[SR-3164] Save previous swiftdeps files during a build

[belkadan]

Previous ID	SR-3164
Radar	None
Original Reporter	@belkadan
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Driver, StarterBug
Assignee	@modocache
Priority	Medium

md5: 39ac8470de717c1a23eb228c5bf25162

Issue Description:

The Swift compiler uses files with an extension of ".swiftdeps" to store information about cross-file dependencies within a single module, plus an extra "swiftdeps" file for the "build record" (information about the whole compilation). These files are read in at the start of compilation to compute a dependency graph, and updated (and re-read) as compilation proceeds. However, because these files are updated on every build, an issue with dependency analysis is hard to reproduce—the inputs have been lost.

A simple answer for this that will help in the common case is to move the previous swiftdeps file aside just before writing the new one. I'd suggest just renaming them from "foo.swiftdeps" to "foo.swiftdeps~". If this fails for any reason, just ignore the failure and continue on. (Maybe the directory isn't writable, only the one file. Maybe this is the first build. Maybe the user write-protected the previous "foo.swiftdeps~" for some reason.)

The LLVM APIs to mess with the filesystem are in "llvm/Support/FileSystem.h"; the one I'd expect to use is llvm::sys::fs::rename.

[modocache]

I'd like to try my hand at this, if that's alright. 🙂

[modocache]

@belkadan, hopefully this is what you had in mind: #5816

This doesn't do anything special with '.d' output files. Should those be preserved as '.d~' as well?

[modocache]

Thanks for the starter task, Jordan! I think this one is done.