Skip linking against Flutter for CocoaPods transitive dependencies

[jmagman]

Background

Flutter plugins need to link against Flutter.framework or FlutterMacOS.framework. That work is done in the app's Podfile like:

post_install do |installer|
  installer.pods_project.targets.each do |target|
    flutter_additional_ios_build_settings(target)
  end
end

flutter_additional_ios_build_settings and flutter_additional_macos_build_settings sets up the search paths to point to the bin/cache version of Flutter.framework or FlutterMacOS.framework, and for iOS additionally links against Flutter.

flutter/packages/flutter_tools/bin/podhelper.rb

Line 63 in efbde44

build_configuration.build_settings['OTHER_LDFLAGS'] = '$(inherited) -framework Flutter'

ALL pods, even non-Flutter plugins transitive dependencies, were therefore linking against Flutter.
For example, Flutter plugin firebase_admob needs to link against Flutter. That plugin has a transitive dependency on Google-Mobile-Ads-SDK which being incorrectly also linked to Flutter.framework.

The problem

#72151 causes an issue where, if a transitive dependency contained bitcode, it fails when linking against the debug version of Flutter (which isn't necessary since the transitive dependencies don't need to know anything about Flutter) because debug Flutter only contains a bitcode marker, not full bitcode.

ld: bitcode bundle could not be generated because 'bin/cache/artifacts/engine/ios/Flutter.xcframework/ios-armv7_arm64/Flutter.framework/Flutter'
was built without full bitcode.

The fix

Change the podhelper logic to only set up all the Flutter build settings ONLY for Flutter plugins and not their transitive dependencies.

Add a a plugin with a transitive dependency to build_ios_framework_module_test and test that the dependency doesn't link against Flutter.
https://ci.chromium.org/p/flutter/builders/try/Mac%20build_ios_framework_module_test/4725
On master this test fails:

Task result:
{
  "success": false,
  "reason": "Transitive dependency /var/folders/bx/pyplw7v92lj58gm3_lztz5sc00mfq2/T/flutter_module_test.o1howC/hello_module/flutter-frameworks/Debug/Reachability.xcframework/ios-arm64_armv7/Reachability.framework/Reachability unexpectedly links on Flutter"
}

Fixes #78589

[stuartmorgan-g]

LGTM. (I'm assuming "links on" means the same thing as "links against"? I've not encountered that terminology.)

[jmagman]

LGTM. (I'm assuming "links on" means the same thing as "links against"? I've not encountered that terminology.)

Yeah it was late. 😄

[stuartmorgan-g]

LGTM. (I'm assuming "links on" means the same thing as "links against"? I've not encountered that terminology.)

Yeah it was late. 😄

In that case: link on, and be excellent to your libraries 😁