fix(span): make child span clock relative to root span (#628)

* fix(span): make child span clock relative to root span

* test(span): improve comment

* test(span): fix

packages/opencensus-core/src/internal/clock.ts

@@ -27,9 +27,11 @@ export class Clock {
   /** The duration between start and end of the clock. */
   private diff: [number, number] = [0, 0];
 
-  /** Constructs a new SamplerImpl instance. */
-  constructor() {
-    this.startTimeLocal = new Date();
+  /** Constructs a new Clock instance. */
+  constructor(startTime?: Date) {
+    // In some cases clocks need to be relative to other resources, passing a
+    // startTime makes it possible.
+    this.startTimeLocal = startTime || new Date();
     this.hrtimeLocal = process.hrtime();
   }
 
@@ -42,6 +44,14 @@ export class Clock {
     this.endedLocal = true;
   }
 
+  /** Gets the current date from ellapsed milliseconds and start time. */
+  get currentDate(): Date {
+    const diff = process.hrtime(this.hrtimeLocal);
+    const ns = diff[0] * 1e9 + diff[1];
+    const ellapsed = ns / 1e6;
+    return new Date(this.startTime.getTime() + ellapsed);
+  }
+
   /** Gets the duration of the clock. */
   get duration(): number {
     if (!this.endedLocal) {

packages/opencensus-core/src/trace/model/span.ts

@@ -316,7 +316,12 @@ export class Span implements types.Span {
       );
       return;
     }
-    this.clock = new Clock();
+    // start child span's clock from root's current time to preserve integrity.
+    if (this.parentSpan) {
+      this.clock = new Clock(this.parentSpan.clock.currentDate);
+    } else {
+      this.clock = new Clock();
+    }
     this.startedLocal = true;
     this.logger.debug('starting %s  %o', this.className, {
       traceId: this.traceId,

packages/opencensus-core/test/test-span.ts

@@ -47,6 +47,14 @@ describe('Span', () => {
       const span = new Span(tracer, rootSpan);
       assert.ok(span instanceof Span);
     });
+
+    it('should use relative clock for child spans', () => {
+      const rootSpan = new RootSpan(tracer, name, kind, traceId, parentSpanId);
+      rootSpan.start();
+      const span = new Span(tracer, rootSpan);
+      span.start();
+      assert.ok(rootSpan.startTime.getTime() <= span.startTime.getTime());
+    });
   });
 
   /**