Firestore: use C++ Executor for user queue. (#1238)

FSTDispatchQueue enforces serial execution, which is inappropriate for
user queue, because a user may configure usage of a concurrent queue in
settings, breaking FSTDispatchQueue invariants. Instead, use C++
ExecutorLibdispatch directly.

8 files changed, 74 insertions, 61 deletions

diff --git a/Firestore/Example/Tests/Core/FSTQueryListenerTests.mm b/Firestore/Example/Tests/Core/FSTQueryListenerTests.mm
index 7ae9704..5629075 100644
--- a/Firestore/Example/Tests/Core/FSTQueryListenerTests.mm
+++ b/Firestore/Example/Tests/Core/FSTQueryListenerTests.mm
@@ -15,6 +15,7 @@
  */
 
 #import <XCTest/XCTest.h>
+#include <memory>
 
 #import "Firestore/Source/Core/FSTEventManager.h"
 #import "Firestore/Source/Core/FSTQuery.h"
@@ -23,23 +24,29 @@
 #import "Firestore/Source/Model/FSTDocumentSet.h"
 #import "Firestore/Source/Remote/FSTRemoteEvent.h"
 #import "Firestore/Source/Util/FSTAsyncQueryListener.h"
-#import "Firestore/Source/Util/FSTDispatchQueue.h"
 
 #import "Firestore/Example/Tests/Util/FSTHelpers.h"
 
+#include "Firestore/core/src/firebase/firestore/util/executor_libdispatch.h"
+#include "absl/memory/memory.h"
+
+using firebase::firestore::util::internal::ExecutorLibdispatch;
 using firebase::firestore::model::DocumentKeySet;
 
 NS_ASSUME_NONNULL_BEGIN
 
 @interface FSTQueryListenerTests : XCTestCase
-@property(nonatomic, strong, readonly) FSTDispatchQueue *asyncQueue;
 @end
 
-@implementation FSTQueryListenerTests
+@implementation FSTQueryListenerTests {
+  std::unique_ptr<ExecutorLibdispatch> _executor;
+}
 
 - (void)setUp {
-  _asyncQueue = [FSTDispatchQueue
-      queueWith:dispatch_queue_create("FSTQueryListenerTests Queue", DISPATCH_QUEUE_SERIAL)];
+  // TODO(varconst): moving this test to C++, it should be possible to store Executor as a value,
+  // not a pointer, and initialize it in the constructor.
+  _executor = absl::make_unique<ExecutorLibdispatch>(
+      dispatch_queue_create("FSTQueryListenerTests Queue", DISPATCH_QUEUE_SERIAL));
 }
 
 - (void)testRaisesCollectionEvents {
@@ -131,12 +138,12 @@ NS_ASSUME_NONNULL_BEGIN
   FSTDocument *doc1 = FSTTestDoc("rooms/Eros", 3, @{@"name" : @"Eros"}, NO);
   FSTDocument *doc2 = FSTTestDoc("rooms/Eros", 4, @{@"name" : @"Eros2"}, NO);
 
-  __block FSTAsyncQueryListener *listener = [[FSTAsyncQueryListener alloc]
-      initWithDispatchQueue:self.asyncQueue
-            snapshotHandler:^(FSTViewSnapshot *snapshot, NSError *error) {
-              [accum addObject:snapshot];
-              [listener mute];
-            }];
+  __block FSTAsyncQueryListener *listener =
+      [[FSTAsyncQueryListener alloc] initWithExecutor:_executor.get()
+                                      snapshotHandler:^(FSTViewSnapshot *snapshot, NSError *error) {
+                                        [accum addObject:snapshot];
+                                        [listener mute];
+                                      }];
 
   FSTView *view = [[FSTView alloc] initWithQuery:query remoteDocuments:DocumentKeySet{}];
   FSTViewSnapshot *viewSnapshot1 = FSTTestApplyChanges(view, @[ doc1 ], nil);
@@ -148,9 +155,7 @@ NS_ASSUME_NONNULL_BEGIN
 
