Xamarin.iOS linker removes assembly attributes, but large & useful libraries (e.g. EF Core 2.0) rely on such being preserved

[cwrea]

I've been attempting to get Microsoft Entity Framework Core 2.0 working in a Xamarin.iOS application with full linking enabled. EF Core is a large library — exactly the kind that could benefit from linking. Discovering all the critical types/methods to preserve in a LinkDescription XML file has been tedious, owing to extensive reflection and use of DI in EF Core 2.0, but I've been making progress, one crash at a time. (There's also hope that ER Core could one day have linker hints built in.)

However, I hit a major blocker: When EF Core creates a database from a model, it reads its own AssemblyInformationalVersion attribute to preserve in generated metadata. But the Xamarin.iOS linker strips assembly attributes from an assembly that is not link-skipped. So, in an app with full linking enabled, EF Core is crashing as it gets nothing back from reading the desired attribute value.

This behaviour appears to be an intentional linker feature, perhaps desirable and harmless in many cases — but for cases where it is a problem, there exists no fine-grained directive to preserve assembly attributes, as one could do with plain types/methods in a LinkDescription XML file.

The only workaround at present is to go back to link-skipping the large EF Core main assembly, and put up with the resulting bloat. For a small app that doesn't use EF Core's more advanced features, the unnecessary bloat may be considerable. (I haven't measured potential avoidable bloat yet, as I'm blocked from getting to the point where I could compare a working iOS app with full linking vs. one that link-skips EF Core.)

Steps to Reproduce (without EF Core; just the essential issue)

1. Create a new Xamarin.iOS project using the Single View app template.
2. In the project's iOS build settings for platform "iPhone" (real device), set linker behavior to "Link All".
3. In AppDelegate.cs' FinishedLaunching() method, add the following line before the return:
   System.Diagnostics.Debug.WriteLine("TODO");
4. Change the build to target a real iOS device, then build and run with debugging. Notice "TODO" is printed in the debug output. i.e. The basic application works with full linking.
5. In Properties/AssemblyInfo.cs, append the following line:
   [assembly: AssemblyInformationalVersion("1.0.0.0")]
6. From the ProductInfo.cs file from the EF Core 2.0.0 repository, copy and paste just the ProductInfo class definition into AppDelegate.cs, before the AppDelegate class definition.
7. Change the Debug.WriteLine() call added in step 1 to read:
   System.Diagnostics.Debug.WriteLine("Version: " + ProductInfo.GetVersion());.
8. Build and run with debugging. Instead of seeing "Informational version: 1.0.0.0" in the debugger output, the application crashes with System.NullReferenceException: Object reference not set to an instance of an object.

If you were to expand the one-liner from EF Core's GetVersion() method into multiple statements and intermediate values instead, you would see that the null reference is the attribute object from which the InformationalVersion property would have been accessed. If you were to change linking back to "Link SDK Assemblies Only", you would avoid the crash and see the expected output.

Expected Behavior

It should be possible to configure the linker to specifically preserve assembly attributes when the need arises — i.e. without completely opting out of linking an assembly.

Developers of .NET Standard class libraries should be able to rely on reading their own assembly's attribute information. Linking should aim to exclude only unnecessary types, methods, and other data from an assembly. Beyond types and methods, when the linker assumes that a kind of thing can safely be excluded, it should provide a fine-grained directive to preserve some or all of the kinds-of-things, for cases where the assumption has been proven unsafe for a specific assembly.

Actual Behavior

Crash. The present workaround requires completely opting out of linking for an assembly that relies on reading its own assembly attributes.

However, linking remains highly desirable for large library dependencies used in mobile applications. The case in point is not only a very large .NET Standard library, but a popular data access framework promoted by Microsoft as cross-platform capable, which ideally would include mobile applications with on-device data access needs.

