diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 4ffcebb..a4a3466 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,30 @@
+2019-10-16  Kate Cheney  <katherine_cheney@apple.com>
+
+        WKWebsiteDataStoreStatisticsResetToConsistentState() should call store.setUseITPDatabase(false)
+        https://bugs.webkit.org/show_bug.cgi?id=203020
+        <rdar://problem/56314484>
+
+        Reviewed by Chris Dumez.
+
+        The resourceLoadStatisticsStore should be reset between tests to use
+        a memory store, and any test that needs to use the database store
+        can use the setUseITPDatabase function.
+
+        * NetworkProcess/NetworkProcess.cpp:
+        (WebKit::NetworkProcess::setUseITPDatabase):
+        * NetworkProcess/NetworkProcess.h:
+        * NetworkProcess/NetworkProcess.messages.in:
+        * UIProcess/API/C/WKWebsiteDataStoreRef.cpp:
+        (WKWebsiteDataStoreSetUseITPDatabase):
+        (WKWebsiteDataStoreStatisticsResetToConsistentState):
+        * UIProcess/API/C/WKWebsiteDataStoreRef.h:
+        * UIProcess/Network/NetworkProcessProxy.cpp:
+        (WebKit::NetworkProcessProxy::setUseITPDatabase):
+        * UIProcess/Network/NetworkProcessProxy.h:
+        * UIProcess/WebsiteData/WebsiteDataStore.cpp:
+        (WebKit::WebsiteDataStore::setUseITPDatabase):
+        * UIProcess/WebsiteData/WebsiteDataStore.h:
+
 2019-10-16  Chris Dumez  <cdumez@apple.com>
 
         [WK2] Handle back/forward cache entry expiration in the UIProcess instead of the WebProcess
diff --git a/Source/WebKit/NetworkProcess/NetworkProcess.cpp b/Source/WebKit/NetworkProcess/NetworkProcess.cpp
index 14259b1..810e748 100644
--- a/Source/WebKit/NetworkProcess/NetworkProcess.cpp
+++ b/Source/WebKit/NetworkProcess/NetworkProcess.cpp
@@ -666,15 +666,18 @@
     }
 }
 
-void NetworkProcess::setUseITPDatabase(PAL::SessionID sessionID, bool value)
+void NetworkProcess::setUseITPDatabase(PAL::SessionID sessionID, bool value, CompletionHandler<void()>&& completionHandler)
 {
     if (auto* networkSession = this->networkSession(sessionID)) {
         if (m_isITPDatabaseEnabled != value) {
             m_isITPDatabaseEnabled = value;
             networkSession->recreateResourceLoadStatisticStore();
         }
-    } else
+        completionHandler();
+    } else {
         ASSERT_NOT_REACHED();
+        completionHandler();
+    }
 }
 
