fix: maintain a ref count for objects sent over remote (#17464)

* spec: clean up after a failed window count assertion

Previously when this assertion failed all tests that ran after the
failed assertion also failed.  This ensure that the assertion fails for
the test that actually caused the issue but cleans up the left-over
windows so that future tests do not fail.

* fix: maintain a ref count for objects sent over remote

Previously there was a race condition where a GC could occur in the
renderer process between the main process sending a meta.id and the
renderer pulling the proxy out its weakmap to stop it being GC'ed.

This fixes that race condition by maintaining a "sent" ref count in the
object registry and a "received" ref count in the object cache on the
renderer side.  The deref request now sends the number of refs the
renderer thinks it owns, if the number does not match the value in the
object registry it is assumed that there is an IPC message containing a
new reference in flight and this race condition was hit.

The browser side ref count is then reduced and we wait for the new deref
message.  This guaruntees that an object will only be removed from the
registry if every reference we sent has been guarunteed to be unreffed.

atom/common/api/atom_api_v8_util.cc

@@ -115,6 +115,7 @@ void Initialize(v8::Local<v8::Object> exports,
   dict.SetMethod("takeHeapSnapshot", &TakeHeapSnapshot);
   dict.SetMethod("setRemoteCallbackFreer", &atom::RemoteCallbackFreer::BindTo);
   dict.SetMethod("setRemoteObjectFreer", &atom::RemoteObjectFreer::BindTo);
+  dict.SetMethod("addRemoteObjectRef", &atom::RemoteObjectFreer::AddRef);
   dict.SetMethod("createIDWeakMap", &atom::api::KeyWeakMap<int32_t>::Create);
   dict.SetMethod(
       "createDoubleIDWeakMap",

atom/common/api/remote_object_freer.cc

@@ -36,6 +36,14 @@ void RemoteObjectFreer::BindTo(v8::Isolate* isolate,
   new RemoteObjectFreer(isolate, target, context_id, object_id);
 }
 
+// static
+void RemoteObjectFreer::AddRef(const std::string& context_id, int object_id) {
+  ref_mapper_[context_id][object_id]++;
+}
+
+// static
+std::map<std::string, std::map<int, int>> RemoteObjectFreer::ref_mapper_;
+
 RemoteObjectFreer::RemoteObjectFreer(v8::Isolate* isolate,
                                      v8::Local<v8::Object> target,
                                      const std::string& context_id,
@@ -62,6 +70,12 @@ void RemoteObjectFreer::RunDestructor() {
   base::ListValue args;
   args.AppendString(context_id_);
   args.AppendInteger(object_id_);
+  args.AppendInteger(ref_mapper_[context_id_][object_id_]);
+  // Reset our local ref count in case we are in a GC race condition and will
+  // get more references in an inbound IPC message
+  ref_mapper_[context_id_].erase(object_id_);
+  if (ref_mapper_[context_id_].empty())
+    ref_mapper_.erase(context_id_);
 
   mojom::ElectronBrowserAssociatedPtr electron_ptr;
   render_frame->GetRemoteAssociatedInterfaces()->GetInterface(

atom/common/api/remote_object_freer.h

@@ -5,6 +5,7 @@
 #ifndef ATOM_COMMON_API_REMOTE_OBJECT_FREER_H_
 #define ATOM_COMMON_API_REMOTE_OBJECT_FREER_H_
 
+#include <map>
 #include <string>
 
 #include "atom/common/api/object_life_monitor.h"
@@ -17,6 +18,7 @@ class RemoteObjectFreer : public ObjectLifeMonitor {
                      v8::Local<v8::Object> target,
                      const std::string& context_id,
                      int object_id);
+  static void AddRef(const std::string& context_id, int object_id);
 
  protected:
   RemoteObjectFreer(v8::Isolate* isolate,
@@ -27,6 +29,9 @@ class RemoteObjectFreer : public ObjectLifeMonitor {
 
   void RunDestructor() override;
 
+  // { context_id => { object_id => ref_count }}
+  static std::map<std::string, std::map<int, int>> ref_mapper_;
+
  private:
   std::string context_id_;
   int object_id_;

lib/browser/objects-registry.js

@@ -14,8 +14,8 @@ class ObjectsRegistry {
     // (id) => {object, count}
     this.storage = {}
 
-    // Stores the IDs of objects referenced by WebContents.
-    // (ownerKey) => [id]
+    // Stores the IDs + refCounts of objects referenced by WebContents.
+    // (ownerKey) => { id: refCount }
     this.owners = {}
   }
 
@@ -29,14 +29,16 @@ class ObjectsRegistry {
     const ownerKey = getOwnerKey(webContents, contextId)
     let owner = this.owners[ownerKey]
     if (!owner) {
-      owner = this.owners[ownerKey] = new Set()
+      owner = this.owners[ownerKey] = new Map()
       this.registerDeleteListener(webContents, contextId)
     }
     if (!owner.has(id)) {
-      owner.add(id)
+      owner.set(id, 0)
       // Increase reference count if not referenced before.
       this.storage[id].count++
     }
+
+    owner.set(id, owner.get(id) + 1)
     return id
   }
 
