bench: run rsync over localhost network for fair comparison

Previously rsync ran as a direct local copy (rsync -aH source/ dest/), bypassing the
loopback interface entirely. This meant tc netem latency/loss/rate limits were never
applied to rsync, making the comparison fundamentally unfair.

Now rsync goes through the network via an rsync daemon on localhost:
  rsync --daemon --no-detach --config=rsyncd.conf
  rsync rsync://localhost:PORT/source/ dest/

The daemon is started before rsync tests and killed afterward. Both the rsync client
and FastSync client run under the same systemd-run disk I/O limits when applicable.

test.py

@@ -6,6 +6,7 @@ import subprocess
 import sys
 import tempfile
 import time
+import socket
 
 TEST_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), "test_data")
 DEFAULT_SOURCE_DIR = os.path.join(TEST_DIR, "source")
@@ -86,6 +87,12 @@ def netem_reset():
     )
 
 
+def find_free_port():
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+        s.bind(('', 0))
+        return s.getsockname()[1]
+
+
 def generate_test_files(source_dir):
     if os.path.exists(source_dir):
         shutil.rmtree(source_dir)
@@ -262,6 +269,27 @@ def run_profile(profile_name, source_dir, dest_dir):
                         server_process.kill()
                         server_process.wait()
 
+        # Rsync tests (over network via daemon, so tc netem applies)
+        rsync_port = find_free_port()
+        rsyncd_conf = os.path.join(tempfile.gettempdir(), f"rsyncd-{rsync_port}.conf")
+        with open(rsyncd_conf, "w") as f:
+            f.write(f"""use chroot = no
+max connections = 5
+read only = yes
+port = {rsync_port}
+
+[source]
+path = {source_dir}
+""")
+
+        rsync_daemon = None
+        try:
+            rsync_daemon = subprocess.Popen(
+                ["rsync", "--daemon", "--no-detach", f"--config={rsyncd_conf}"],
+                stdout=subprocess.DEVNULL, stderr=None
+            )
+            time.sleep(0.5)
+
             for case in RSYNC_CASES:
                 name = case["name"]
                 rsync_args = case["args"]
@@ -272,9 +300,10 @@ def run_profile(profile_name, source_dir, dest_dir):
 
                 try:
                     rsync_cmd = (
-                    ["rsync"]
+                        client_prefix
+                        + ["rsync"]
                         + rsync_args
-                    + [f"{source_dir}/", f"{dest_dir}/"]
+                        + [f"rsync://localhost:{rsync_port}/source/", f"{dest_dir}/"]
                     )
                     print(f"    Running: {' '.join(rsync_cmd)}")
 
@@ -321,6 +350,18 @@ def run_profile(profile_name, source_dir, dest_dir):
                         {"name": name, "suite": profile_name, "status": "Error", "time": "N/A", "error": str(e)}
                     )
 
+        finally:
+            if rsync_daemon:
+                try:
+                    rsync_daemon.wait(timeout=5)
+                except subprocess.TimeoutExpired:
+                    rsync_daemon.kill()
+                    rsync_daemon.wait()
+            try:
+                os.unlink(rsyncd_conf)
+            except Exception:
+                pass
+
     except subprocess.CalledProcessError as e:
         print(f"  Error running netem command: {' '.join(e.cmd)}")
     finally: