[Core, Android] Fix to address the elusive 'Sharing violation on path PropertyStore.forms.tmp' (#1075)

* use a semaphor for actual saving piece
make reading serialized propertystore readonly to help with collisions.

* change over to async Task and await OnStateChanged() method to try and mitigate what might be the race condition causing the ' Sharing violation on path PropertyStore.forms.tmp' error

* update Semaphore name

* remove private specifier per the style guide

* Update docs and add warning suppression for broken mono warning

* ... and adding the warning suppression to Release mode, as well.

* -add try...finally block so we can guarantee that the semaphore is released if there's an exception while saving the properties.
-remove the legacy code that the semaphore replaces

* Update docs

* Attempting to fix docs

* Trying again to get docs to pass

3 files changed, 32 insertions, 26 deletions

diff --git a/Xamarin.Forms.Platform.Android/AppCompat/FormsAppCompatActivity.cs b/Xamarin.Forms.Platform.Android/AppCompat/FormsAppCompatActivity.cs
index 26024613..ed28656e 100644
--- a/Xamarin.Forms.Platform.Android/AppCompat/FormsAppCompatActivity.cs
+++ b/Xamarin.Forms.Platform.Android/AppCompat/FormsAppCompatActivity.cs
@@ -22,6 +22,7 @@ using AColor = Android.Graphics.Color;
 using AlertDialog = Android.Support.V7.App.AlertDialog;
 using ARelativeLayout = Android.Widget.RelativeLayout;
 using Xamarin.Forms.Internals;
+using System.Threading.Tasks;
 
 #endregion
 
@@ -169,7 +170,7 @@ namespace Xamarin.Forms.Platform.Android
 				callback(resultCode, data);
 		}
 
-		protected override void OnCreate(Bundle savedInstanceState)
+		protected override async void OnCreate(Bundle savedInstanceState)
 		{
 			if (!AllowFragmentRestore)
 			{
@@ -201,7 +202,7 @@ namespace Xamarin.Forms.Platform.Android
 			_previousState = _currentState;
 			_currentState = AndroidApplicationLifecycleState.OnCreate;
 
-			OnStateChanged();
+			await OnStateChanged();
 
 			if (Forms.IsLollipopOrNewer)
 			{
@@ -228,7 +229,7 @@ namespace Xamarin.Forms.Platform.Android
 			CheckForAppLink(intent);
 		}
 
-		protected override void OnPause()
+		protected override async void OnPause()
 		{
 			_layout.HideKeyboard(true);
 
@@ -241,20 +242,20 @@ namespace Xamarin.Forms.Platform.Android
 			_previousState = _currentState;
 			_currentState = AndroidApplicationLifecycleState.OnPause;
 
-			OnStateChanged();
+			await OnStateChanged();
 		}
 
-		protected override void OnRestart()
+		protected override async void OnRestart()
 		{
 			base.OnRestart();
 
 			_previousState = _currentState;
 			_currentState = AndroidApplicationLifecycleState.OnRestart;
 
-			OnStateChanged();
+			await OnStateChanged();
 		}
 
-		protected override void OnResume()
+		protected override async void OnResume()
 		{
 			// counterpart to OnPause
 			base.OnResume();
@@ -270,24 +271,24 @@ namespace Xamarin.Forms.Platform.Android
 			_previousState = _currentState;
 			_currentState = AndroidApplicationLifecycleState.OnResume;
 
-			OnStateChanged();
+			await OnStateChanged();
 		}
 
-		protected override void OnStart()
+		protected override async void OnStart()
 		{
 			base.OnStart();
 
 			_previousState = _currentState;
 			_currentState = AndroidApplicationLifecycleState.OnStart;
 
-			OnStateChanged();
+			await OnStateChanged();
 		}
 
 		// Scenarios that stop and restart your app
 		// -- Switches from your app to another app, activity restarts when clicking on the app again.
 		// -- Action in your app that starts a new Activity, the current activity is stopped and the second is created, pressing back restarts the activity
 		// -- The user receives a phone call while using your app on his or her phone
-		protected override void OnStop()
+		protected override async void OnStop()
 		{
 			// writing to storage happens here!
 			// full UI obstruction
@@ -300,7 +301,7 @@ namespace Xamarin.Forms.Platform.Android
 			_previousState = _currentState;
 			_currentState = AndroidApplicationLifecycleState.OnStop;
 
-			OnStateChanged();
+			await OnStateChanged();
 		}
 
 		void AppOnPropertyChanged(object sender, PropertyChangedEventArgs args)
@@ -387,7 +388,7 @@ namespace Xamarin.Forms.Platform.Android
 			UpdateProgressBarVisibility(_busyCount > 0);
 		}
 
-		async void OnStateChanged()
+		async Task OnStateChanged()
 		{
 			if (_application == null)
 				return;
diff --git a/Xamarin.Forms.Platform.Android/Deserializer.cs b/Xamarin.Forms.Platform.Android/Deserializer.cs
index c85aaa73..ccce9cdf 100644
--- a/Xamarin.Forms.Platform.Android/Deserializer.cs
+++ b/Xamarin.Forms.Platform.Android/Deserializer.cs
@@ -20,22 +20,28 @@ namespace Xamarin.Forms.Platform.Android
 			return Task.Run(() =>
 			{
 				using (IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication())
-				using (IsolatedStorageFileStream stream = store.OpenFile(PropertyStoreFile, System.IO.FileMode.OpenOrCreate))
-				using (XmlDictionaryReader reader = XmlDictionaryReader.CreateBinaryReader(stream, XmlDictionaryReaderQuotas.Max))
 				{
-					if (stream.Length == 0)
+					if (!store.FileExists(PropertyStoreFile))
 						return null;
 
-					try
+					using (IsolatedStorageFileStream stream = store.OpenFile(PropertyStoreFile, System.IO.FileMode.Open, System.IO.FileAccess.Read))
+					using (XmlDictionaryReader reader = XmlDictionaryReader.CreateBinaryReader(stream, XmlDictionaryReaderQuotas.Max))
 					{
-						var dcs = new DataContractSerializer(typeof(Dictionary<string, object>));
-						return (IDictionary<string, object>)dcs.ReadObject(reader);
-					}
-					catch (Exception e)
-					{
-						Debug.WriteLine("Could not deserialize properties: " + e.Message);
-						Log.Warning("Xamarin.Forms PropertyStore", $"Exception while reading Application properties: {e}");
+						if (stream.Length == 0)
+							return null;
+
+						try
+						{
+							var dcs = new DataContractSerializer(typeof(Dictionary<string, object>));
+							return (IDictionary<string, object>)dcs.ReadObject(reader);
+						}
+						catch (Exception e)
+						{
+							Debug.WriteLine("Could not deserialize properties: " + e.Message);
+							Log.Warning("Xamarin.Forms PropertyStore", $"Exception while reading Application properties: {e}");
+						}
 					}
+
 				}
 
 				return null;
diff --git a/Xamarin.Forms.Platform.Android/Xamarin.Forms.Platform.Android.csproj b/Xamarin.Forms.Platform.Android/Xamarin.Forms.Platform.Android.csproj
index aa91dedb..5dd03486 100644
--- a/Xamarin.Forms.Platform.Android/Xamarin.Forms.Platform.Android.csproj
+++ b/Xamarin.Forms.Platform.Android/Xamarin.Forms.Platform.Android.csproj
@@ -42,8 +42,7 @@
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
-    <NoWarn>
-    </NoWarn>
+    <NoWarn>CS0109</NoWarn>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Turkey|AnyCPU'">
     <DebugSymbols>true</DebugSymbols>