tools: an end-to-end check that flowing water pushes a boat

A headless single-player game builds a source-fed trench, summons an oak boat
on the flowing water and reads its Pos every ten ticks: vanilla's
Entity.baseTick pushes a boat by the water current (0.014) before floatBoat
scales the velocity, so the boat must drift downstream. Without that step the
boat sits still (#6435).

tools/boat_current_e2e.py

@@ -0,0 +1,145 @@
+#!/usr/bin/env python3
+"""Check that a boat in flowing water drifts with the current (#6435).
+
+A headless single-player game opens a scratch world; a one-deep, three-wide stone
+trench is filled from a single source block at its head, an oak boat is summoned
+onto the flowing water, and its `Pos` is read every ten ticks. Vanilla's
+`Entity.updateFluidInteraction` pushes a boat by the water current it sits in
+(`AbstractBoat` does not opt out of `isPushedByFluid`), with water's 0.014 scale,
+and `AbstractBoat.tick` runs it before `floatBoat` scales the velocity; a boat
+that stays put is missing that step.
+
+The reported case rides the boat, but a ridden boat ticks through the same
+`tick_boat` (the rider's client moves it), so the unridden boat covers the same
+current step.
+
+    python3 tools/boat_current_e2e.py --bin-dir target/fast
+"""
+import argparse
+import json
+import shutil
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+try:
+    from . import lab
+except ImportError:
+    import lab
+
+WORLD = 'BoatCurrent'
+SAMPLE_TICKS = 10
+SAMPLES = 4
+MIN_DRIFT = 1.0
+BOAT = '@e[type=minecraft:oak_boat,limit=1,sort=nearest]'
+
+
+class Failed(Exception):
+    pass
+
+
+def ctl(ctl_path, socket, cwd, env, *words):
+    result = subprocess.run([str(ctl_path), '--socket', str(socket), *map(str, words)], cwd=cwd, env=env,
+                            capture_output=True, text=True, timeout=900)
+    lines = result.stdout.strip().splitlines()
+    reply = json.loads(lines[-1]) if lines else {'error': result.stderr.strip()}
+    if result.returncode or 'error' in reply:
+        raise Failed(f'{" ".join(map(str, words))}: exit {result.returncode}: {reply}')
+    return reply
+
+
+def command(ctl_path, socket, cwd, env, text):
+    return ctl(ctl_path, socket, cwd, env, 'command', text)
+
+
+def boat_pos(ctl_path, socket, cwd, env):
+    reply = command(ctl_path, socket, cwd, env, f'data get entity {BOAT} Pos')
+    for line in reply.get('chat') or []:
+        if '[' in line and ']' in line:
+            body = line[line.index('[') + 1: line.rindex(']')]
+            return [float(v.strip().removesuffix('d')) for v in body.split(',')]
+    raise Failed(f'no boat position in {reply}')
+
+
+def build_trench(ctl_path, socket, cwd, env):
+    # The chunk has to be loaded before a fill reaches it: aim the player at the
+    # trench, then retry until the floor lands.
+    command(ctl_path, socket, cwd, env, 'tp @s 2.5 160 1')
+    ctl(ctl_path, socket, cwd, env, 'wait-for', 'loaded', '--timeout', '120')
+    ctl(ctl_path, socket, cwd, env, 'wait', '60')
+    for _ in range(5):
+        reply = command(ctl_path, socket, cwd, env, 'fill 0 150 -1 9 150 3 stone')
+        if not any('not loaded' in line for line in reply.get('chat') or []):
+            break
+        ctl(ctl_path, socket, cwd, env, 'wait', '40')
+    else:
+        raise Failed('the trench floor never loaded')
+    command(ctl_path, socket, cwd, env, 'fill -1 151 -1 -1 151 3 stone')
+    command(ctl_path, socket, cwd, env, 'fill 0 151 -1 9 151 -1 stone')
