Building SwiftProtobuf with BUILD_LIBRARY_FOR_DISTRIBUTION=YES is not supported

[acecilia]

[REQUIRED] Step 1: Describe your environment

• Xcode version: 12.3
• Firebase SDK version: 8.1.0
• Installation method: Carthage
• Firebase Component: FirebaseMLModelDownloader

[REQUIRED] Step 2: Describe the problem

• This PR added SwiftProtobuf as a dependency of FirebaseMLModelDownloader
• The carthage distribution seems like it uses BUILD_LIBRARY_FOR_DISTRIBUTION=YES
• But the problem is that SwiftProtobuf does not support being built with BUILD_LIBRARY_FOR_DISTRIBUTION=YES. From the linked comment:

Swift Protobuf does not support itself being built in library evolution mode, so it cannot be present in the public ABI of any framework that is built in library evolution mode. That means it can only ever be imported @_implementationOnly, and its types can never be public.

I have other third parties in my app that depend on SwiftProtobuf (grpc), so when using FirebaseMLModelDownloader I am building my own SwiftProtobuf xcframework using Carthage, and that uses BUILD_LIBRARY_FOR_DISTRIBUTION=NO. When doing that, multiple link errors show up (Undefined symbols). See the following issue for an extensive description of the problem: apple/swift-protobuf#1101

Steps to reproduce:

See: apple/swift-protobuf#1101 (comment)

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[paulb777]

@acecilia Thanks for the report and sharing the thorough investigation.

We'll have to investigate if there are any alternatives for the FirebaseMLModelDownloader binary distributions.

In the meantime, is it possible for you to integrate FirebaseMLModelDownloader via a source distribution? CocoaPods, Swift Package Manager, or the source itself?

[acecilia]

@paulb777 Thanks for the response! 🙌 I sadly cant integrate source and need a prebuilt binary, but I have a couple alternatives in mind:

• Stop using my own SwiftProtobuf xcframework and instead use yours (which is using the not supported BUILD_LIBRARY_FOR_DISTRIBUTION=YES, but if it builds in my setup I think it is the easiest way)
• Rebuild myself all the Firebase dependencies using BUILD_LIBRARY_FOR_DISTRIBUTION=NO

[acecilia]

@paulb777 there is another problem:

• FirebaseMLModelDownloader zip contains two xcframeworks: the FirebaseMLModelDownloader and the SwiftProtobuf one
• If using Carthage for integration, it is impossible to use your own SwiftProtobuf, as in the final Carthage/Build folder its undefined which of the xcframeworks will be used: will your own copy of SwiftProtobuf.xcframework prevail, or will the Firebase SwiftProtobuf.xcframework? Seems like Carthage is not raising an error in this case

This could be solved by distributing SwiftProtobuf.xcframework using a dedicated zip

[paulb777]

@acecilia Would you explain more about your workflow? My understanding is after installing with Carthage, there is still an integration step into an Xcode project in which you would choose which SwiftProtobuf.xcframework to integrate.

[acecilia]

@paulb777 Mmm Maybe I am missing something but: aren't both my SwiftProtobuf.xcframework and Firebase SwiftProtobuf.xcframework installed in the same path Carthage/Build/SwiftProtobuf.xcframework?

So one would overwrite the other when Carthage installs them

[paulb777]

I understand now. The other SwiftProtobuf is also installed from Carthage. Thanks.