Share ExternalCommitHelpers between Realm instances for a single path

apple/external_commit_helper.cpp

@@ -86,10 +86,8 @@ void ExternalCommitHelper::FdHolder::close()
 // written to the anonymous pipe the background thread removes the runloop
 // source from the runloop and and shuts down.
 ExternalCommitHelper::ExternalCommitHelper(Realm* realm)
-: m_realm(realm)
-, m_run_loop(CFRunLoopGetCurrent())
 {
-    CFRetain(m_run_loop);
+    add_realm(realm);
 
     m_kq = kqueue();
     if (m_kq == -1) {
@@ -160,23 +158,49 @@ ExternalCommitHelper::ExternalCommitHelper(Realm* realm)
 
 ExternalCommitHelper::~ExternalCommitHelper()
 {
+    REALM_ASSERT_DEBUG(m_realms.empty());
     notify_fd(m_shutdown_write_fd);
     pthread_join(m_thread, nullptr); // Wait for the thread to exit
 }
 
+void ExternalCommitHelper::add_realm(realm::Realm* realm)
+{
+    std::lock_guard<std::mutex> lock(m_realms_mutex);
+
+    // Create the runloop source
+    CFRunLoopSourceContext ctx{};
+    ctx.info = realm;
+    ctx.perform = [](void* info) {
+        static_cast<Realm*>(info)->notify();
+    };
+
+    CFRunLoopRef runloop = CFRunLoopGetCurrent();
+    CFRetain(runloop);
+    CFRunLoopSourceRef signal = CFRunLoopSourceCreate(kCFAllocatorDefault, 0, &ctx);
+    CFRunLoopAddSource(runloop, signal, kCFRunLoopDefaultMode);
+
+    m_realms.push_back({realm, runloop, signal});
+}
+
+void ExternalCommitHelper::remove_realm(realm::Realm* realm)
+{
+    std::lock_guard<std::mutex> lock(m_realms_mutex);
+    for (auto it = m_realms.begin(); it != m_realms.end(); ++it) {
+        if (it->realm == realm) {
+            CFRunLoopSourceInvalidate(it->signal);
+            CFRelease(it->signal);
+            CFRelease(it->runloop);
+            m_realms.erase(it);
+            return;
+        }
+    }
+    REALM_TERMINATE("Realm not registered");
+}
+
 void ExternalCommitHelper::listen()
 {
     pthread_setname_np("RLMRealm notification listener");
 
-    // Create the runloop source
-    CFRunLoopSourceContext ctx{};
-    ctx.info = this;
-    ctx.perform = [](void *info) {
-        static_cast<ExternalCommitHelper *>(info)->m_realm->notify();
-    };
-
-    CFRunLoopSourceRef signal = CFRunLoopSourceCreate(kCFAllocatorDefault, 0, &ctx);
-    CFRunLoopAddSource(m_run_loop, signal, kCFRunLoopDefaultMode);
 
     // Set up the kqueue
     // EVFILT_READ indicates that we care about data being available to read
@@ -204,18 +228,18 @@ void ExternalCommitHelper::listen()
         // pipe, then someone called -stop; otherwise it's the named pipe
         // and someone committed a write transaction
         if (event.ident == (uint32_t)m_shutdown_read_fd) {
-            CFRunLoopSourceInvalidate(signal);
-            CFRelease(signal);
-            CFRelease(m_run_loop);
             return;
         }
         assert(event.ident == (uint32_t)m_notify_fd);
 
-        CFRunLoopSourceSignal(signal);
-        // Signalling the source makes it run the next time the runloop gets
-        // to it, but doesn't make the runloop start if it's currently idle
-        // waiting for events
-        CFRunLoopWakeUp(m_run_loop);
+        std::lock_guard<std::mutex> lock(m_realms_mutex);
+        for (auto const& realm : m_realms) {
+            CFRunLoopSourceSignal(realm.signal);
+            // Signalling the source makes it run the next time the runloop gets
+            // to it, but doesn't make the runloop start if it's currently idle
+            // waiting for events
+            CFRunLoopWakeUp(realm.runloop);
+        }
     }
 }
 

apple/external_commit_helper.hpp

@@ -20,6 +20,8 @@
 #define REALM_EXTERNAL_COMMIT_HELPER_HPP
 
 #include <CoreFoundation/CFRunLoop.h>
+#include <mutex>
+#include <vector>
 
 namespace realm {
 class Realm;
@@ -30,6 +32,8 @@ public:
     ~ExternalCommitHelper();
 
     void notify_others();
+    void add_realm(Realm* realm);
+    void remove_realm(Realm* realm);
 
 private:
     // A RAII holder for a file descriptor which automatically closes the wrapped
@@ -54,13 +58,20 @@ private:
         FdHolder(FdHolder const&) = delete;
     };
 
+    struct PerRealmInfo {
+        Realm* realm;
+        CFRunLoopRef runloop;
+        CFRunLoopSourceRef signal;
+    };
+
     void listen();
 
-    // This is owned by the realm, so it needs to not retain the realm
-    Realm *const m_realm;
+    // Currently registered realms and the signal for delivering notifications
+    // to them
+    std::vector<PerRealmInfo> m_realms;
 
-    // Runloop which notifications are delivered on
-    CFRunLoopRef m_run_loop;
+    // Mutex which guards m_realms
+    std::mutex m_realms_mutex;
 
     // The listener thread
     pthread_t m_thread;

shared_realm.cpp

@@ -55,7 +55,6 @@ Realm::Config& Realm::Config::operator=(realm::Realm::Config const& c)
 
 Realm::Realm(Config config)
 : m_config(std::move(config))
-, m_notifier(new ExternalCommitHelper(this))
 {
     try {
         if (m_config.read_only) {
@@ -85,6 +84,12 @@ Realm::Realm(Config config)
     }
 }
 
+Realm::~Realm() {
+    if (m_notifier) { // might not exist yet if an error occurred during init
+        m_notifier->remove_realm(this);
+    }
+}
+
 Group *Realm::read_group()
 {
     if (!m_group) {
@@ -132,8 +137,13 @@ SharedRealm Realm::get_shared_realm(Config config)
         // if there is an existing realm at the current path steal its schema/column mapping
         // FIXME - need to validate that schemas match
         realm->m_config.schema = std::make_unique<Schema>(*existing->m_config.schema);
+
+        realm->m_notifier = existing->m_notifier;
+        realm->m_notifier->add_realm(realm.get());
     }
     else {
+        realm->m_notifier = std::make_shared<ExternalCommitHelper>(realm.get());
+
         // otherwise get the schema from the group
         realm->m_config.schema = std::make_unique<Schema>(ObjectStore::schema_from_group(realm->read_group()));
 

shared_realm.hpp

@@ -97,6 +97,8 @@ namespace realm {
         std::thread::id thread_id() const { return m_thread_id; }
         void verify_thread();
 
+        ~Realm();
+
       private:
         Realm(Config config);
 
@@ -111,7 +113,7 @@ namespace realm {
 
         Group *m_group = nullptr;
 
-        std::unique_ptr<ExternalCommitHelper> m_notifier;
+        std::shared_ptr<ExternalCommitHelper> m_notifier;
 
       public:
         std::unique_ptr<RealmDelegate> m_delegate;