 void NetworkProcess::setPrevalentResource(PAL::SessionID sessionID, const RegistrableDomain& domain, CompletionHandler<void()>&& completionHandler)
diff --git a/Source/WebKit/NetworkProcess/NetworkProcess.h b/Source/WebKit/NetworkProcess/NetworkProcess.h
index c130cac..c89df84 100644
--- a/Source/WebKit/NetworkProcess/NetworkProcess.h
+++ b/Source/WebKit/NetworkProcess/NetworkProcess.h
@@ -221,7 +221,7 @@
     void isRegisteredAsSubFrameUnder(PAL::SessionID, const SubFrameDomain&, const TopFrameDomain&, CompletionHandler<void(bool)>&&);
     void isRegisteredAsSubresourceUnder(PAL::SessionID, const SubResourceDomain&, const TopFrameDomain&, CompletionHandler<void(bool)>&&);
     void setGrandfathered(PAL::SessionID, const RegistrableDomain&, bool isGrandfathered, CompletionHandler<void()>&&);
-    void setUseITPDatabase(PAL::SessionID, bool value);
+    void setUseITPDatabase(PAL::SessionID, bool value, CompletionHandler<void()>&&);
     void setMaxStatisticsEntries(PAL::SessionID, uint64_t maximumEntryCount, CompletionHandler<void()>&&);
     void setPrevalentResource(PAL::SessionID, const RegistrableDomain&, CompletionHandler<void()>&&);
     void setPrevalentResourceForDebugMode(PAL::SessionID, const RegistrableDomain&, CompletionHandler<void()>&&);
diff --git a/Source/WebKit/NetworkProcess/NetworkProcess.messages.in b/Source/WebKit/NetworkProcess/NetworkProcess.messages.in
index ce5b6ba..c35d113 100644
--- a/Source/WebKit/NetworkProcess/NetworkProcess.messages.in
+++ b/Source/WebKit/NetworkProcess/NetworkProcess.messages.in
@@ -112,7 +112,7 @@
     SubmitTelemetry(PAL::SessionID sessionID) -> () Async
     SetCacheMaxAgeCapForPrevalentResources(PAL::SessionID sessionID, Seconds seconds) -> () Async
     SetGrandfathered(PAL::SessionID sessionID, WebCore::RegistrableDomain resourceDomain, bool isGrandfathered) -> () Async
-    SetUseITPDatabase(PAL::SessionID sessionID, bool value)
+    SetUseITPDatabase(PAL::SessionID sessionID, bool value) -> () Async
     SetGrandfatheringTime(PAL::SessionID sessionID, Seconds seconds) -> () Async
     SetMaxStatisticsEntries(PAL::SessionID sessionID, uint64_t maximumEntryCount) -> () Async
     SetMinimumTimeBetweenDataRecordsRemoval(PAL::SessionID sessionID, Seconds seconds) -> () Async
diff --git a/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.cpp b/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.cpp
index 744fcba..d335060 100644
--- a/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.cpp
+++ b/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.cpp
@@ -289,10 +289,14 @@
 #endif
 }
 
-void WKWebsiteDataStoreSetUseITPDatabase(WKWebsiteDataStoreRef dataStoreRef, bool value)
+void WKWebsiteDataStoreSetUseITPDatabase(WKWebsiteDataStoreRef dataStoreRef, bool value, void* context, WKWebsiteDataStoreSetUseITPDatabaseFunction completionHandler)
 {
 #if ENABLE(RESOURCE_LOAD_STATISTICS)
-    WebKit::toImpl(dataStoreRef)->setUseITPDatabase(value);
+    WebKit::toImpl(dataStoreRef)->setUseITPDatabase(value, [context, completionHandler] {
+        completionHandler(context);
+    });
+#else
+    completionHandler(context);
 #endif
 }
 
