gh-75572: Speed up test_xpickle

Run a long living subprocess which handles multiple requests instead of running a new subprocess for each request.
python · serhiy-storchaka · Feb 2, 2026 · Feb 2, 2026 · Feb 2, 2026 · Feb 2, 2026
commit c7bd567e41fb747ec38487256caa18a11166bf98
diff --git a/Lib/test/test_xpickle.py b/Lib/test/test_xpickle.py
@@ -3,6 +3,7 @@
 import io
 import os
 import pickle
+import struct
 import subprocess
 import sys
 import unittest
@@ -83,16 +84,27 @@ def have_python_version(py_version):
     return py_executable_map.get(py_version, None)
 
 
-@support.requires_resource('cpu')
+def read_exact(f, n):
+    buf = b''
+    while len(buf) < n:
+        chunk = f.read(n - len(buf))
+        if not chunk:
+            raise EOFError
+        buf += chunk
+    return buf
+
+
 class AbstractCompatTests(pickletester.AbstractPickleTests):
     py_version = None
+    worker = None
 
     @classmethod
     def setUpClass(cls):
         assert cls.py_version is not None, 'Needs a python version tuple'
         if not have_python_version(cls.py_version):
             py_version_str = ".".join(map(str, cls.py_version))
             raise unittest.SkipTest(f'Python {py_version_str} not available')
+        cls.addClassCleanup(cls.close_worker)
         # Override the default pickle protocol to match what xpickle worker
         # will be running.
         highest_protocol = highest_proto_for_py_version(cls.py_version)
@@ -101,8 +113,31 @@ def setUpClass(cls):
         cls.enterClassContext(support.swap_attr(pickle, 'HIGHEST_PROTOCOL',
                                                 highest_protocol))
 
-    @staticmethod
-    def send_to_worker(python, data):
+    @classmethod
+    def start_worker(cls):
+        target = os.path.join(os.path.dirname(__file__), 'xpickle_worker.py')
+        worker = subprocess.Popen([*python, target],
+                                  stdin=subprocess.PIPE,
+                                  stdout=subprocess.PIPE,
+                                  stderr=subprocess.PIPE,
+                                  # For windows bpo-17023.
+                                  shell=is_windows)
+        cls.worker = worker
+
+    @classmethod
+    def close_worker(cls):
+        worker = cls.worker
+        if worker is None:
+            return
+        cls.worker = None
+        worker.stdin.close()
+        worker.stdout.close()
+        worker.stderr.close()
+        worker.terminate()
+        worker.wait()
+
+    @classmethod
+    def send_to_worker(cls, python, data):
         """Bounce a pickled object through another version of Python.
         This will send data to a child process where it will
         be unpickled, then repickled and sent back to the parent process.
@@ -112,33 +147,40 @@ def send_to_worker(python, data):
         Returns:
             The pickled data received from the child process.
         """
-        target = os.path.join(os.path.dirname(__file__), 'xpickle_worker.py')
-        worker = subprocess.Popen([*python, target],
-                                  stdin=subprocess.PIPE,
-                                  stdout=subprocess.PIPE,
-                                  stderr=subprocess.PIPE,
-                                  # For windows bpo-17023.
-                                  shell=is_windows)
-        stdout, stderr = worker.communicate(data)
-        if worker.returncode == 0:
-            return stdout
-        # if the worker fails, it will write the exception to stdout
+        worker = cls.worker
+        if worker is None:
+            target = os.path.join(os.path.dirname(__file__), 'xpickle_worker.py')
+            worker = subprocess.Popen([*python, target],
+                                      stdin=subprocess.PIPE,
+                                      stdout=subprocess.PIPE,
+                                      stderr=subprocess.PIPE,
+                                      # For windows bpo-17023.
+                                      shell=is_windows)
+            cls.worker = worker
+
         try:
-            exception = pickle.loads(stdout)
-        except (pickle.UnpicklingError, EOFError):
+            worker.stdin.write(struct.pack('!i', len(data)) + data)
+            worker.stdin.flush()
+
+            size, = struct.unpack('!i', read_exact(worker.stdout, 4))
+            if size > 0:
+                return read_exact(worker.stdout, size)
+            # if the worker fails, it will write the exception to stdout
+            if size < 0:
+                stdout = read_exact(worker.stdout, -size)
+                try:
+                    exception = pickle.loads(stdout)
+                except (pickle.UnpicklingError, EOFError):
+                    pass
+                else:
+                    if isinstance(exception, Exception):
+                        # To allow for tests which test for errors.
+                        raise exception
+            _, stderr = worker.communicate()
             raise RuntimeError(stderr)
-        else:
-            if support.verbose > 1:
-                print()
-                print(f'{data   = }')
-                print(f'{stdout = }')
-                print(f'{stderr = }')
-            if isinstance(exception, Exception):
-                # To allow for tests which test for errors.
-                raise exception
-            else:
-                raise RuntimeError(stderr)
-
+        except:
+            cls.close_worker()
+            raise
 
     def dumps(self, arg, proto=0, **kwargs):
         # Skip tests that require buffer_callback arguments since
@@ -148,9 +190,8 @@ def dumps(self, arg, proto=0, **kwargs):
             self.skipTest('Test does not support "buffer_callback" argument.')
         f = io.BytesIO()
         p = self.pickler(f, proto, **kwargs)
-        p.dump((proto, arg))
-        f.seek(0)
-        data = bytes(f.read())
+        p.dump(arg)
+        data = struct.pack('!i', proto) + f.getvalue()
         python = py_executable_map[self.py_version]
         return self.send_to_worker(python, data)
 

diff --git a/Lib/test/xpickle_worker.py b/Lib/test/xpickle_worker.py
@@ -2,6 +2,7 @@
 # pickles in a different Python version.
 import os
 import pickle
+import struct
 import sys
 
 
@@ -24,16 +25,38 @@
         sources = f.read()
     exec(sources, vars(test_module))
 
+def read_exact(f, n):
+    buf = b''
+    while len(buf) < n:
+        chunk = f.read(n - len(buf))
+        if not chunk:
+            raise EOFError
+        buf += chunk
+    return buf
 
 in_stream = getattr(sys.stdin, 'buffer', sys.stdin)
 out_stream = getattr(sys.stdout, 'buffer', sys.stdout)
 
 try:
-    message = pickle.load(in_stream)
-    protocol, obj = message
-    pickle.dump(obj, out_stream, protocol)
-except Exception as e:
+    while True:
+        size, = struct.unpack('!i', read_exact(in_stream, 4))
+        if not size:
+            break
+        data = read_exact(in_stream, size)
+        protocol, = struct.unpack('!i', data[:4])
+        obj = pickle.loads(data[4:])
+        data = pickle.dumps(obj, protocol)
+        out_stream.write(struct.pack('!i', len(data)) + data)
+        out_stream.flush()
+except Exception as exc:
     # dump the exception to stdout and write to stderr, then exit
-    pickle.dump(e, out_stream)
-    sys.stderr.write(repr(e))
+    try:
+        data = pickle.dumps(exc)
+        out_stream.write(struct.pack('!i', -len(data)) + data)
+        out_stream.flush()
+    except Exception:
+        out_stream.write(struct.pack('!i', 0))
+        out_stream.flush()
+        sys.stderr.write(repr(exc))
+        sys.stderr.flush()
     sys.exit(1)