Don't wait 8 minutes to suspend background tabs on iOS

https://bugs.webkit.org/show_bug.cgi?id=265064
rdar://118578232

Reviewed by Chris Dumez.

Disable the NearSuspended state, which allows background WebContent processes to suspend immediately
rather than after 8 minutes.

To do this, we need to fix the heuristic in `prepareToDropLastAssertion`, which fires just before a
process task_suspends. Previously it was generally called after a process ended up in the process
cache. Now that we are calling it earlier, we want it to prevent clearing out the caches from
processes that might end up in the process cache in the future, so we have to consult
`canBeAddedToWebProcessCache` about that.

Another change is that the `SetIsInProcessCache` message might be sent while the process is
task_suspended. To fix this, change that message to one with an async reply handler so that it takes
out a background activity while the IPC is being handled.

Finally, the `WebProcess::releaseMemory` IPC handler was often causing the process to completely exit,
since it invokes the memory pressure handler, and the memory pressure handler exits if there are no
active pages in the process. This seems undesired (if the UIProcess really wanted to terminate the
WebProcess, it could just send the `AuxiliaryProcess::ShutDown` message). So we now suppress the
memory pressure handler called by `WebProcess::releaseMemory` from exiting the process.

* Source/WebKit/UIProcess/WebProcessProxy.cpp:
(WebKit::WebProcessProxy::setIsInProcessCache):
(WebKit::WebProcessProxy::prepareToDropLastAssertion):
* Source/WebKit/WebProcess/WebProcess.cpp:
(WebKit::WebProcess::initializeWebProcess):
(WebKit::WebProcess::setIsInProcessCache):
(WebKit::WebProcess::releaseMemory):
(WebKit::WebProcess::prepareToSuspend):
* Source/WebKit/WebProcess/WebProcess.h:
* Source/WebKit/WebProcess/WebProcess.messages.in:

Canonical link: https://commits.webkit.org/272937@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -6160,7 +6160,7 @@ ShouldTakeNearSuspendedAssertions:
     WebKitLegacy:
       default: true
     WebKit:
-      default: true
+      default: defaultShouldTakeNearSuspendedAssertion()
     WebCore:
       default: true
 

Source/WTF/wtf/cocoa/RuntimeApplicationChecksCocoa.h

@@ -55,6 +55,7 @@ enum class SDKAlignedBehavior {
     ExpiredOnlyReloadBehavior,
     ForbidsDotPrefixedFonts,
     FullySuspendsBackgroundContent,
+    FullySuspendsBackgroundContentImmediately,
     HasUIContextMenuInteraction,
     HTMLDocumentSupportedPropertyNames,
     InitializeWebKit2MainThreadAssertion,

Source/WebKit/Shared/WebPreferencesDefaultValues.cpp

@@ -232,6 +232,16 @@ bool defaultShouldDropNearSuspendedAssertionAfterDelay()
 #endif
 }
 
+bool defaultShouldTakeNearSuspendedAssertion()
+{
+#if PLATFORM(IOS_FAMILY)
+    static bool newSDK = linkedOnOrAfterSDKWithBehavior(SDKAlignedBehavior::FullySuspendsBackgroundContentImmediately);
+    return !newSDK;
+#else
+    return true;
+#endif
+}
+
 bool defaultLiveRangeSelectionEnabled()
 {
 #if PLATFORM(IOS_FAMILY)

Source/WebKit/Shared/WebPreferencesDefaultValues.h

@@ -101,6 +101,7 @@ bool defaultGamepadVibrationActuatorEnabled();
 
 bool defaultRunningBoardThrottlingEnabled();
 bool defaultShouldDropNearSuspendedAssertionAfterDelay();
+bool defaultShouldTakeNearSuspendedAssertion();
 bool defaultShowModalDialogEnabled();
 bool defaultLiveRangeSelectionEnabled();
 

Source/WebKit/UIProcess/WebProcessProxy.cpp

@@ -390,7 +390,9 @@ void WebProcessProxy::setIsInProcessCache(bool value, WillShutDown willShutDown)
     if (willShutDown == WillShutDown::Yes)
         return;
 
