diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 91dbaf5..648254c 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,43 @@
+2019-05-15  Chris Dumez  <cdumez@apple.com>
+
+        [WK2][iOS] UIProcess may get killed because it is taking too long to release its background task after expiration
+        https://bugs.webkit.org/show_bug.cgi?id=197893
+        <rdar://problem/50234105>
+
+        Reviewed by Alex Christensen.
+
+        The UIProcess may get killed because it is taking too long to release its background task after its expiration handler
+        was called. The reason is that the background task's expiration handler was sequentially sending a ProcessWillSuspendImminently
+        synchronous IPC to each of its child processes and only then ends the background task. By the time we receive the response from
+        all child processes, it may be too late and we get killed.
+
+        To address the issue, we now:
+        1. Send the ProcessWillSuspendImminently asynchronously so that all processes can do their processing in parallel
+        2. After 2 seconds, the UIProcess releases the background task (We get killed after ~5 seconds)
+
+        Also, to make sure that the UIProcess supends promptly, we now make sure we never start a new background task *after*
+        the app has been backgrounded. The intention of our background task is too finish critical work (like releasing locked
+        files) after the app gets backgrounded, not to start new work and delay process suspension.
+
+        * NetworkProcess/NetworkProcess.cpp:
+        (WebKit::NetworkProcess::processWillSuspendImminently):
+        * NetworkProcess/NetworkProcess.h:
+        * NetworkProcess/NetworkProcess.messages.in:
+        * UIProcess/Network/NetworkProcessProxy.cpp:
+        (WebKit::NetworkProcessProxy::sendProcessWillSuspendImminently):
+        * UIProcess/WebProcessProxy.cpp:
+        (WebKit::WebProcessProxy::sendProcessWillSuspendImminently):
+        * UIProcess/ios/ProcessAssertionIOS.mm:
+        (-[WKProcessAssertionBackgroundTaskManager init]):
+        (-[WKProcessAssertionBackgroundTaskManager _scheduleReleaseTask]):
+        (-[WKProcessAssertionBackgroundTaskManager _cancelPendingReleaseTask]):
+        (-[WKProcessAssertionBackgroundTaskManager _updateBackgroundTask]):
+        * WebProcess/WebProcess.cpp:
+        (WebKit::WebProcess::didReceiveSyncMessage):
+        (WebKit::WebProcess::processWillSuspendImminently):
+        * WebProcess/WebProcess.h:
+        * WebProcess/WebProcess.messages.in:
+
 2019-05-15  Jiewen Tan  <jiewen_tan@apple.com>
 
         [WebAuthN] Make WebAuthN default on
diff --git a/Source/WebKit/NetworkProcess/NetworkProcess.cpp b/Source/WebKit/NetworkProcess/NetworkProcess.cpp
index 9c64844..b775889 100644
--- a/Source/WebKit/NetworkProcess/NetworkProcess.cpp
+++ b/Source/WebKit/NetworkProcess/NetworkProcess.cpp
@@ -1949,15 +1949,15 @@
 #endif
 }
 
-void NetworkProcess::processWillSuspendImminently(CompletionHandler<void(bool)>&& completionHandler)
+void NetworkProcess::processWillSuspendImminently()
 {
-    RELEASE_LOG(ProcessSuspension, "%p - NetworkProcess::processWillSuspendImminently()", this);
+    RELEASE_LOG(ProcessSuspension, "%p - NetworkProcess::processWillSuspendImminently() BEGIN", this);
 #if PLATFORM(IOS_FAMILY) && ENABLE(INDEXED_DATABASE)
     for (auto& server : m_idbServers.values())
         server->tryStop(IDBServer::ShouldForceStop::Yes);
 #endif
     actualPrepareToSuspend(ShouldAcknowledgeWhenReadyToSuspend::No);
-    completionHandler(true);
+    RELEASE_LOG(ProcessSuspension, "%p - NetworkProcess::processWillSuspendImminently() END", this);
 }
 
