Avoid loading xcframeworks as Bundles

[elliottwilliams]

(NS)Bundle caches bundles as it loads them, so if a loaded bundle's metadata changes during program execution, it will not be reflected by the Bundle API. Carthage does this when it updates a preexisting XCFramework using mergeIntoXCFramework(in:settings:).

This leads to situations where a variant framework inside an XCFramework is "missed" by Carthage and not copied by extractXCFrameworks(in:for:) until carthage build is run a second time. For example, consider the following dependencies:

A: builds for iOS, watchOS
B: depends on A, builds for iOS

When carthage build --use-xcframeworks --platform iOS,watchOS with an empty Carthage/Build directory:

1. A's watchOS variant builds, and is stored in a new xcframework, Carthage/Build/A.xcframework.
2. A's iOS variant builds. Carthage loads the Bundle for A.xcframework but does not extract anything (because A.xcframework's Info.plist only indicates watchOS variant frameworks).
3. B's iOS variant builds. Carthage has already loaded the Bundle for A.xcframework, and does not extract anything (because, as cached by the system, A.xcframework's Info.plist only indicates watchOS variant frameworks). Building B fails.

If I run carthage build --use-xcframeworks --platform iOS,watchOS again, Carthage loads the Bundle for A.xcframework and sees both the watchOS and iOS variants, so building B succeeds.

While there's not an official way to flush the Bundle system's metadata cache, there's a long-standing private CoreFoundation call that does it. We can use that, and assertion-fail in Debug builds if it ever goes away.

[stale]

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