boolean,
       annotationToNanometers: Float64Array): Promise<number[][]> {
     const {l2CacheUrl, table} = this.info.app;
-    let l2CacheAvailable = false;
-    try {
-      const tableMapping = await cancellableFetchSpecialOk(
-          undefined, `${l2CacheUrl}/table_mapping`, {}, responseJson);
-      l2CacheAvailable = tableMapping && tableMapping[table];
-    } catch (e) {
-      console.error('e', e);
-    }
+    const l2CacheAvailable = precisionMode &&
+        await this.isL2CacheUrlAvailable();  // don't check if available if we don't need it
     let {centroids, l2_path} = await this.graphServer.findPath(
         selectionInNanometers(first, annotationToNanometers),
         selectionInNanometers(second, annotationToNanometers), precisionMode && !l2CacheAvailable);