test: differential st_blocks for --sparse/--preallocate and --ignore-existing wire volume

- Assert FastSync's sparse/preallocate block accounting matches rsync 3.4.1
  for a hole file (preallocate wins over sparse, exactly like rsync).
- Assert --ignore-existing is answered by the receiver before the sender
  transmits the payload (CountingProxy wire volume near-zero).
- CountingProxy.run gains a bounded join_timeout so tests that only need the
  client->server count do not wait for the server's idle socket.
- Fix the stale protocol 2.24.0 comment in test_config.c (golden is 2.26.0).

tests/integration/common.py

@@ -101,9 +101,15 @@ class CountingProxy:
                 return
             counter[0] += len(data)
 
-    def run(self, cmd):
+    def run(self, cmd, join_timeout=20):
         """Forward one client run (the full command list) to the real server and
-        return the CompletedProcess after the counts have settled."""
+        return the CompletedProcess after the counts have settled.
+
+        ``join_timeout`` bounds how long to wait for the forwarding threads.  The
+        client->server count is published as soon as the client side reaches EOF
+        (i.e. once the client process has exited), so callers that only need that
+        count can pass a small value instead of waiting for the server to close
+        its idle socket."""
 
         def serve():
             try:
@@ -119,15 +125,15 @@ class CountingProxy:
             a.start()
             b.start()
             a.join()
-            b.join()
             self.client_to_server = c2s[0]
+            b.join()
             self.server_to_client = s2c[0]
             self._listener.close()
 
-        thread = threading.Thread(target=serve)
+        thread = threading.Thread(target=serve, daemon=True)
         thread.start()
         result = subprocess.run(cmd, capture_output=True, text=True, timeout=180)
-        thread.join(20)
+        thread.join(join_timeout)
         return result
 
 

tests/integration/test_parity_quickwins.py

@@ -12,6 +12,8 @@ import pytest
 
 sys.path.insert(0, os.path.dirname(__file__))
 from common import (
+    CLIENT_CMD,
+    CountingProxy,
     TEST_DATA_DIR,
     ServerManager,
     run_client,
@@ -593,6 +595,57 @@ class TestVerifyAndFlip:
         assert result.returncode == 0, result.stderr[:300]
         _assert_same_tree(rdst, get_dest_received_dir(dest, source), "(--preallocate)")
 
+    @requires_rsync
+    @pytest.mark.ci
+    def test_sparse_preallocate_blocks_match_rsync(self, shared_server):
+        """rsync lets --preallocate win over --sparse: the hole file gets its
+        full space reserved (st_blocks ~ size/512) even though the sparse writer
+        seeks over the zero run.  FastSync must agree both on the block count and
+        with rsync's exact value for each flag combination."""
+        source = self._src("sparsepre")
+        dest = self._dst("sparsepre")
+        rdst = self._dst("sparsepre_r")
+        total = 1024 * 1024
+        # A zero run >= the 4 KiB sparse threshold in the middle of the image.
+        with open(os.path.join(source, "hole.bin"), "wb") as fh:
+            fh.write(os.urandom(64 * 1024))
+            fh.write(b"\x00" * (768 * 1024))
+            fh.write(os.urandom(total - 64 * 1024 - 768 * 1024))
+
+        def run_both(flags, tag):
+            rsync_dst = self._dst(f"sparsepre_{tag}_r")
+            fs_dst = self._dst(f"sparsepre_{tag}_f")
+            assert _rsync(["-a"] + flags + [source + "/", rsync_dst + "/"]).returncode == 0
+            result, _ = run_client(source, fs_dst, flags=["-a"] + flags,
+                                   port=shared_server.port)
+            assert result.returncode == 0, result.stderr[:300]
+            rfile = os.path.join(rsync_dst, "hole.bin")
+            ffile = os.path.join(get_dest_received_dir(fs_dst, source), "hole.bin")
+            with open(rfile, "rb") as rfh, open(ffile, "rb") as ffh:
+                assert rfh.read() == ffh.read(), f"{tag}: content diverged"
+            return os.stat(rfile).st_blocks, os.stat(ffile).st_blocks
+
+        r_sparse, f_sparse = run_both(["--sparse"], "sparse")
+        r_both, f_both = run_both(["--sparse", "--preallocate"], "both")
+
+        assert f_sparse == r_sparse, (
+            f"--sparse st_blocks: fastsync={f_sparse} rsync={r_sparse}"
+        )
+        assert f_both == r_both, (
+            f"--sparse --preallocate st_blocks: fastsync={f_both} rsync={r_both}"
+        )
+        # Only meaningful where the filesystem actually reports holes; otherwise
+        # both sides simply allocate the full size and the equality above holds.
+        has_holes = r_sparse * 512 < total
+        if has_holes:
+            assert f_both > f_sparse, (
+                "preallocate must win over sparse: "
+                f"sparse={f_sparse} both={f_both} blocks"
+            )
+            assert r_both > r_sparse, (
+                f"rsync preallocate must win: sparse={r_sparse} both={r_both}"
+            )
+
     @requires_rsync
     def test_fuzzy_content_matches_rsync(self, shared_server):
         source = self._src("fuzzy")
@@ -717,6 +770,58 @@ class TestVerifyAndFlip:
             "--link-dest must hard-link to the basis file"
 
