Whenever a shared state package is written out, so is a corresponding .siginfo file. This practice results in a pickled python database of all the metadata that went into creating the hash for a given shared state package.

If BitBake is run with the --dump-signatures (or -S) option, BitBake dumps out .siginfo files in the stamp directory for every task it would have executed instead of building the specified target package.

There is a bitbake-diffsigs command that can process these .siginfo files. If one file is specified, it will dump out the dependency information in the file. If two files are specified, it will compare the two files and dump out the differences between the two. This allows the question of "What changed between X and Y?" to be answered easily.