Changed cid-impl to handle a racy condition.

When the cid is requested twice in a row and no cookie is present, the
cid in the first call is different from the seconed one.

src/service/cid-impl.js

@@ -73,6 +73,13 @@ class Cid {
      * @private {?string}
      */
     this.baseCid_ = null;
+
+    /**
+     * Cache to store external cids. Scope is used as the key and cookie value
+     * is the value.
+     * @private {!Object.<string, string>}
+     */
+    this.externalCidCache_ = Object.create(null);
   }
 
   /**
@@ -169,6 +176,10 @@ function getOrCreateCookie(cid, getCidStruct, persistenceConsent) {
     return Promise.resolve(null);
   }
 
+  if (cid.externalCidCache_[scope]) {
+    return Promise.resolve(cid.externalCidCache_[scope]);
+  }
+
   if (existingCookie) {
     // If we created the cookie, update it's expiration time.
     if (/^amp-/.test(existingCookie)) {
@@ -181,6 +192,7 @@ function getOrCreateCookie(cid, getCidStruct, persistenceConsent) {
   // the value whether we created it.
   const newCookie = 'amp-' + cid.sha384Base64_(getEntropy(win));
 
+  cid.externalCidCache_[scope] = newCookie;
   // Store it as a cookie based on the persistence consent.
   persistenceConsent.then(() => {
     // The initial CID generation is inherently racy. First one that gets
@@ -247,6 +259,8 @@ function getBaseCid(cid, persistenceConsent) {
   // We need to make a new one.
   const seed = getEntropy(win);
   const newVal = cid.sha384Base64_(seed);
+
+  cid.baseCid_ = newVal;
   // Storing the value may require consent. We wait for the respective
   // promise.
   persistenceConsent.then(() => {

test/functional/test-cid.js

@@ -382,6 +382,34 @@ describe('cid', () => {
     });
   });
 
+  it('should return same value for multiple calls on non-proxied urls', () => {
+    fakeWin.location.href = 'https://abc.org/foo/?f=0';
+    fakeWin.location.hostname = 'foo.abc.org';
+    const cid1 = cid.get({scope: 'cookie', createCookieIfNotPresent: true},
+        hasConsent);
+    const cid2 = cid.get({scope: 'cookie', createCookieIfNotPresent: true},
+        hasConsent);
+    return cid1.then(c1 => {
+      return cid2.then(c2 => {
+        expect(c1).to.equal(c2);
+      });
+    });
+  });
+
+  it('should return same value for multiple calls on proxied urls', () => {
+    fakeWin.location.href = 'https://cdn.ampproject.org/v/abc.org/foo/?f=0';
+    fakeWin.location.hostname = 'cdn.ampproject.org';
+    const cid1 = cid.get({scope: 'cookie', createCookieIfNotPresent: true},
+        hasConsent);
+    const cid2 = cid.get({scope: 'cookie', createCookieIfNotPresent: true},
+        hasConsent);
+    return cid1.then(c1 => {
+      return cid2.then(c2 => {
+        expect(c1).to.equal(c2);
+      });
+    });
+  });
+
   function compare(externalCidScope, compareValue) {
     return cid.get(externalCidScope, hasConsent).then(c => {
       expect(c).to.equal(compareValue);