   // Drain queue
   XCTestExpectation *expectation = [self expectationWithDescription:@"Queue drained"];
-  [self.asyncQueue dispatchAsync:^{
-    [expectation fulfill];
-  }];
+  _executor->Execute([=] { [expectation fulfill]; });
 
   [self waitForExpectationsWithTimeout:4.0
                                handler:^(NSError *_Nullable expectationError) {
diff --git a/Firestore/Source/API/FIRDocumentReference.mm b/Firestore/Source/API/FIRDocumentReference.mm
index da67a5b..5ad606c 100644
--- a/Firestore/Source/API/FIRDocumentReference.mm
+++ b/Firestore/Source/API/FIRDocumentReference.mm
@@ -280,8 +280,8 @@ addSnapshotListenerInternalWithOptions:(FSTListenOptions *)internalOptions
   };
 
   FSTAsyncQueryListener *asyncListener =
-      [[FSTAsyncQueryListener alloc] initWithDispatchQueue:self.firestore.client.userDispatchQueue
-                                           snapshotHandler:snapshotHandler];
+      [[FSTAsyncQueryListener alloc] initWithExecutor:self.firestore.client.userExecutor
+                                      snapshotHandler:snapshotHandler];
 
   FSTQueryListener *internalListener =
       [firestore.client listenToQuery:query
diff --git a/Firestore/Source/API/FIRFirestore.mm b/Firestore/Source/API/FIRFirestore.mm
index fe461d6..e5f0c12 100644
--- a/Firestore/Source/API/FIRFirestore.mm
+++ b/Firestore/Source/API/FIRFirestore.mm
@@ -45,12 +45,16 @@
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 #include "absl/memory/memory.h"
 
+#include "Firestore/core/src/firebase/firestore/util/executor_libdispatch.h"
+
 namespace util = firebase::firestore::util;
 using firebase::firestore::auth::CredentialsProvider;
 using firebase::firestore::auth::FirebaseCredentialsProvider;
 using firebase::firestore::core::DatabaseInfo;
 using firebase::firestore::model::DatabaseId;
 using firebase::firestore::model::ResourcePath;
+using util::internal::Executor;
+using util::internal::ExecutorLibdispatch;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -272,12 +276,13 @@ extern "C" NSString *const FIRFirestoreErrorDomain = @"FIRFirestoreErrorDomain";
       const DatabaseInfo database_info(*self.databaseID, util::MakeStringView(_persistenceKey),
                                        util::MakeStringView(_settings.host), _settings.sslEnabled);
 
-      FSTDispatchQueue *userDispatchQueue = [FSTDispatchQueue queueWith:_settings.dispatchQueue];
+      std::unique_ptr<Executor> userExecutor =
+          absl::make_unique<ExecutorLibdispatch>(_settings.dispatchQueue);
 
       _client = [FSTFirestoreClient clientWithDatabaseInfo:database_info
                                             usePersistence:_settings.persistenceEnabled
                                        credentialsProvider:_credentialsProvider.get()
-                                         userDispatchQueue:userDispatchQueue
+                                              userExecutor:std::move(userExecutor)
                                        workerDispatchQueue:_workerDispatchQueue];
     }
   }
diff --git a/Firestore/Source/API/FIRQuery.mm b/Firestore/Source/API/FIRQuery.mm
index c83af9c..596f6ac 100644
--- a/Firestore/Source/API/FIRQuery.mm
+++ b/Firestore/Source/API/FIRQuery.mm
@@ -183,8 +183,8 @@ addSnapshotListenerInternalWithOptions:(FSTListenOptions *)internalOptions
   };
 
   FSTAsyncQueryListener *asyncListener =
-      [[FSTAsyncQueryListener alloc] initWithDispatchQueue:self.firestore.client.userDispatchQueue
-                                           snapshotHandler:snapshotHandler];
+      [[FSTAsyncQueryListener alloc] initWithExecutor:self.firestore.client.userExecutor
+                                      snapshotHandler:snapshotHandler];
 
