Fix TSAN violations in FSTTransactionTests (#5922)

TSAN uncovered cases where we were reading values in the transaction
thread that were set on the main thread and were unprotected by lock. In
practice this is unlikely to cause an harm, but it's good to keep TSAN
clean for when there are real issues.

Author: wilhuff

Firestore/Example/Tests/Integration/FSTTransactionTests.mm

@@ -119,13 +119,14 @@ - (FSTTransactionTester *)runWithStages:(NSArray<TransactionStage> *)stages;
 - (void)expectDoc:(NSObject *)expected;
 - (void)expectNoDoc;
 - (void)expectError:(FIRFirestoreErrorCode)expected;
+
+@property(atomic, strong, readwrite) NSArray<TransactionStage> *stages;
+@property(atomic, strong, readwrite) FIRDocumentReference *docRef;
+@property(atomic, assign, readwrite) BOOL fromExistingDoc;
 @end
 
 @implementation FSTTransactionTester {
   FIRFirestore *_db;
-  FIRDocumentReference *_docRef;
-  BOOL _fromExistingDoc;
-  NSArray<TransactionStage> *_stages;
   FSTTransactionTests *_testCase;
   NSMutableArray<XCTestExpectation *> *_testExpectations;
 }
@@ -141,17 +142,17 @@ - (instancetype)initWithDb:(FIRFirestore *)db testCase:(FSTTransactionTests *)te
 }
 
 - (FSTTransactionTester *)withExistingDoc {
-  _fromExistingDoc = YES;
+  self.fromExistingDoc = YES;
   return self;
 }
 
 - (FSTTransactionTester *)withNonexistentDoc {
-  _fromExistingDoc = NO;
+  self.fromExistingDoc = NO;
   return self;
 }
 
 - (FSTTransactionTester *)runWithStages:(NSArray<TransactionStage> *)stages {
-  _stages = stages;
+  self.stages = stages;
   return self;
 }
 
@@ -160,7 +161,7 @@ - (void)expectDoc:(NSObject *)expected {
   [self runSuccessfulTransaction];
 
   XCTestExpectation *expectation = [_testCase expectationWithDescription:@"expectDoc"];
-  [_docRef getDocumentWithCompletion:^(FIRDocumentSnapshot *snapshot, NSError *error) {
+  [self.docRef getDocumentWithCompletion:^(FIRDocumentSnapshot *snapshot, NSError *error) {
     [self->_testCase assertSnapshot:snapshot equalsObject:expected error:error];
     [expectation fulfill];
   }];
@@ -174,7 +175,7 @@ - (void)expectNoDoc {
   [self runSuccessfulTransaction];
 
   XCTestExpectation *expectation = [_testCase expectationWithDescription:@"expectNoDoc"];
-  [_docRef getDocumentWithCompletion:^(FIRDocumentSnapshot *snapshot, NSError *error) {
+  [self.docRef getDocumentWithCompletion:^(FIRDocumentSnapshot *snapshot, NSError *error) {
     [self->_testCase assertDoesNotExistWithSnapshot:snapshot error:error];
     [expectation fulfill];
   }];
@@ -191,9 +192,9 @@ - (void)expectError:(FIRFirestoreErrorCode)expected {
 }
 
 - (void)prepareDoc {
-  _docRef = [[_db collectionWithPath:@"nonexistent"] documentWithAutoID];
+  self.docRef = [[_db collectionWithPath:@"nonexistent"] documentWithAutoID];
   if (_fromExistingDoc) {
-    NSError *setError = [self writeDocumentRef:_docRef data:@{@"foo" : @"bar"}];
+    NSError *setError = [self writeDocumentRef:self.docRef data:@{@"foo" : @"bar"}];
     NSString *message = [NSString stringWithFormat:@"Failed set at %@", [self stageNames]];
     [_testCase assertNilError:setError message:message];
   }
@@ -217,8 +218,8 @@ - (void)runSuccessfulTransaction {
       [_testCase expectationWithDescription:@"runSuccessfulTransaction"];
   [_db
       runTransactionWithBlock:^id _Nullable(FIRTransaction *transaction, NSError **) {
-        for (TransactionStage stage in self->_stages) {
-          stage(transaction, self->_docRef);
+        for (TransactionStage stage in self.stages) {
+          stage(transaction, self.docRef);
         }
         return @YES;
       }
@@ -239,8 +240,8 @@ - (void)runFailingTransactionWithError:(FIRFirestoreErrorCode)expected {
       [_testCase expectationWithDescription:@"runFailingTransactionWithError"];
   [_db
       runTransactionWithBlock:^id _Nullable(FIRTransaction *transaction, NSError **) {
-        for (TransactionStage stage in self->_stages) {
-          stage(transaction, self->_docRef);
+        for (TransactionStage stage in self.stages) {
+          stage(transaction, self.docRef);
         }
         return @YES;
       }
@@ -256,14 +257,14 @@ - (void)runFailingTransactionWithError:(FIRFirestoreErrorCode)expected {
 }
 
 - (void)cleanupTester {
-  _stages = [NSArray array];
+  self.stages = [NSArray array];
   // Set the docRef to something else to lose the original reference.
-  _docRef = [[self->_db collectionWithPath:@"reset"] documentWithAutoID];
+  self.docRef = [[self->_db collectionWithPath:@"reset"] documentWithAutoID];
 }
 
 - (NSString *)stageNames {
   NSMutableArray<NSString *> *seqList = [NSMutableArray array];
-  for (TransactionStage stage in _stages) {
+  for (TransactionStage stage in self.stages) {
     if (stage == delete1) {
       [seqList addObject:@"delete"];
     } else if (stage == update1 || stage == update2) {