 void NetworkProcess::prepareToSuspend()
diff --git a/Source/WebKit/NetworkProcess/NetworkProcess.h b/Source/WebKit/NetworkProcess/NetworkProcess.h
index d8563d2..40a1acb 100644
--- a/Source/WebKit/NetworkProcess/NetworkProcess.h
+++ b/Source/WebKit/NetworkProcess/NetworkProcess.h
@@ -181,7 +181,7 @@
 
     bool canHandleHTTPSServerTrustEvaluation() const { return m_canHandleHTTPSServerTrustEvaluation; }
 
-    void processWillSuspendImminently(CompletionHandler<void(bool)>&&);
+    void processWillSuspendImminently();
     void prepareToSuspend();
     void cancelPrepareToSuspend();
     void processDidResume();
diff --git a/Source/WebKit/NetworkProcess/NetworkProcess.messages.in b/Source/WebKit/NetworkProcess/NetworkProcess.messages.in
index 35bc82b..74c5025 100644
--- a/Source/WebKit/NetworkProcess/NetworkProcess.messages.in
+++ b/Source/WebKit/NetworkProcess/NetworkProcess.messages.in
@@ -77,7 +77,7 @@
     ProcessDidTransitionToBackground()
     ProcessDidTransitionToForeground()
 
-    ProcessWillSuspendImminently() -> (bool handled) Synchronous
+    ProcessWillSuspendImminently()
     PrepareToSuspend()
     CancelPrepareToSuspend()
     ProcessDidResume()
diff --git a/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp b/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp
index 0ee81be..81c4f0e 100644
--- a/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp
+++ b/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp
@@ -957,11 +957,8 @@
 
 void NetworkProcessProxy::sendProcessWillSuspendImminently()
 {
-    if (!canSendMessage())
-        return;
-
-    bool handled = false;
-    sendSync(Messages::NetworkProcess::ProcessWillSuspendImminently(), Messages::NetworkProcess::ProcessWillSuspendImminently::Reply(handled), 0, 1_s);
+    if (canSendMessage())
+        send(Messages::NetworkProcess::ProcessWillSuspendImminently(), 0);
 }
     
 void NetworkProcessProxy::sendPrepareToSuspend()
diff --git a/Source/WebKit/UIProcess/WebProcessProxy.cpp b/Source/WebKit/UIProcess/WebProcessProxy.cpp
index 4248ec9..8203465 100644
--- a/Source/WebKit/UIProcess/WebProcessProxy.cpp
+++ b/Source/WebKit/UIProcess/WebProcessProxy.cpp
@@ -1146,11 +1146,8 @@
 
 void WebProcessProxy::sendProcessWillSuspendImminently()
 {
-    if (!canSendMessage())
-        return;
-
-    bool handled = false;
-    sendSync(Messages::WebProcess::ProcessWillSuspendImminently(), Messages::WebProcess::ProcessWillSuspendImminently::Reply(handled), 0, 1_s);
+    if (canSendMessage())
+        send(Messages::WebProcess::ProcessWillSuspendImminently(), 0);
 }
 
 void WebProcessProxy::sendPrepareToSuspend()
diff --git a/Source/WebKit/UIProcess/ios/ProcessAssertionIOS.mm b/Source/WebKit/UIProcess/ios/ProcessAssertionIOS.mm
index dc35acc..2aca868 100644
--- a/Source/WebKit/UIProcess/ios/ProcessAssertionIOS.mm
+++ b/Source/WebKit/UIProcess/ios/ProcessAssertionIOS.mm
@@ -37,6 +37,11 @@
 
 using WebKit::ProcessAndUIAssertion;
 
+// This gives some time to our child processes to process the ProcessWillSuspendImminently IPC but makes sure we release
+// the background task before the UIKit timeout (We get killed if we do not release the background task within 5 seconds
+// on the expiration handler getting called).
+static const Seconds releaseBackgroundTaskAfterExpirationDelay { 2_s };
+
 @interface WKProcessAssertionBackgroundTaskManager : NSObject
 