   FSTQueryListener *internalListener =
       [firestore.client listenToQuery:query
diff --git a/Firestore/Source/Core/FSTFirestoreClient.h b/Firestore/Source/Core/FSTFirestoreClient.h
index 7285e65..94c2284 100644
--- a/Firestore/Source/Core/FSTFirestoreClient.h
+++ b/Firestore/Source/Core/FSTFirestoreClient.h
@@ -15,6 +15,7 @@
  */
 
 #import <Foundation/Foundation.h>
+#include <memory>
 
 #import "Firestore/Source/Core/FSTTypes.h"
 #import "Firestore/Source/Core/FSTViewSnapshot.h"
@@ -23,6 +24,7 @@
 #include "Firestore/core/src/firebase/firestore/auth/credentials_provider.h"
 #include "Firestore/core/src/firebase/firestore/core/database_info.h"
 #include "Firestore/core/src/firebase/firestore/model/database_id.h"
+#include "Firestore/core/src/firebase/firestore/util/executor.h"
 
 @class FIRDocumentReference;
 @class FIRDocumentSnapshot;
@@ -50,14 +52,15 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Creates and returns a FSTFirestoreClient with the given parameters.
  *
- * All callbacks and events will be triggered on the provided userDispatchQueue.
+ * All callbacks and events will be triggered on the provided userExecutor.
  */
-+ (instancetype)clientWithDatabaseInfo:(const firebase::firestore::core::DatabaseInfo &)databaseInfo
-                        usePersistence:(BOOL)usePersistence
-                   credentialsProvider:(firebase::firestore::auth::CredentialsProvider *)
-                                           credentialsProvider  // no passing ownership
-                     userDispatchQueue:(FSTDispatchQueue *)userDispatchQueue
-                   workerDispatchQueue:(FSTDispatchQueue *)workerDispatchQueue;
++ (instancetype)
+clientWithDatabaseInfo:(const firebase::firestore::core::DatabaseInfo &)databaseInfo
+        usePersistence:(BOOL)usePersistence
+   credentialsProvider:(firebase::firestore::auth::CredentialsProvider *)
+                           credentialsProvider  // no passing ownership
+          userExecutor:(std::unique_ptr<firebase::firestore::util::internal::Executor>)userExecutor
+   workerDispatchQueue:(FSTDispatchQueue *)workerDispatchQueue;
 
 - (instancetype)init __attribute__((unavailable("Use static constructor method.")));
 
@@ -111,7 +114,7 @@ NS_ASSUME_NONNULL_BEGIN
  * Dispatch queue for user callbacks / events. This will often be the "Main Dispatch Queue" of the
  * app but the developer can configure it to a different queue if they so choose.
  */
-@property(nonatomic, strong, readonly) FSTDispatchQueue *userDispatchQueue;
+- (firebase::firestore::util::internal::Executor *)userExecutor;
 
 @end
 
diff --git a/Firestore/Source/Core/FSTFirestoreClient.mm b/Firestore/Source/Core/FSTFirestoreClient.mm
index 658cf57..93d1f7c 100644
--- a/Firestore/Source/Core/FSTFirestoreClient.mm
+++ b/Firestore/Source/Core/FSTFirestoreClient.mm
@@ -18,6 +18,7 @@
 
 #include <future>  // NOLINT(build/c++11)
 #include <memory>
+#include <utility>
 
 #import "FIRFirestoreErrors.h"
 #import "Firestore/Source/API/FIRDocumentReference+Internal.h"
@@ -58,6 +59,8 @@ using firebase::firestore::core::DatabaseInfo;
 using firebase::firestore::model::DatabaseId;
 using firebase::firestore::model::DocumentKeySet;
 
