bpo-39850: Add support for abstract sockets in multiprocessing (GH-18866)

Author: pablogsal

Lib/multiprocessing/connection.py

@@ -73,6 +73,11 @@ def arbitrary_address(family):
     if family == 'AF_INET':
         return ('localhost', 0)
     elif family == 'AF_UNIX':
+        # Prefer abstract sockets if possible to avoid problems with the address
+        # size.  When coding portable applications, some implementations have
+        # sun_path as short as 92 bytes in the sockaddr_un struct.
+        if util.abstract_sockets_supported:
+            return f"\0listener-{os.getpid()}-{next(_mmap_counter)}"
         return tempfile.mktemp(prefix='listener-', dir=util.get_temp_dir())
     elif family == 'AF_PIPE':
         return tempfile.mktemp(prefix=r'\\.\pipe\pyc-%d-%d-' %
@@ -102,7 +107,7 @@ def address_type(address):
         return 'AF_INET'
     elif type(address) is str and address.startswith('\\\\'):
         return 'AF_PIPE'
-    elif type(address) is str:
+    elif type(address) is str or util.is_abstract_socket_namespace(address):
         return 'AF_UNIX'
     else:
         raise ValueError('address type of %r unrecognized' % address)
@@ -597,7 +602,8 @@ def __init__(self, address, family, backlog=1):
         self._family = family
         self._last_accepted = None
 
-        if family == 'AF_UNIX':
+        if family == 'AF_UNIX' and not util.is_abstract_socket_namespace(address):
+            # Linux abstract socket namespaces do not need to be explicitly unlinked
             self._unlink = util.Finalize(
                 self, os.unlink, args=(address,), exitpriority=0
                 )

Lib/multiprocessing/forkserver.py

@@ -55,7 +55,8 @@ def _stop_unlocked(self):
         os.waitpid(self._forkserver_pid, 0)
         self._forkserver_pid = None
 
-        os.unlink(self._forkserver_address)
+        if not util.is_abstract_socket_namespace(self._forkserver_address):
+            os.unlink(self._forkserver_address)
         self._forkserver_address = None
 
     def set_forkserver_preload(self, modules_names):
@@ -135,7 +136,8 @@ def ensure_running(self):
             with socket.socket(socket.AF_UNIX) as listener:
                 address = connection.arbitrary_address('AF_UNIX')
                 listener.bind(address)
-                os.chmod(address, 0o600)
+                if not util.is_abstract_socket_namespace(address):
+                    os.chmod(address, 0o600)
                 listener.listen()
 
                 # all client processes own the write end of the "alive" pipe;

Lib/multiprocessing/managers.py

@@ -1262,8 +1262,12 @@ class SharedMemoryServer(Server):
 
         def __init__(self, *args, **kwargs):
             Server.__init__(self, *args, **kwargs)
+            address = self.address
+            # The address of Linux abstract namespaces can be bytes
+            if isinstance(address, bytes):
+                address = os.fsdecode(address)
             self.shared_memory_context = \
-                _SharedMemoryTracker(f"shmm_{self.address}_{getpid()}")
+                _SharedMemoryTracker(f"shm_{address}_{getpid()}")
             util.debug(f"SharedMemoryServer started by pid {getpid()}")
 
         def create(self, c, typeid, /, *args, **kwargs):

Lib/multiprocessing/util.py

@@ -102,6 +102,29 @@ def log_to_stderr(level=None):
     _log_to_stderr = True
     return _logger
 
+
+# Abstract socket support
+
+def _platform_supports_abstract_sockets():
+    if sys.platform == "linux":
+        return True
+    if hasattr(sys, 'getandroidapilevel'):
+        return True
+    return False
+
+
+def is_abstract_socket_namespace(address):
+    if not address:
+        return False
+    if isinstance(address, bytes):
+        return address[0] == 0
+    elif isinstance(address, str):
+        return address[0] == "\0"
+    raise TypeError('address type of {address!r} unrecognized')
+
+
+abstract_sockets_supported = _platform_supports_abstract_sockets()
+
 #
 # Function returning a temp directory which will be removed on exit
 #

Lib/test/_test_multiprocessing.py

@@ -3274,6 +3274,19 @@ def test_context(self):
         if self.TYPE == 'processes':
             self.assertRaises(OSError, l.accept)
 
+    @unittest.skipUnless(util.abstract_sockets_supported,
+                         "test needs abstract socket support")
+    def test_abstract_socket(self):
+        with self.connection.Listener("\0something") as listener:
+            with self.connection.Client(listener.address) as client:
+                with listener.accept() as d:
+                    client.send(1729)
+                    self.assertEqual(d.recv(), 1729)
+
+        if self.TYPE == 'processes':
+            self.assertRaises(OSError, listener.accept)
+
+
 class _TestListenerClient(BaseTestCase):
 
     ALLOWED_TYPES = ('processes', 'threads')

Misc/NEWS.d/next/Library/2020-03-09-01-45-06.bpo-39850.eaJNIE.rst

@@ -0,0 +1,5 @@
+:mod:`multiprocessing` now supports abstract socket addresses (if abstract sockets
+are supported in the running platform). When creating arbitrary addresses (like when
+default-constructing :class:`multiprocessing.connection.Listener` objects) abstract
+sockets are preferred to avoid the case when the temporary-file-generated address is
+too large for an AF_UNIX socket address. Patch by Pablo Galindo.