I wrote last year about problems arising from Developer ID signed apps and their dependence upon an Apple “timestamp server” during the code signing phase. In the article, I describe a workaround for scenarios where the timestamp server is either down or there is no internet connection: disable the functionality by adding “–timestamp=none” to the OTHER_CODE_SIGN_FLAGS Xcode build setting.

Swift based apps currently install a complete copy of the Swift standard libraries in the built app’s bundle, and during this phase, Xcode implicitly signs the Swift libraries with whatever code signing identity is defined by the project and/or target. Unfortunately in doing so, it ignores the OTHER_CODE_SIGN_FLAGS build setting, removing the opportunity to finesse code signing of the Swift libraries in the way I described.

The long and short of it? You can’t build a Developer ID based Swift application when you’re offline. I discovered this recently when I dug into a Swift application in a jet, high above the Pacific. Hours of uninterrupted, focused development time lay before me, but I was met with this annoying build failure:

The timestamp service is not available.
*** error: Couldn't codesign [...]/My.app/Contents/Frameworks/libswiftCore.dylib: codesign failed with exit code 1
Command [...]/XcodeDefault.xctoolchain/usr/bin/swift-stdlib-tool failed with exit code 1

The workaround is to turn off code signing completely, or to switch to a Mac App Store code signing identity (which doesn’t rely upon the timestamp service feature of the code sign tool). I filed this bug as a Radar #21891588.