[msbuild] Use the full path to the symbols list file. Fixes #15046. (#…

…15105)

Give the full path to the symbols list file in the extension project to the
main project, so that strip can find it.

When building a solution remotely from Windows, we can't compute a relative
path between projects, because they're laid out differently on disk. This
means that we have to use full paths when passing paths between projects (such
as the path to the symbol list file).

Fixes #15046.

msbuild/Xamarin.Shared/Xamarin.Shared.targets

@@ -2389,6 +2389,17 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 			<Output TaskParameter="Items" ItemName="_WatchAppPostProcessingItems" />
 		</ReadItemsFromFile>
 
+		<!--
+			For App Extensions, we delay running dsymutil & strip until it has been copied into the main app bundle.
+			This means that we need the path to the symbols list in the main app's project, and it needs to be a full path when executing remotely from Windows,
+			because the projects are not in the same position relative to eachother when building remotely (so a relative path doesn't work).
+			This needs to be executed in a task (and not here inside the target) so the execution always occur on the mac, even when the build is done from Windows.
+			Ref: https://github.com/xamarin/xamarin-macios/issues/15046
+		-->
+		<GetFullPath SessionId="$(BuildSessionId)" Condition="'$(IsMacEnabled)' == 'true'" RelativePath="$(_MtouchSymbolsList)">
+			<Output TaskParameter="FullPath" PropertyName="_SymbolsListFullPath" />
+		</GetFullPath>
+
 		<ItemGroup>
 			<!-- add frameworks, with the path relative to the app bundle -->
 			<_PostProcessingItem Include="@(_ResolvedNativeReference->'$(_AppBundleName)$(AppBundleExtension/$(_AppFrameworksRelativePath)%(Filename).framework/%(Filename)')" Condition="'%(_ResolvedNativeReference.Kind' == 'Framework'">
@@ -2409,7 +2420,7 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 				<DSymName>%(_ResolvedNativeReference.Filename)%(_ResolvedNativeReference.Extension).dSYM</DSymName>
 			</_PostProcessingItem>
 			<_PostProcessingItem Include="$([System.IO.Path]::GetFileName('$(AppBundleDir)'))/$(_NativeExecutableRelativePath)" Condition="'$(IsWatchApp)' != 'true'">
-				<SymbolFile>$(_MtouchSymbolsList)</SymbolFile>
+				<SymbolFile>$(_SymbolsListFullPath)</SymbolFile>
 				<DSymName>$(_AppBundleName)$(AppBundleExtension).dSYM</DSymName>
 			</_PostProcessingItem>
 
@@ -2425,10 +2436,7 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 			</_PostProcessingItem>
 
 			<!-- Add any items from app extensions -->
-			<!-- We must update metadata with paths relative to the root of the contained app bundle to be relative to the root of the current app bundle -->
-			<_PostProcessingItem Include="@(_AppExtensionPostProcessingItems -> '$(_AppBundleName)$(AppBundleExtension)\$(_AppPlugInsRelativePath)%(Identity)')" Condition="@(_AppExtensionPostProcessingItems->Count()) &gt; 0"> <!-- The condition here shouldn't be necessary, but https://github.com/dotnet/msbuild/issues/4056 -->
-				<SymbolFile>$([MSBuild]::MakeRelative('$(MSBuildProjectDirectory)','%(_AppExtensionPostProcessingItems.SourceProjectPath)/%(_AppExtensionPostProcessingItems.SymbolFile)'))</SymbolFile>
-			</_PostProcessingItem>
+			<_PostProcessingItem Include="@(_AppExtensionPostProcessingItems -> '$(_AppBundleName)$(AppBundleExtension)\$(_AppPlugInsRelativePath)%(Identity)')" Condition="@(_AppExtensionPostProcessingItems->Count()) &gt; 0" /> <!-- The condition here shouldn't be necessary, but https://github.com/dotnet/msbuild/issues/4056 -->
 
 			<!-- Add any items from watch app -->
 			<!-- We must update metadata with paths relative to the root of the app bundle to be relative to the root of the current app bundle -->