 + (WKProcessAssertionBackgroundTaskManager *)shared;
@@ -50,7 +55,8 @@
 {
     UIBackgroundTaskIdentifier _backgroundTask;
     HashSet<ProcessAndUIAssertion*> _assertionsNeedingBackgroundTask;
-    BOOL _assertionHasExpiredInTheBackground;
+    BOOL _applicationIsBackgrounded;
+    dispatch_block_t _pendingTaskReleaseTask;
 }
 
 + (WKProcessAssertionBackgroundTaskManager *)shared
@@ -68,10 +74,15 @@
     _backgroundTask = UIBackgroundTaskInvalid;
 
     [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationWillEnterForegroundNotification object:[UIApplication sharedApplication] queue:nil usingBlock:^(NSNotification *) {
-        _assertionHasExpiredInTheBackground = NO;
+        _applicationIsBackgrounded = NO;
+        [self _cancelPendingReleaseTask];
         [self _updateBackgroundTask];
     }];
 
+    [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationDidEnterBackgroundNotification object:[UIApplication sharedApplication] queue:nil usingBlock:^(NSNotification *) {
+        _applicationIsBackgrounded = YES;
+    }];
+
     return self;
 }
 
@@ -101,12 +112,40 @@
         assertion->uiAssertionWillExpireImminently();
 }
 
+
+- (void)_scheduleReleaseTask
+{
+    ASSERT(!_pendingTaskReleaseTask);
+    if (_pendingTaskReleaseTask)
+        return;
+
+    RELEASE_LOG(ProcessSuspension, "%p - WKProcessAssertionBackgroundTaskManager - _scheduleReleaseTask because the expiration handler has been called", self);
+    _pendingTaskReleaseTask = dispatch_block_create((dispatch_block_flags_t)0, ^{
+        _pendingTaskReleaseTask = nil;
+        [self _releaseBackgroundTask];
+    });
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, releaseBackgroundTaskAfterExpirationDelay.value() * NSEC_PER_SEC), dispatch_get_main_queue(), _pendingTaskReleaseTask);
+#if !__has_feature(objc_arc)
+    // dispatch_async() does a Block_copy() / Block_release() on behalf of the caller.
+    Block_release(_pendingTaskReleaseTask);
+#endif
+}
+
+- (void)_cancelPendingReleaseTask
+{
+    if (!_pendingTaskReleaseTask)
+        return;
+
+    RELEASE_LOG(ProcessSuspension, "%p - WKProcessAssertionBackgroundTaskManager - _cancelPendingReleaseTask because the application is foreground again", self);
+    dispatch_block_cancel(_pendingTaskReleaseTask);
+    _pendingTaskReleaseTask = nil;
+}
+
 - (void)_updateBackgroundTask
 {
     if (!_assertionsNeedingBackgroundTask.isEmpty() && _backgroundTask == UIBackgroundTaskInvalid) {
-        if (_assertionHasExpiredInTheBackground) {
-            RELEASE_LOG_ERROR(ProcessSuspension, "%p - WKProcessAssertionBackgroundTaskManager: Ignored request to start a background task because we're still in the background and the previous task expired", self);
-            // Our invalidation handler would not get called if we tried to re-take a new background assertion at this point, and the UIProcess would get killed (rdar://problem/50001505).
+        if (_applicationIsBackgrounded) {
+            RELEASE_LOG_ERROR(ProcessSuspension, "%p - WKProcessAssertionBackgroundTaskManager: Ignored request to start a new background task because the application is already in the background", self);
             return;
         }
         RELEASE_LOG(ProcessSuspension, "%p - WKProcessAssertionBackgroundTaskManager - beginBackgroundTaskWithName", self);
@@ -121,9 +160,7 @@
                 });
             }
 