+    command(ctl_path, socket, cwd, env, 'fill 0 151 3 9 151 3 stone')
+    command(ctl_path, socket, cwd, env, 'fill 0 151 0 0 151 2 water')
+    # A source spreads one block every five ticks; nine blocks settle in 45.
+    ctl(ctl_path, socket, cwd, env, 'wait', '120')
+    # Stand on the south wall, out of the current, next to the boat.
+    command(ctl_path, socket, cwd, env, 'tp @s 2.5 152 3.5')
+    ctl(ctl_path, socket, cwd, env, 'wait', '20')
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('--bin-dir', default='target/fast', help='folder with horizon-client and horizon-ctl')
+    parser.add_argument('--resources', default=str(lab.RESOURCES), help='the prepared resources (default: .local)')
+    parser.add_argument('--keep', action='store_true', help='keep the scratch game home')
+    args = parser.parse_args()
+    bindir = Path(args.bin_dir).resolve()
+    client, ctl_path = bindir / 'horizon-client', bindir / 'horizon-ctl'
+    for binary in (client, ctl_path):
+        if not binary.exists():
+            raise Failed(f'{binary} is missing; build it (cargo build --profile fast -p horizon-client '
+                         '-p horizon-client-lib --bins) or pass --bin-dir')
+    resources = Path(args.resources).resolve()
+    if not resources.exists():
+        raise Failed(f'{resources} is missing; run tools/lab.py prepare')
+    work = Path(tempfile.mkdtemp(prefix='hz-boat-current-'))
+    (work / '.local/horizon-client').mkdir(parents=True)
+    lab.link_dir(resources, work / '.local/horizon-client' / lab.VERSION)
+    (work / 'options.txt').write_text('onboardAccessibility:false\n')
+    env = lab.game_env(home=work, HORIZON_TELEMETRY_URL='off')
+    socket = work / 'c.sock'
+    launch = subprocess.run([str(ctl_path), '--socket', str(socket), 'launch', '--world', WORLD, '--seed', '7',
+                             '--creative', '--headless', '--wait'],
+                            cwd=work, env=env, capture_output=True, text=True, timeout=900)
+    if launch.returncode:
+        log = next(iter(sorted(work.glob('*.log'))), None)
+        tail = log.read_text(errors='replace')[-2000:] if log else launch.stderr[-2000:]
+        raise Failed(f'launch failed ({launch.returncode}): {tail}')
+    try:
+        ctl(ctl_path, socket, work, env, 'wait', '20')
+        build_trench(ctl_path, socket, work, env)
+        command(ctl_path, socket, work, env, 'summon minecraft:oak_boat 2.5 151.0 1.9')
+        ctl(ctl_path, socket, work, env, 'wait', '5')
+        xs = []
+        for _ in range(SAMPLES):
+            xs.append(boat_pos(ctl_path, socket, work, env)[0])
+            ctl(ctl_path, socket, work, env, 'wait', str(SAMPLE_TICKS))
+        drift = xs[-1] - xs[0]
+        print(f'boat x over {(SAMPLES - 1) * SAMPLE_TICKS} ticks: {xs[0]:.3f} -> {xs[-1]:.3f} ({drift:+.3f})')
+        if drift < MIN_DRIFT:
+            raise Failed(f'the flowing water did not push the boat (drift {drift:+.3f}, want >= {MIN_DRIFT}); '
+                         'Entity.baseTick\'s fluid step is missing from the boat tick')
+        print('PASS: the boat drifts with the flowing water')
+        return 0
+    finally:
+        subprocess.run([str(ctl_path), '--socket', str(socket), 'quit'], cwd=work, env=env,
+                       capture_output=True, timeout=120)
+        if not args.keep:
+            shutil.rmtree(work, ignore_errors=True)
+
+
+if __name__ == '__main__':
+    try:
+        sys.exit(main())
+    except Failed as error:
+        print(f'FAIL: {error}')
+        sys.exit(1)