Superfluous rebuilds for Skip apps containing multiple targets

[marcprux]

A Skip-enabled app that contains multiple targets, such as the https://github.com/skiptools/skipapp-weather.git sample app (which contains a separate SkipWeather and SkipWeatherModel targets), can sometimes be slow to rebuild, even if no changes have been made. Other times, the build results are cached and the build is very fast.

This isn't the case for a Skip app that contains only a single target, such as https://github.com/skiptools/skipapp-hello.git. In those cases, rebuilds are always very fast.

To illustrate, see the following series of build logs there are taken from 3 subsequent builds (with no changes in between). The first (leftmost build) takes 47.5 seconds, the second (middle) takes 5.8 seconds, and then the third (rightmost) takes 43.1 seconds:

Whether it takes 5 seconds or 45 seconds is random, but a correlating factor can be seen by scrolling to the top of the build log and observing that the slow builds correspond to a message about "Removed stale file…" during the "Prepare build" phase:

This is random, but it is speculated that it has to do with a timing issue around the generation of the plugin scripts for projects that contain more than one target. Since unnecessary rebuild will result in the skipstone plugin being run on every dependent package – resulting in the parsing of every single .swift file in those dependencies – this bug can lead to extremely slow build times even for projects that haven't made any changes at all to any of their source files.

[marcprux]

This is a bug in Xcode and swift-build: swiftlang/swift-build#312. It can be reproduced using any build plugin, not just Skip.