-            // Remember that the assertion has expired in the background so we do not try to re-take it until the application becomes foreground again.
-            _assertionHasExpiredInTheBackground = YES;
-            [self _releaseBackgroundTask];
+            [self _scheduleReleaseTask];
         }];
     } else if (_assertionsNeedingBackgroundTask.isEmpty())
         [self _releaseBackgroundTask];
diff --git a/Source/WebKit/WebProcess/WebProcess.cpp b/Source/WebKit/WebProcess/WebProcess.cpp
index 97b8305..d21bbf6 100644
--- a/Source/WebKit/WebProcess/WebProcess.cpp
+++ b/Source/WebKit/WebProcess/WebProcess.cpp
@@ -732,8 +732,6 @@
 {
     if (messageReceiverMap().dispatchSyncMessage(connection, decoder, replyEncoder))
         return;
-
-    didReceiveSyncWebProcessMessage(connection, decoder, replyEncoder);
 }
 
 void WebProcess::didReceiveMessage(IPC::Connection& connection, IPC::Decoder& decoder)
@@ -1490,19 +1488,11 @@
     });
 }
 
-void WebProcess::processWillSuspendImminently(CompletionHandler<void(bool)>&& completionHandler)
+void WebProcess::processWillSuspendImminently()
 {
-    if (parentProcessConnection()->inSendSync()) {
-        // Avoid reentrency bugs such as rdar://problem/21605505 by just bailing
-        // if we get an incoming ProcessWillSuspendImminently message when waiting for a
-        // reply to a sync message.
-        // FIXME: ProcessWillSuspendImminently should not be a sync message.
-        return completionHandler(false);
-    }
-
-    RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processWillSuspendImminently()", this);
+    RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processWillSuspendImminently() BEGIN", this);
     actualPrepareToSuspend(ShouldAcknowledgeWhenReadyToSuspend::No);
-    completionHandler(true);
+    RELEASE_LOG(ProcessSuspension, "%p - WebProcess::processWillSuspendImminently() END", this);
 }
 
 void WebProcess::prepareToSuspend()
diff --git a/Source/WebKit/WebProcess/WebProcess.h b/Source/WebKit/WebProcess/WebProcess.h
index 4b6ddc6..96d3ce9 100644
--- a/Source/WebKit/WebProcess/WebProcess.h
+++ b/Source/WebKit/WebProcess/WebProcess.h
@@ -216,7 +216,7 @@
 
     void setHiddenPageDOMTimerThrottlingIncreaseLimit(int milliseconds);
 
-    void processWillSuspendImminently(CompletionHandler<void(bool)>&&);
+    void processWillSuspendImminently();
     void prepareToSuspend();
     void cancelPrepareToSuspend();
     void processDidResume();
@@ -420,7 +420,6 @@
 
     // Implemented in generated WebProcessMessageReceiver.cpp
     void didReceiveWebProcessMessage(IPC::Connection&, IPC::Decoder&);
-    void didReceiveSyncWebProcessMessage(IPC::Connection&, IPC::Decoder&, std::unique_ptr<IPC::Encoder>&);
 
 #if PLATFORM(MAC)
     void setScreenProperties(const WebCore::ScreenProperties&);
diff --git a/Source/WebKit/WebProcess/WebProcess.messages.in b/Source/WebKit/WebProcess/WebProcess.messages.in
index 3847412..b39b38c 100644
--- a/Source/WebKit/WebProcess/WebProcess.messages.in
+++ b/Source/WebKit/WebProcess/WebProcess.messages.in
@@ -96,7 +96,7 @@
     EnsureAutomationSessionProxy(String sessionIdentifier)
     DestroyAutomationSessionProxy()
 
-    ProcessWillSuspendImminently() -> (bool handled) Synchronous
+    ProcessWillSuspendImminently()
     PrepareToSuspend()
     CancelPrepareToSuspend()
     ProcessDidResume()