horizon-prepare: the release check's repeat run compares the manifest too; four JVMs with the split

The repeat run's file check skipped horizon-resources.json, so a manifest the repeat rewrote differently passed. The comments that said at most three JVMs run at once now say four with the second server JVM.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-prepare/src/java.rs

@@ -15,7 +15,7 @@ pub struct Jdk {
 const TIMEOUT: Duration = Duration::from_secs(3600);
 
 /// Options for every JVM preparing starts (the exporters, the data generator, javac with `-J`).
-/// They are short batch jobs that run three at a time: the C2 compiler's background work costs
+/// They are short batch jobs that run side by side: the C2 compiler's background work costs
 /// more CPU than it wins back, and the serial collector keeps their heaps small. Measured on the
 /// pinned jars, a fresh preparation took 105 s with the defaults and 63-66 s with these, its
 /// JVMs together at most 1.25 GB (2 GB with the parallel collector, as fast), same output.

crates/horizon-prepare/src/lib.rs

@@ -754,7 +754,8 @@ type Entries = Vec<(&'static str, Value)>;
 /// The exporters, the data generator and the asset copy, on four lanes that run at once:
 /// the server exporter JVM's steps in a fixed order (they share its state), the client
 /// exporter JVM's then the command registries' JVM, the data generator's then the creative
-/// inventory's, and the copying, which needs no Java. At most three JVMs run at a time.
+/// inventory's, and the copying, which needs no Java. At most three JVMs run at a time, four
+/// with the split below.
 /// From this many cores (`pool::jobs`) the generator's block behaviour gets its own server JVM.
 /// It pays a second Bootstrap and ~400 MB, which only pays off when the other lanes leave cores
 /// idle: a 6-core laptop finished its other JVMs 20 s before the server lane.

docs/launcher-preparation.md

@@ -41,13 +41,13 @@ Fresh release preparation therefore starts five exporter/data-generator JVMs, do
 (six with 6 cores or more, below), and invokes no compiler. `HORIZON_PREPARE_ISOLATED=1`
 retains separate exporter JVMs for output comparisons and diagnosis.
 
-Those JVMs run on four lanes at once, so at most three run at a time: the server worker's
-steps in their fixed order; the client worker's, then the command registries'; the data
-generator, then the creative tabs'; and the Rust-only copying (client assets and data, the
-world generator's bundle), spread over several threads. The world generator's tables, which
-need the block facts and the reports, and the manifest come after all four. A worker stops
-when its lane is done. The two sides compile at once too. Imports and checks hash the pinned
-files on several threads; every file is still checked in full.
+Those JVMs run on four lanes at once, so at most three run at a time (four with the split
+below): the server worker's steps in their fixed order; the client worker's, then the command
+registries'; the data generator, then the creative tabs'; and the Rust-only copying (client
+assets and data, the world generator's bundle), spread over several threads. The world
+generator's tables, which need the block facts and the reports, and the manifest come after
+all four. A worker stops when its lane is done. The two sides compile at once too. Imports and
+checks hash the pinned files on several threads; every file is still checked in full.
 
 `HORIZON_PREPARE_JOBS` is how many things run at once: the processors by default, at most 8.
 `1` runs it all one step after another (for a machine short on memory). From 6, the world

tools/check_prepare_exporters.py

@@ -146,8 +146,9 @@ def main():
             times.append(round(time.monotonic() - started, 2))
             print(f'{name}: {times[-1]}s, prepared', flush=True)
         verify(results)
-        # Nothing changed since the last run: it starts no JVM and rewrites nothing.
-        before = hashes(resources)
+        # Nothing changed since the last run: it starts no JVM and leaves every file as it was.
+        manifest = resources / 'horizon-resources.json'
+        before = hashes(resources), manifest.read_bytes()
         trace.write_text('')
         with (work / 'repeat.log').open('w') as log:
             subprocess.run([str(binary), '--root', str(folder)], cwd=folder, env=env,
@@ -155,7 +156,7 @@ def main():
         launches = [line for line in trace.read_text().splitlines() if line != '-version']
         if launches:
             raise SystemExit(f'A repeat preparation started {len(launches)} JVMs')
-        if hashes(resources) != before:
+        if (hashes(resources), manifest.read_bytes()) != before:
             raise SystemExit('A repeat preparation changed the resources')
         print('Repeat preparation: no JVM, same files')
         print(f'Isolated {times[:2]}s, batched {times[2:]}s')