+using firebase::firestore::util::internal::Executor;
+
 NS_ASSUME_NONNULL_BEGIN
 
 @interface FSTFirestoreClient () {
@@ -68,7 +71,7 @@ NS_ASSUME_NONNULL_BEGIN
                       usePersistence:(BOOL)usePersistence
                  credentialsProvider:
                      (CredentialsProvider *)credentialsProvider  // no passing ownership
-                   userDispatchQueue:(FSTDispatchQueue *)userDispatchQueue
+                        userExecutor:(std::unique_ptr<Executor>)userExecutor
                  workerDispatchQueue:(FSTDispatchQueue *)queue NS_DESIGNATED_INITIALIZER;
 
 @property(nonatomic, assign, readonly) const DatabaseInfo *databaseInfo;
@@ -91,18 +94,24 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
-@implementation FSTFirestoreClient
+@implementation FSTFirestoreClient {
+  std::unique_ptr<Executor> _userExecutor;
+}
+
+- (Executor *)userExecutor {
+  return _userExecutor.get();
+}
 
 + (instancetype)clientWithDatabaseInfo:(const DatabaseInfo &)databaseInfo
                         usePersistence:(BOOL)usePersistence
                    credentialsProvider:
                        (CredentialsProvider *)credentialsProvider  // no passing ownership
-                     userDispatchQueue:(FSTDispatchQueue *)userDispatchQueue
+                          userExecutor:(std::unique_ptr<Executor>)userExecutor
                    workerDispatchQueue:(FSTDispatchQueue *)workerDispatchQueue {
   return [[FSTFirestoreClient alloc] initWithDatabaseInfo:databaseInfo
                                            usePersistence:usePersistence
                                       credentialsProvider:credentialsProvider
-                                        userDispatchQueue:userDispatchQueue
+                                             userExecutor:std::move(userExecutor)
                                       workerDispatchQueue:workerDispatchQueue];
 }
 
@@ -110,12 +119,12 @@ NS_ASSUME_NONNULL_BEGIN
                       usePersistence:(BOOL)usePersistence
                  credentialsProvider:
                      (CredentialsProvider *)credentialsProvider  // no passing ownership
-                   userDispatchQueue:(FSTDispatchQueue *)userDispatchQueue
+                        userExecutor:(std::unique_ptr<Executor>)userExecutor
                  workerDispatchQueue:(FSTDispatchQueue *)workerDispatchQueue {
   if (self = [super init]) {
     _databaseInfo = databaseInfo;
     _credentialsProvider = credentialsProvider;
-    _userDispatchQueue = userDispatchQueue;
+    _userExecutor = std::move(userExecutor);
     _workerDispatchQueue = workerDispatchQueue;
 
     auto userPromise = std::make_shared<std::promise<User>>();
@@ -230,9 +239,7 @@ NS_ASSUME_NONNULL_BEGIN
   [self.workerDispatchQueue dispatchAsync:^{
     [self.remoteStore disableNetwork];
     if (completion) {
-      [self.userDispatchQueue dispatchAsync:^{
-        completion(nil);
-      }];
+      _userExecutor->Execute([=] { completion(nil); });
     }
   }];
 }
@@ -241,9 +248,7 @@ NS_ASSUME_NONNULL_BEGIN
   [self.workerDispatchQueue dispatchAsync:^{
     [self.remoteStore enableNetwork];
     if (completion) {
-      [self.userDispatchQueue dispatchAsync:^{
-        completion(nil);
-      }];
+      _userExecutor->Execute([=] { completion(nil); });
     }
   }];
 }
@@ -255,9 +260,7 @@ NS_ASSUME_NONNULL_BEGIN
     [self.remoteStore shutdown];
     [self.persistence shutdown];
     if (completion) {
-      [self.userDispatchQueue dispatchAsync:^{
-        completion(nil);
-      }];
+      _userExecutor->Execute([=] { completion(nil); });
     }
   }];
 }
@@ -338,18 +341,14 @@ NS_ASSUME_NONNULL_BEGIN
   [self.workerDispatchQueue dispatchAsync:^{
     if (mutations.count == 0) {
       if (completion) {
-        [self.userDispatchQueue dispatchAsync:^{
-          completion(nil);
-        }];
+        _userExecutor->Execute([=] { completion(nil); });
       }
     } else {
       [self.syncEngine writeMutations:mutations
                            completion:^(NSError *error) {
                              // Dispatch the result back onto the user dispatch queue.
                              if (completion) {
-                               [self.userDispatchQueue dispatchAsync:^{
-                                 completion(error);
-                               }];
+                               _userExecutor->Execute([=] { completion(error); });
                              }
                            }];
     }
@@ -366,9 +365,7 @@ NS_ASSUME_NONNULL_BEGIN
                                  completion:^(id _Nullable result, NSError *_Nullable error) {
                                    // Dispatch the result back onto the user dispatch queue.
                                    if (completion) {
-                                     [self.userDispatchQueue dispatchAsync:^{
-                                       completion(result, error);
-                                     }];
+                                     _userExecutor->Execute([=] { completion(result, error); });
                                    }
                                  }];
   }];
diff --git a/Firestore/Source/Util/FSTAsyncQueryListener.h b/Firestore/Source/Util/FSTAsyncQueryListener.h
index 4888268..06471d8 100644
--- a/Firestore/Source/Util/FSTAsyncQueryListener.h
+++ b/Firestore/Source/Util/FSTAsyncQueryListener.h
@@ -18,6 +18,8 @@
 