-    send(Messages::WebProcess::SetIsInProcessCache(m_isInProcessCache), 0);
+    // The WebProcess might be task_suspended at this point, so use sendWithAsyncReply to resume
+    // the process via a background activity long enough to process the IPC if necessary.
+    sendWithAsyncReply(Messages::WebProcess::SetIsInProcessCache(m_isInProcessCache), []() { });
 
     if (m_isInProcessCache) {
         // WebProcessProxy objects normally keep the process pool alive but we do not want this to be the case
@@ -1758,16 +1760,25 @@ void WebProcessProxy::didDropLastAssertion()
 
 void WebProcessProxy::prepareToDropLastAssertion(CompletionHandler<void()>&& completionHandler)
 {
-#if PLATFORM(MAC)
-    if (isInProcessCache()) {
-        // We don't free caches in cached WebProcesses on macOS for performance reasons.
-        // Cached WebProcess will anyway shutdown on memory pressure.
+#if ENABLE(WEBPROCESS_CACHE)
+    if (isInProcessCache() || !m_suspendedPages.isEmptyIgnoringNullReferences() || (canTerminateAuxiliaryProcess() && canBeAddedToWebProcessCache())) {
+        // We avoid freeing caches if:
+        //
+        //  1. The process is already in the WebProcess cache.
+        //  2. The process is already in the back/forward cache.
+        //  3. The process might end up in the process cache (canTerminateAuxiliaryProcess() && canBeAddedToWebProcessCache())
+        //
+        // The idea here is that we want these cached processes to retain useful data if they're
+        // reused. They have a low jetsam priority and will be killed by our low memory handler or
+        // the kernel if necessary.
         return completionHandler();
     }
-#endif
-    // We don't slim down the process in the PrepareToSuspend IPC, we delay clearing the
-    // caches until we release the suspended assertion.
+    // When the WebProcess cache is enabled, instead of freeing caches in the PrepareToSuspend
+    // we free caches here just before we drop our last process assertion.
     sendWithAsyncReply(Messages::WebProcess::ReleaseMemory(), WTFMove(completionHandler), 0, { }, ShouldStartProcessThrottlerActivity::No);
+#else
+    completionHandler();
+#endif
 }
 
 String WebProcessProxy::environmentIdentifier() const

Source/WebKit/WebProcess/WebProcess.cpp

@@ -436,7 +436,7 @@ void WebProcess::initializeWebProcess(WebProcessCreationParameters&& parameters)
 
 #if PLATFORM(COCOA)
             // If this is a process we keep around for performance, kill it on memory pressure instead of trying to free up its memory.
-            if (!m_isSuspending && (m_processType == ProcessType::CachedWebContent || m_processType == ProcessType::PrewarmedWebContent || areAllPagesSuspended())) {
+            if (m_allowExitOnMemoryPressure && (m_processType == ProcessType::CachedWebContent || m_processType == ProcessType::PrewarmedWebContent || areAllPagesSuspended())) {
                 if (m_processType == ProcessType::CachedWebContent)
                     WEBPROCESS_RELEASE_LOG(Process, "initializeWebProcess: Cached WebProcess is exiting due to memory pressure");
                 else if (m_processType == ProcessType::PrewarmedWebContent)
@@ -448,8 +448,8 @@ void WebProcess::initializeWebProcess(WebProcessCreationParameters&& parameters)
             }
 #endif
 
-            auto maintainBackForwardCache = m_isSuspending ? WebCore::MaintainBackForwardCache::Yes : WebCore::MaintainBackForwardCache::No;
-            auto maintainMemoryCache = m_isSuspending && m_hasSuspendedPageProxy ? WebCore::MaintainMemoryCache::Yes : WebCore::MaintainMemoryCache::No;
+            auto maintainBackForwardCache = m_allowExitOnMemoryPressure ? WebCore::MaintainBackForwardCache::No : WebCore::MaintainBackForwardCache::Yes;
+            auto maintainMemoryCache = (m_allowExitOnMemoryPressure || !m_hasSuspendedPageProxy) ? WebCore::MaintainMemoryCache::No : WebCore::MaintainMemoryCache::Yes;
             WebCore::releaseMemory(critical, synchronous, maintainBackForwardCache, maintainMemoryCache);
             for (auto& page : m_pageMap.values())
                 page->releaseMemory(critical);
@@ -674,7 +674,7 @@ void WebProcess::setHasSuspendedPageProxy(bool hasSuspendedPageProxy)
     m_hasSuspendedPageProxy = hasSuspendedPageProxy;
 }
 
