Use crypo RNG for auto ID generation to reduce conflicts. (#2764)

* Use crypo RNG for auto ID generation to reduce conflicts.

* Address comments.

* Fix mistake.

* Declare magic numbers.

Author: wu-hui

packages/firestore/src/platform/platform.ts

@@ -43,6 +43,12 @@ export interface Platform {
   /** Converts a binary string to a Base64 encoded string. */
   btoa(raw: string): string;
 
+  /**
+   * Generates `nBytes` of random bytes. If `nBytes` is negative, an empty array
+   * will be returned.
+   */
+  randomBytes(nBytes: number): Uint8Array;
+
   /** The Platform's 'window' implementation or null if not available. */
   readonly window: Window | null;
 

packages/firestore/src/platform_browser/browser_platform.ts

@@ -67,4 +67,14 @@ export class BrowserPlatform implements Platform {
   btoa(raw: string): string {
     return btoa(raw);
   }
+
+  randomBytes(nBytes: number): Uint8Array {
+    if (nBytes <= 0) {
+      return new Uint8Array();
+    }
+
+    const v = new Uint8Array(nBytes);
+    crypto.getRandomValues(v);
+    return v;
+  }
 }

packages/firestore/src/platform_node/node_platform.ts

@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+import { randomBytes } from 'crypto';
 import * as util from 'util';
 
 import { DatabaseId, DatabaseInfo } from '../core/database_info';
@@ -74,4 +75,12 @@ export class NodePlatform implements Platform {
   btoa(raw: string): string {
     return new Buffer(raw, 'binary').toString('base64');
   }
+
+  randomBytes(nBytes: number): Uint8Array {
+    if (nBytes <= 0) {
+      return new Uint8Array();
+    }
+
+    return randomBytes(nBytes);
+  }
 }

packages/firestore/src/util/misc.ts

@@ -16,6 +16,7 @@
  */
 
 import { assert } from './assert';
+import { PlatformSupport } from '../platform/platform';
 
 export type EventHandler<E> = (value: E) => void;
 export interface Indexable {
@@ -28,8 +29,17 @@ export class AutoId {
     const chars =
       'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
     let autoId = '';
-    for (let i = 0; i < 20; i++) {
-      autoId += chars.charAt(Math.floor(Math.random() * chars.length));
+    while (autoId.length < 20) {
+      const bytes = PlatformSupport.getPlatform().randomBytes(40);
+      bytes.forEach(b => {
+        // Length of `chars` is 62. We only take bytes between 0 and 62*4-1
+        // (both inclusive). The value is then evenly mapped to indices of `char`
+        // via a modulo operation.
+        const maxValue = 62 * 4 - 1;
+        if (autoId.length < 20 && b <= maxValue) {
+          autoId += chars.charAt(b % 62);
+        }
+      });
     }
     assert(autoId.length === 20, 'Invalid auto ID: ' + autoId);
     return autoId;

packages/firestore/test/util/test_platform.ts

@@ -262,6 +262,10 @@ export class TestPlatform implements Platform {
   btoa(raw: string): string {
     return this.basePlatform.btoa(raw);
   }
+
+  randomBytes(nBytes: number): Uint8Array {
+    return this.basePlatform.randomBytes(nBytes);
+  }
 }
 
 /** Returns true if we are running under Node. */