 #import "Firestore/Source/Core/FSTViewSnapshot.h"
 
+#include "Firestore/core/src/firebase/firestore/util/executor.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
 @class FSTDispatchQueue;
@@ -28,9 +30,8 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface FSTAsyncQueryListener : NSObject
 
-- (instancetype)initWithDispatchQueue:(FSTDispatchQueue *)dispatchQueue
-                      snapshotHandler:(FSTViewSnapshotHandler)snapshotHandler
-    NS_DESIGNATED_INITIALIZER;
+- (instancetype)initWithExecutor:(firebase::firestore::util::internal::Executor*)executor
+                 snapshotHandler:(FSTViewSnapshotHandler)snapshotHandler NS_DESIGNATED_INITIALIZER;
 
 - (instancetype)init NS_UNAVAILABLE;
 
diff --git a/Firestore/Source/Util/FSTAsyncQueryListener.mm b/Firestore/Source/Util/FSTAsyncQueryListener.mm
index b72ac57..426594b 100644
--- a/Firestore/Source/Util/FSTAsyncQueryListener.mm
+++ b/Firestore/Source/Util/FSTAsyncQueryListener.mm
@@ -18,16 +18,18 @@
 
 #import "Firestore/Source/Util/FSTDispatchQueue.h"
 
+using firebase::firestore::util::internal::Executor;
+
 @implementation FSTAsyncQueryListener {
   volatile BOOL _muted;
   FSTViewSnapshotHandler _snapshotHandler;
-  FSTDispatchQueue *_dispatchQueue;
+  Executor *_executor;
 }
 
-- (instancetype)initWithDispatchQueue:(FSTDispatchQueue *)dispatchQueue
-                      snapshotHandler:(FSTViewSnapshotHandler)snapshotHandler {
+- (instancetype)initWithExecutor:(Executor *)executor
+                 snapshotHandler:(FSTViewSnapshotHandler)snapshotHandler {
   if (self = [super init]) {
-    _dispatchQueue = dispatchQueue;
+    _executor = executor;
     _snapshotHandler = snapshotHandler;
   }
   return self;
@@ -40,11 +42,11 @@
   // users just want to turn on notifications "forever" and don't want to have
   // to keep track of our handle to keep them going.
   return ^(FSTViewSnapshot *_Nullable snapshot, NSError *_Nullable error) {
-    [self->_dispatchQueue dispatchAsync:^{
+    _executor->Execute([self, snapshot, error] {
       if (!self->_muted) {
         self->_snapshotHandler(snapshot, error);
       }
-    }];
+    });
   };
 }