The Java byte-vector harness is rebuilt from scratch by every agent that needs test vectors

[andrewgazelka]

Three slices of the 26.2 protocol port so far (tests/item.rs, and now tests/configuration.rs and tests/play_login.rs in #968) have each stood up the same Java harness to print real encoder bytes, and none of them is committed. The recipe is identical every time:

unzip -q "$JAR" 'META-INF/versions/*/server-*.jar' -d bundle
unzip -q "$JAR" 'META-INF/libraries/*' -d libs
CP="$(find bundle -name 'server-*.jar'):$(find libs -name '*.jar' | tr '\n' :)"
javac -cp "$CP" -d out Harness.java && java -cp "$CP:out" Harness

plus SharedConstants.tryDetectVersion(); Bootstrap.bootStrap(); at the top of main, without which the first touch of BuiltInRegistries throws IllegalArgumentException: Not bootstrapped.

Rediscovering that costs 15-30 minutes each time, and it means the vectors in the tests cannot be regenerated after a protocol bump without rewriting the harness.

Suggested fix: a minecraftHarness derivation in nix/minecraft-data.nix that assembles the classpath from the pinned jar, plus a committed Harness.java per test module (or one with a subcommand). nix run .#minecraft-vectors would then reprint every vector on demand, and a bump would show up as a diff in the printed hex rather than as silently stale fixtures.

Related: #964 (the decompiled set is too narrow for the same reason -- every slice re-runs cfr by hand).

Filed by Claude while implementing #968.

[andrewgazelka]

The harness is a nix app on feat/proto-framing-world (#973), so it no longer gets rebuilt by hand.

nix run .#minecraft-encode -- fixtures out.json     # hex from the real StreamCodecs and netty handlers
nix run .#minecraft-encode -- registries out/       # network NBT for every synchronised registry

Source is nix/java/VanillaEncoder.java; nix compiles it against the pinned server jar via a new minecraft-server-classpath derivation that unpacks the bundler and builds the classpath.

Two derivations consume it, so its output is checked rather than just available: minecraft-encoder-fixtures produces the JSON the Rust tests assert against, and minecraft-registry-contents produces the registry blobs the generated tables embed. Flake checks minecraft-encoder-fixtures and minecraft-registry-data fail when the committed copies drift from what the jar produces.

Adding a fixture for a future slice is a put call in the Java and a matching assertion in Rust.