crush depth

Reproducible Builds

Considering moving to producing 100% reproducible builds for all of my packages.

It seems fairly easy. The following changes are required for the primogenitor:

  • Stop using The commit ID is enough!

  • Use the reproducible-build-maven-plugin to strip manifest headers such as Built-By, Build-JDK, etc, and repack jar files such that the timestamps of entries are set to known constant values and the entries are placed into the jar in a deterministic order.

  • Strip Bnd-LastModified and Tool headers from bundle manifests using the <_removeheaders> instruction in the maven-bundle-plugin configuration.

  • Stop using version ranges. This may be too painful.

Some early experiments show that this yields byte-for-byte identical jar files on each compile. This is pretty impressive.

The one open issue: Oracle (or OpenJDK's) javac appears to produce completely deterministic output; there aren't any embedded timestamps or other nonsense. However, someone building the packages from source isn't guaranteed to be using an Oracle JDK. I could use the Enforcer plugin to check that the user is using a known-deterministic JDK, but it would be pretty obnoxious to break builds if they aren't. Perhaps a warning message ("JDK is not known to produce deterministic output: Build may not be reproducible!") is enough.