Remove the global outer scope. (tensorflow#1425)

BUG

Author: Nikhil Thorat

src/engine.ts

@@ -97,7 +97,7 @@ export class Engine implements TensorManager, DataMover {
 
   // Keep Tensors that parallel the tapes.
   private activeScope: ScopeState;
-  private scopeStack: ScopeState[];
+  private scopeStack: ScopeState[] = [];
   private keepTensors: Set<number> = new Set();
   private profiler: Profiler;
 
@@ -112,9 +112,6 @@ export class Engine implements TensorManager, DataMover {
   constructor(
       public backend: KernelBackend, public safeMode: boolean,
       private debugMode: () => boolean) {
-    // Create a default outer scope.
-    this.activeScope = {track: [], name: 'default scope'};
-    this.scopeStack = [this.activeScope];
     this.profiler = new Profiler(backend);
     this.activeProfile =
         {newBytes: 0, newTensors: 0, peakBytes: 0, kernels: [], result: null};
@@ -454,7 +451,7 @@ export class Engine implements TensorManager, DataMover {
 
     const oldScope = this.scopeStack.pop();
     this.activeScope = this.scopeStack.length === 0 ?
-        {track: [], name: 'default scope'} :
+        null :
         this.scopeStack[this.scopeStack.length - 1];
 
     // Track the current result in the parent scope.
@@ -612,7 +609,9 @@ export class Engine implements TensorManager, DataMover {
           'Safe mode is ON. Enclose all tensor operations inside tf.tidy(): ' +
           'tf.tidy(() => {op();...}); to avoid memory leaks.');
     }
-    this.activeScope.track.push(result);
+    if (this.activeScope != null) {
+      this.activeScope.track.push(result);
+    }
     return result;
   }
 }

src/engine_test.ts

@@ -649,3 +649,19 @@ describeWithFlags('Switching WebGL + CPU backends', WEBGL_ENVS, () => {
     expect(tf.memory().numTensors).toBe(0);
   });
 });
+
+// NOTE: This describe is purposefully not a describeWithFlags so that we test
+// tensor allocation where no scopes have been created. The backend here must be
+// set to CPU because we cannot allocate GPU tensors outside a
+// describeWithFlags because the default webgl backend and the test backends
+// share a WebGLContext. When backends get registered, global WebGL state is
+// initialized, which causes the two backends to step on each other and get in a
+// bad state.
+describe('Memory allocation outside a test scope', () => {
+  it('constructing a tensor works', () => {
+    tf.setBackend('cpu');
+    const a = tf.tensor1d([1, 2, 3]);
+    expectArraysClose(a, [1, 2, 3]);
+    a.dispose();
+  });
+});

src/environment_test.ts

@@ -61,11 +61,6 @@ describeWithFlags(
     });
 
 describeWithFlags('WEBGL_PAGING_ENABLED', WEBGL_ENVS, testEnv => {
-  afterEach(() => {
-    ENV.reset();
-    ENV.setFeatures(testEnv.features);
-  });
-
   it('should be true if in a browser', () => {
     const features: Features = {'IS_BROWSER': true};
     const env = new Environment(features);