-void WebProcess::setIsInProcessCache(bool isInProcessCache)
+void WebProcess::setIsInProcessCache(bool isInProcessCache, CompletionHandler<void()>&& completionHandler)
 {
 #if PLATFORM(COCOA)
     if (isInProcessCache) {
@@ -690,6 +690,7 @@ void WebProcess::setIsInProcessCache(bool isInProcessCache)
 #else
     UNUSED_PARAM(isInProcessCache);
 #endif
+    completionHandler();
 }
 
 void WebProcess::markIsNoLongerPrewarmed()
@@ -1523,6 +1524,7 @@ void WebProcess::pageActivityStateDidChange(PageIdentifier, OptionSet<WebCore::A
 void WebProcess::releaseMemory(CompletionHandler<void()>&& completionHandler)
 {
     WEBPROCESS_RELEASE_LOG(ProcessSuspension, "releaseMemory: BEGIN");
+    SetForScope allowExitScope(m_allowExitOnMemoryPressure, false);
     MemoryPressureHandler::singleton().releaseMemory(Critical::Yes, Synchronous::Yes);
     for (auto& page : m_pageMap.values())
         page->releaseMemory(Critical::Yes);
@@ -1537,7 +1539,7 @@ void WebProcess::prepareToSuspend(bool isSuspensionImminent, MonotonicTime estim
     double remainingRunTime = nowTime > estimatedSuspendTime ? (nowTime - estimatedSuspendTime).value() : 0.0;
 #endif
     WEBPROCESS_RELEASE_LOG(ProcessSuspension, "prepareToSuspend: isSuspensionImminent=%d, remainingRunTime=%fs", isSuspensionImminent, remainingRunTime);
-    SetForScope suspensionScope(m_isSuspending, true);
+    SetForScope allowExitScope(m_allowExitOnMemoryPressure, false);
     m_processIsSuspended = true;
 
     flushResourceLoadStatistics();

Source/WebKit/WebProcess/WebProcess.h

@@ -470,7 +470,7 @@ class WebProcess : public AuxiliaryProcess
     void platformTerminate();
 
     void setHasSuspendedPageProxy(bool);
-    void setIsInProcessCache(bool);
+    void setIsInProcessCache(bool, CompletionHandler<void()>&&);
     void markIsNoLongerPrewarmed();
 
     void registerURLSchemeAsEmptyDocument(const String&);
@@ -781,7 +781,7 @@ class WebProcess : public AuxiliaryProcess
 #endif
 
     bool m_hasSuspendedPageProxy { false };
-    bool m_isSuspending { false };
+    bool m_allowExitOnMemoryPressure { true };
     bool m_isLockdownModeEnabled { false };
 
 #if ENABLE(MEDIA_STREAM) && ENABLE(SANDBOX_EXTENSIONS)

Source/WebKit/WebProcess/WebProcess.messages.in

@@ -105,7 +105,7 @@ messages -> WebProcess LegacyReceiver NotRefCounted {
     EstablishRemoteWorkerContextConnectionToNetworkProcess(enum:uint8_t WebKit::RemoteWorkerType workerType, WebKit::PageGroupIdentifier pageGroupID, WebKit::WebPageProxyIdentifier webPageProxyID, WebCore::PageIdentifier pageID, struct WebKit::WebPreferencesStore store, WebCore::RegistrableDomain domain, std::optional<WebCore::ScriptExecutionContextIdentifier> serviceWorkerPageIdentifier, struct WebKit::RemoteWorkerInitializationData initializationData) -> ()
 
     SetHasSuspendedPageProxy(bool hasSuspendedPageProxy);
-    SetIsInProcessCache(bool isInProcessCache)
+    SetIsInProcessCache(bool isInProcessCache) -> ()
     MarkIsNoLongerPrewarmed()
     GetActivePagesOriginsForTesting() -> (Vector<String> activeOrigins)