Environment

Microsoft Visual Studio Community 2017 
Version 15.5.7
VisualStudio.15.Release/15.5.7+27130.2036
Microsoft .NET Framework
Version 4.7.02556

Installed Version: Community

Visual Basic 2017   00369-60000-00001-AA398
Microsoft Visual Basic 2017

Visual C# 2017   00369-60000-00001-AA398
Microsoft Visual C# 2017

Visual C++ 2017   00369-60000-00001-AA398
Microsoft Visual C++ 2017

Visual F# 4.1   00369-60000-00001-AA398
Microsoft Visual F# 4.1

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Application Insights Tools for Visual Studio Package   8.10.01106.1
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ASP.NET Web Frameworks and Tools 2012   4.0.20601.0
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ASP.NET Web Frameworks and Tools 2017   5.2.51007.0
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Azure App Service Tools v3.0.0   15.0.31106.0
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Microsoft Continuous Delivery Tools for Visual Studio   0.3
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Mono Debugging for Visual Studio   4.8.4-pre (3fe64e3)
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NuGet Package Manager   4.5.0
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Xamarin   4.8.0.760 (fc93f3f5b)
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Xamarin.Android SDK   8.1.5.0 (HEAD/75f8c6838)
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Xamarin.iOS and Xamarin.Mac SDK   11.6.1.4 (db807ec)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

Build Logs

1>------ Rebuild All started: Project: TestXamLinkIssue, Configuration: Debug iPhone ------
1>  Connecting to Mac server cwrea-mini...
1>  TestXamLinkIssue -> D:\Projects\TestXamLinkIssueSoln\TestXamLinkIssue\bin\iPhone\Debug\TestXamLinkIssue.exe
1>  Detected signing identity:
1>    Code Signing Key: "iPhone Developer: [...]" (...)
1>    Provisioning Profile: "cwrea iOS Dev Prov. Profile" (...)
1>    Bundle Id: com.companyname.TestXamLinkIssue
1>    App Id: J7ADM89KY9.com.companyname.TestXamLinkIssue
1>  /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip: removing global symbols from a final linked no longer supported.  Use -exported_symbols_list at link time when building: /Users/cwrea/Library/Caches/Xamarin/mtbs/builds/TestXamLinkIssue/a101bae86211f0bd1960357a25c90492/bin/iPhone/Debug/TestXamLinkIssue.app/TestXamLinkIssue
1>  bin/iPhone/Debug/TestXamLinkIssue.app: valid on disk
1>  bin/iPhone/Debug/TestXamLinkIssue.app: satisfies its Designated Requirement
1>  bin/iPhone/Debug/TestXamLinkIssue.app: explicit requirement satisfied
========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ==========

Projects build report:
  Status    | Project [Config|platform]
 -----------|---------------------------------------------------------------------------------------------------
  Succeeded | TestXamLinkIssue\TestXamLinkIssue.csproj [Debug|iPhone]

Time Elapsed 00:00:22.323

[mandel-macaque]

This looks like an issue with the linker since we should be able to use the xml configuration file to state that attributes should be preserved in the case of libraries like EF.

[mandel-macaque]

@spouliot any feedback for this issue? Should we move it to the linker project?

[spouliot]

@mandel-macaque no (moving), it's a XI/XM specific step

@cwrea the XML file is used to mark code, not metadata. We'll look to see how we can make this more flexible.

[divega]

@marek-safar some time ago we talked about things that could enable EF Core to work better with Xamarin, but I think I did not mention this issue specifically. Having fine-grained control to preserve assembly-level attributes would unblock anyone (us, @cwrea, another contributor, or an application developer) that wants to use EF Core with full linking.

[marek-safar]

@spouliot that's bug in the hardcoded override of attributes in https://github.com/xamarin/xamarin-macios/blob/master/tools/linker/MonoTouch.Tuner/RemoveAttributes.cs I think it'd be better to remove that customization completely and rely on original linker to do the work (I am happy to help with that)

[tipa]

I am using EF7 and was able to enable full linking on Android using a LinkDescription.xml. However, this issue now blocks me enabling full linking on iOS, so I am stuck with a 110 MB app size :(

[tipa]

What's the current status of this? Don't want to be annoying, but I think this could be a small fix that unblocks & helps a lot of us working with iOS & EF Core

[spouliot]

This is an old step but it removes a lot of unused metadata out of every applications: reducing size and saving build and deploy times.

There's a recent mechanism to enable/disable optimizations (using Additional mtouch arguments). That would be an easy, first fix/hack to disable the step (entirely) when needed (i.e. the extra cost being paid only if needed).

There's a newer, smarter metadata remover step - which checks if 3rd party code use a feature before removing it (e.g. parameters names are removed from metadata only if ParamaterInfo.Name is not referenced/preserved by the app). This is how the (next/final) fix should should be applied.

I have an upcoming trip to Redmond next week so I'll have a look at it (at least the first part) during the flights.