fix(database): serialize push ID generation

FirebaseDatabase/CHANGELOG.md

@@ -1,4 +1,6 @@
 # 13.1.0
+- [fixed] Fixed a race where calling childByAutoId() concurrently from multiple
+  threads could generate duplicate push IDs. (#16768)
 - [fixed] Fixed an issue where transactions could fail with a maxretry error when
   every child of a node had a priority. (#16769)
 - [fixed] Fixed an issue where observers could miss events, and keep showing stale

FirebaseDatabase/Sources/Utilities/FNextPushId.m

@@ -37,38 +37,42 @@ @implementation FNextPushId
 + (NSString *)get:(NSTimeInterval)currentTime {
     static long long lastPushTime = 0;
     static int lastRandChars[12];
+    @synchronized([FNextPushId class]) {
 
-    long long now = (long long)(currentTime * 1000);
+        long long now = (long long)(currentTime * 1000);
 
-    BOOL duplicateTime = now == lastPushTime;
-    lastPushTime = now;
+        BOOL duplicateTime = now == lastPushTime;
+        lastPushTime = now;
 
-    unichar timeStampChars[8];
-    for (int i = 7; i >= 0; i--) {
-        timeStampChars[i] = [PUSH_CHARS characterAtIndex:(now % 64)];
-        now = (long long)floor(now / 64);
-    }
+        unichar timeStampChars[8];
+        for (int i = 7; i >= 0; i--) {
+            timeStampChars[i] = [PUSH_CHARS characterAtIndex:(now % 64)];
+            now = (long long)floor(now / 64);
+        }
 
-    NSMutableString *id = [[NSMutableString alloc] init];
-    [id appendString:[NSString stringWithCharacters:timeStampChars length:8]];
+        NSMutableString *id = [[NSMutableString alloc] init];
+        [id appendString:[NSString stringWithCharacters:timeStampChars
+                                                 length:8]];
 
-    if (!duplicateTime) {
-        for (int i = 0; i < 12; i++) {
-            lastRandChars[i] = (int)floor(arc4random() % 64);
+        if (!duplicateTime) {
+            for (int i = 0; i < 12; i++) {
+                lastRandChars[i] = (int)floor(arc4random() % 64);
+            }
+        } else {
+            int i = 0;
+            for (i = 11; i >= 0 && lastRandChars[i] == 63; i--) {
+                lastRandChars[i] = 0;
+            }
+            lastRandChars[i]++;
         }
-    } else {
-        int i = 0;
-        for (i = 11; i >= 0 && lastRandChars[i] == 63; i--) {
-            lastRandChars[i] = 0;
+
+        for (int i = 0; i < 12; i++) {
+            [id appendFormat:@"%C",
+                             [PUSH_CHARS characterAtIndex:lastRandChars[i]]];
         }
-        lastRandChars[i]++;
-    }
 
-    for (int i = 0; i < 12; i++) {
-        [id appendFormat:@"%C", [PUSH_CHARS characterAtIndex:lastRandChars[i]]];
+        return [NSString stringWithString:id];
     }
-
-    return [NSString stringWithString:id];
 }
 
 + (NSString *)from:(NSString *)fn successor:(NSString *_Nonnull)key {

FirebaseDatabase/Tests/Unit/FNextPushIdTest.m

@@ -49,6 +49,33 @@ - (void)testSuccessorSpecialValues {
                         @"successor(MAX_PUSH_CHAR repeated MAX_KEY_LEN times) == MAX_NAME");
 }
 
+- (void)testConcurrentPushIdsAreUniqueForTheSameTimestamp {
+  const size_t pushIdCount = 1000;
+  NSMutableArray<NSString *> *pushIds = [NSMutableArray arrayWithCapacity:pushIdCount];
+  NSLock *pushIdsLock = [[NSLock alloc] init];
+
+  dispatch_apply(pushIdCount, dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^(size_t index) {
+    (void)index;
+    NSString *pushId = [FNextPushId get:12345.0];
+    [pushIdsLock lock];
+    [pushIds addObject:pushId];
+    [pushIdsLock unlock];
+  });
+
+  XCTAssertEqual([NSSet setWithArray:pushIds].count, pushIdCount);
+}
+
+- (void)testGenerationExceptionDoesNotBlockLaterCalls {
+  XCTAssertThrowsSpecific([FNextPushId get:-0.001], NSException);
+  XCTestExpectation *generated = [self expectationWithDescription:@"Generate after exception"];
+  dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^{
+    NSString *pushId = [FNextPushId get:12345.0];
+    XCTAssertEqual(pushId.length, 20U);
+    [generated fulfill];
+  });
+  [self waitForExpectationsWithTimeout:2.0 handler:nil];
+}
+
 - (void)testSuccessorBasic {
   NSString *actual = [FNextPushId from:@"test" successor:@"abc"];
   NSString *expected = [NSString stringWithFormat:@"abc%@", MIN_PUSH_CHAR];