chore: cache NodeFSTree read

src/resolve/treeContainers.ts

@@ -106,8 +106,14 @@ export class NodeFSTreeContainer extends TreeContainer {
   }
 
   public readFile(fsPath: SourcePath): Promise<Buffer> {
-    // significant enough performance increase using sync instead of fs.promise version
-    return Promise.resolve(readFileSync(fsPath));
+    if (this.fileContentMap.has(fsPath)) {
+      return Promise.resolve(this.fileContentMap.get(fsPath)!);
+    } else {
+      // significant enough performance increase using sync instead of fs.promise version
+      const content = readFileSync(fsPath);
+      this.fileContentMap.set(fsPath, content);
+      return Promise.resolve(content);
+    }
   }
 
   public readFileSync(fsPath: SourcePath): Buffer {

test/resolve/treeContainers.test.ts

@@ -119,12 +119,23 @@ describe('Tree Containers', () => {
     it('should use expected Node API for readFileSync', () => {
       const readFileStub = env.stub(fs, 'readFileSync');
       // @ts-ignore wants Dirents but string[] works as well
-      readFileStub.withArgs(path).returns(Buffer.from('test'));
-      const data = tree.readFileSync(path);
+      readFileStub.withArgs('myNewPath').returns(Buffer.from('test'));
+      const data = tree.readFileSync('myNewPath');
       expect(data.toString()).to.deep.equal('test');
       expect(readFileStub.calledOnce).to.be.true;
     });
 
+    it('should use cached value for readFileSync', () => {
+      const readFileStub = env.stub(fs, 'readFileSync');
+      // @ts-ignore wants Dirents but string[] works as well
+      readFileStub.withArgs('myNewPath').returns(Buffer.from('test'));
+      const data = tree.readFileSync('myNewPath');
+      // returns same value
+      expect(data.toString()).to.deep.equal('test');
+      // didn't re-read the file
+      expect(readFileStub.called).to.be.false;
+    });
+
     it('should use expected Node API for stream', () => {
       const readable = new Readable();
       // @ts-ignore wants ReadStream but Readable works for testing