@@ -49,14 +51,30 @@ class ObjectsRegistry {
   // Dereference an object according to its ID.
   // Note that an object may be double-freed (cleared when page is reloaded, and
   // then garbage collected in old page).
-  remove (webContents, contextId, id) {
+  // rendererSideRefCount is the ref count that the renderer process reported
+  // at time of GC if this is different to the number of references we sent to
+  // the given owner then a GC occurred between a ref being sent and the value
+  // being pulled out of the weak map.
+  // In this case we decrement out ref count and do not delete the stored
+  // object
+  // For more details on why we do renderer side ref counting see
+  // https://github.com/electron/electron/pull/17464
+  remove (webContents, contextId, id, rendererSideRefCount) {
     const ownerKey = getOwnerKey(webContents, contextId)
     const owner = this.owners[ownerKey]
-    if (owner) {
-      // Remove the reference in owner.
-      owner.delete(id)
-      // Dereference from the storage.
-      this.dereference(id)
+    if (owner && owner.has(id)) {
+      const newRefCount = owner.get(id) - rendererSideRefCount
+
+      // Only completely remove if the number of references GCed in the
+      // renderer is the same as the number of references we sent them
+      if (newRefCount <= 0) {
+        // Remove the reference in owner.
+        owner.delete(id)
+        // Dereference from the storage.
+        this.dereference(id)
+      } else {
+        owner.set(id, newRefCount)
+      }
     }
   }
 
@@ -66,7 +84,7 @@ class ObjectsRegistry {
     const owner = this.owners[ownerKey]
     if (!owner) return
 
-    for (const id of owner) this.dereference(id)
+    for (const id of owner.keys()) this.dereference(id)
 
     delete this.owners[ownerKey]
   }

lib/browser/rpc-server.js

@@ -436,8 +436,8 @@ handleRemoteCommand('ELECTRON_BROWSER_MEMBER_GET', function (event, contextId, i
   return valueToMeta(event.sender, contextId, obj[name])
 })
 
-handleRemoteCommand('ELECTRON_BROWSER_DEREFERENCE', function (event, contextId, id) {
-  objectsRegistry.remove(event.sender, contextId, id)
+handleRemoteCommand('ELECTRON_BROWSER_DEREFERENCE', function (event, contextId, id, rendererSideRefCount) {
+  objectsRegistry.remove(event.sender, contextId, id, rendererSideRefCount)
 })
 
 handleRemoteCommand('ELECTRON_BROWSER_CONTEXT_RELEASE', (event, contextId) => {

lib/renderer/api/remote.js

@@ -227,6 +227,7 @@ function metaToValue (meta) {
   } else {
     let ret
     if (remoteObjectCache.has(meta.id)) {
+      v8Util.addRemoteObjectRef(contextId, meta.id)
       return remoteObjectCache.get(meta.id)
     }
 
@@ -254,6 +255,7 @@ function metaToValue (meta) {
     // Track delegate obj's lifetime & tell browser to clean up when object is GCed.
     v8Util.setRemoteObjectFreer(ret, contextId, meta.id)
     v8Util.setHiddenValue(ret, 'atomId', meta.id)
+    v8Util.addRemoteObjectRef(contextId, meta.id)
     remoteObjectCache.set(meta.id, ret)
     return ret
   }

spec-main/window-helpers.ts

@@ -14,7 +14,13 @@ export const closeWindow = async (
   }
 
   if (assertNotWindows) {
-    expect(BrowserWindow.getAllWindows()).to.have.lengthOf(0)
+    const windows = BrowserWindow.getAllWindows()
+    for (const win of windows) {
+      const closePromise = emittedOnce(win, 'closed')
+      win.close()
+      await closePromise
+    }
+    expect(windows).to.have.lengthOf(0)
   }
 }
 

spec/window-helpers.js

@@ -1,5 +1,6 @@
 const { expect } = require('chai')
-const { BrowserWindow } = require('electron').remote
+const { remote } = require('electron')
+const { BrowserWindow } = remote
 
 const { emittedOnce } = require('./events-helpers')
 
@@ -14,7 +15,19 @@ exports.closeWindow = async (window = null,
   }
 
   if (assertSingleWindow) {
-    expect(BrowserWindow.getAllWindows()).to.have.lengthOf(1)
+    // Although we want to assert that no windows were left handing around
+    // we also want to clean up the left over windows so that no future
+    // tests fail as a side effect
+    const currentId = remote.getCurrentWindow().id
+    const windows = BrowserWindow.getAllWindows()
+    for (const win of windows) {
+      if (win.id !== currentId) {
+        const closePromise = emittedOnce(win, 'closed')
+        win.close()
+        await closePromise
+      }
+    }
+    expect(windows).to.have.lengthOf(1)
   }
 }