 
+class TestIgnoreExistingShortCircuit:
+    """#9: --ignore-existing is decided by the receiver during the per-file
+    check, before the sender streams any payload.  A large destination file that
+    is already present must therefore cost almost no wire bytes, not the full
+    file; rsync short-circuits the same way."""
+
+    @requires_rsync
+    @pytest.mark.ci
+    def test_skip_answered_before_payload(self):
+        source = os.path.join(TEST_DATA_DIR, "qw_ie_wire_src")
+        dest = os.path.join(TEST_DATA_DIR, "qw_ie_wire_dst")
+        rdst = os.path.join(TEST_DATA_DIR, "qw_ie_wire_rdst")
+        clean_dir(source)
+        clean_dir(dest)
+        clean_dir(rdst)
+        big = b"S" * (4 * 1024 * 1024)
+        with open(os.path.join(source, "big.bin"), "wb") as fh:
+            fh.write(big)
+        for root in (get_dest_received_dir(dest, source), rdst):
+            os.makedirs(root, exist_ok=True)
+            with open(os.path.join(root, "big.bin"), "wb") as fh:
+                fh.write(b"D" * len(big))
+
+        assert _rsync(["-a", "--ignore-existing", source + "/",
+                       rdst + "/"]).returncode == 0
+
+        # A dedicated one-shot server keeps the proxy's connection teardown
+        # deterministic (the session-wide server can linger on an idle socket).
+        with ServerManager() as server:
+            cmd = CLIENT_CMD + [
+                "--source-dir", source, "--dest-dir", dest, "--save-to-disk",
+                "--server-port", str(server.port), "-a", "--ignore-existing",
+            ]
+            proxy = CountingProxy(server.port)
+            # Only the client->server count matters here; it is final once the
+            # client exits, so do not wait for the server to close its idle
+            # socket (which can take the full default join timeout).
+            result = proxy.run(cmd, join_timeout=1.0)
+        assert result.returncode == 0, (result.stderr or result.stdout)[:300]
+        # The receiver answered the skip before the sender transmitted the file:
+        # only config/path/check frames crossed, not the 4 MiB payload.
+        assert proxy.client_to_server < len(big) // 10, (
+            f"--ignore-existing transmitted {proxy.client_to_server} bytes for a "
+            f"skipped {len(big)}-byte file"
+        )
+        received = get_dest_received_dir(dest, source)
+        with open(os.path.join(received, "big.bin"), "rb") as fh:
+            assert fh.read() == b"D" * len(big), \
+                "--ignore-existing must preserve the destination content"
+        _assert_same_tree(rdst, received, "(--ignore-existing wire short-circuit)")
+
+
 @pytest.mark.skipif(os.geteuid() != 0, reason="ownership mapping requires root")
 class TestOwnershipMapping:
     """#33/#34/#35: --usermap/--groupmap/--chown match rsync's numeric result."""

tests/test_config.c

@@ -2920,7 +2920,7 @@ static unsigned long long capture_wire_hash(const Config* cfg, size_t* out_len)
   return h;
 }
 
-/* Byte-for-byte wire compatibility guard (protocol 2.24.0).  The expected hash
+/* Byte-for-byte wire compatibility guard (protocol 2.26.0).  The expected hash
  * pins the pre-X-macro byte stream; the refactor MUST NOT change it. */
 static void test_config_wire_golden() {
   if (is_running_under_valgrind())