@@ -548,6 +552,7 @@
     store.setResourceLoadStatisticsShouldDowngradeReferrerForTesting(true, [callbackAggregator = callbackAggregator.copyRef()] { });
     store.resetParametersToDefaultValues([callbackAggregator = callbackAggregator.copyRef()] { });
     store.scheduleClearInMemoryAndPersistent(WebKit::ShouldGrandfatherStatistics::No, [callbackAggregator = callbackAggregator.copyRef()] { });
+    store.setUseITPDatabase(false, [callbackAggregator = callbackAggregator.copyRef()] { });
 #else
     UNUSED_PARAM(dataStoreRef);
     completionHandler(context);
diff --git a/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.h b/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.h
index ef43398..ad5c2ce 100644
--- a/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.h
+++ b/Source/WebKit/UIProcess/API/C/WKWebsiteDataStoreRef.h
@@ -75,7 +75,8 @@
 WK_EXPORT void WKWebsiteDataStoreSetStatisticsGrandfathered(WKWebsiteDataStoreRef dataStoreRef, WKStringRef host, bool value);
 typedef void (*WKWebsiteDataStoreIsStatisticsGrandfatheredFunction)(bool isGrandfathered, void* functionContext);
 WK_EXPORT void WKWebsiteDataStoreIsStatisticsGrandfathered(WKWebsiteDataStoreRef dataStoreRef, WKStringRef host, void* context, WKWebsiteDataStoreIsStatisticsGrandfatheredFunction callback);
-WK_EXPORT void WKWebsiteDataStoreSetUseITPDatabase(WKWebsiteDataStoreRef dataStoreRef, bool value);
+typedef void (*WKWebsiteDataStoreSetUseITPDatabaseFunction)(void* functionContext);
+WK_EXPORT void WKWebsiteDataStoreSetUseITPDatabase(WKWebsiteDataStoreRef dataStoreRef, bool value, void* context, WKWebsiteDataStoreSetUseITPDatabaseFunction callback);
 WK_EXPORT void WKWebsiteDataStoreSetStatisticsSubframeUnderTopFrameOrigin(WKWebsiteDataStoreRef dataStoreRef, WKStringRef host, WKStringRef topFrameHost);
 WK_EXPORT void WKWebsiteDataStoreSetStatisticsSubresourceUnderTopFrameOrigin(WKWebsiteDataStoreRef dataStoreRef, WKStringRef host, WKStringRef topFrameHost);
 WK_EXPORT void WKWebsiteDataStoreSetStatisticsSubresourceUniqueRedirectTo(WKWebsiteDataStoreRef dataStoreRef, WKStringRef host, WKStringRef hostRedirectedTo);
diff --git a/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp b/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp
index ca1f5ab..4585cff 100644
--- a/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp
+++ b/Source/WebKit/UIProcess/Network/NetworkProcessProxy.cpp
@@ -786,12 +786,9 @@
     sendWithAsyncReply(Messages::NetworkProcess::SetGrandfathered(sessionID, resourceDomain, isGrandfathered), WTFMove(completionHandler));
 }
 
-void NetworkProcessProxy::setUseITPDatabase(PAL::SessionID sessionID, bool value)
+void NetworkProcessProxy::setUseITPDatabase(PAL::SessionID sessionID, bool value, CompletionHandler<void()>&& completionHandler)
 {
-    if (!canSendMessage())
-        return;
-    
-    send(Messages::NetworkProcess::SetUseITPDatabase(sessionID, value), 0);
+    sendWithAsyncReply(Messages::NetworkProcess::SetUseITPDatabase(sessionID, value), WTFMove(completionHandler));
 }
 
 void NetworkProcessProxy::requestStorageAccessConfirm(WebPageProxyIdentifier pageID, FrameIdentifier frameID, const RegistrableDomain& subFrameDomain, const RegistrableDomain& topFrameDomain, CompletionHandler<void(bool)>&& completionHandler)
diff --git a/Source/WebKit/UIProcess/Network/NetworkProcessProxy.h b/Source/WebKit/UIProcess/Network/NetworkProcessProxy.h
index fbf2ce5..6ab2039 100644
--- a/Source/WebKit/UIProcess/Network/NetworkProcessProxy.h
+++ b/Source/WebKit/UIProcess/Network/NetworkProcessProxy.h
@@ -118,7 +118,7 @@
     void setAgeCapForClientSideCookies(PAL::SessionID, Optional<Seconds>, CompletionHandler<void()>&&);
     void setCacheMaxAgeCap(PAL::SessionID, Seconds, CompletionHandler<void()>&&);
     void setGrandfathered(PAL::SessionID, const RegistrableDomain&, bool isGrandfathered, CompletionHandler<void()>&&);
-    void setUseITPDatabase(PAL::SessionID, bool value);
+    void setUseITPDatabase(PAL::SessionID, bool value, CompletionHandler<void()>&&);
     void setNotifyPagesWhenDataRecordsWereScanned(PAL::SessionID, bool, CompletionHandler<void()>&&);
     void setIsRunningResourceLoadStatisticsTest(PAL::SessionID, bool, CompletionHandler<void()>&&);
     void setNotifyPagesWhenTelemetryWasCaptured(PAL::SessionID, bool, CompletionHandler<void()>&&);
diff --git a/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.cpp b/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.cpp
index 83751e9..3908d7b 100644
--- a/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.cpp
+++ b/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.cpp
@@ -1701,12 +1701,16 @@
     }
 }
 
-void WebsiteDataStore::setUseITPDatabase(bool value)
+void WebsiteDataStore::setUseITPDatabase(bool value, CompletionHandler<void()>&& completionHandler)
 {
     ASSERT(RunLoop::isMain());
-
-    for (auto& processPool : processPools())
-        processPool->ensureNetworkProcess().setUseITPDatabase(m_sessionID, value);
+    
+    auto callbackAggregator = CallbackAggregator::create(WTFMove(completionHandler));
+    
+    for (auto& processPool : processPools()) {
+        if (auto* process = processPool->networkProcess())
+            process->setUseITPDatabase(m_sessionID, value, [callbackAggregator = callbackAggregator.copyRef()] { });
+    }
 }
 
 void WebsiteDataStore::setCrossSiteLoadWithLinkDecorationForTesting(const URL& fromURL, const URL& toURL, CompletionHandler<void()>&& completionHandler)
diff --git a/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.h b/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.h
index 94de9ee..930e305 100644
--- a/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.h
+++ b/Source/WebKit/UIProcess/WebsiteData/WebsiteDataStore.h
@@ -162,7 +162,7 @@
     void submitTelemetry();
     void setGrandfathered(const URL&, bool, CompletionHandler<void()>&&);
     void isGrandfathered(const URL&, CompletionHandler<void(bool)>&&);
-    void setUseITPDatabase(bool);
+    void setUseITPDatabase(bool, CompletionHandler<void()>&&);
     void setGrandfatheringTime(Seconds, CompletionHandler<void()>&&);
     void setLastSeen(const URL&, Seconds, CompletionHandler<void()>&&);
     void mergeStatisticForTesting(const URL&, const URL& topFrameUrl1, const URL& topFrameUrl2, Seconds lastSeen, bool hadUserInteraction, Seconds mostRecentUserInteraction, bool isGrandfathered, bool isPrevalent, bool isVeryPrevalent, unsigned dataRecordsRemoved, CompletionHandler<void()>&&);
diff --git a/Tools/ChangeLog b/Tools/ChangeLog
index 9c9c500..f402594 100644
--- a/Tools/ChangeLog
+++ b/Tools/ChangeLog
@@ -1,3 +1,17 @@
+2019-10-16  Kate Cheney  <katherine_cheney@apple.com>
+
+        WKWebsiteDataStoreStatisticsResetToConsistentState() should call store.setUseITPDatabase(false)
+        https://bugs.webkit.org/show_bug.cgi?id=203020
+        <rdar://problem/56314484>
+
+        Reviewed by Chris Dumez.
+
+        Added a completion handler to setUseITPDatabase() to avoid race
+        conditions.
+
+        * WebKitTestRunner/TestController.cpp:
+        (WTR::TestController::setUseITPDatabase):
+
 2019-10-16  Fujii Hironori  <Hironori.Fujii@sony.com>
 
         DumpRenderTree should clear WTFLogChannelState::OnWithAccumulation state set by testRunner.accummulateLogsForChannel
diff --git a/Tools/WebKitTestRunner/TestController.cpp b/Tools/WebKitTestRunner/TestController.cpp
index e9d428e..4ba4916 100644
--- a/Tools/WebKitTestRunner/TestController.cpp
+++ b/Tools/WebKitTestRunner/TestController.cpp
@@ -3382,7 +3382,9 @@
 
 void TestController::setUseITPDatabase(bool value)
 {
-    WKWebsiteDataStoreSetUseITPDatabase(TestController::websiteDataStore(), value);
+    ResourceStatisticsCallbackContext context(*this);
+    WKWebsiteDataStoreSetUseITPDatabase(TestController::websiteDataStore(), value, &context, resourceStatisticsVoidResultCallback);
+    runUntil(context.done, noTimeout);
 }
 
 void TestController::setStatisticsSubframeUnderTopFrameOrigin(WKStringRef host, WKStringRef topFrameHost)