Use thread ancestry hierarchy to support nested threads

Lint

ipykernel/iostream.py

@@ -451,7 +451,8 @@ def __init__(
             "parent_header"
         )
         self._parent_header.set({})
-        self._thread_parents = {}
+        self._thread_to_parent = {}
+        self._thread_to_parent_header = {}
         self._parent_header_global = {}
         self._master_pid = os.getpid()
         self._flush_pending = False
@@ -509,7 +510,13 @@ def parent_header(self):
         except LookupError:
             try:
                 # thread-specific
-                return self._thread_parents[threading.current_thread().ident]
+                identity = threading.current_thread().ident
+                # retrieve the outermost (oldest ancestor,
+                # discounting the kernel thread) thread identity
+                while identity in self._thread_to_parent:
+                    identity = self._thread_to_parent[identity]
+                # use the header of the oldest ancestor
+                return self._thread_to_parent_header[identity]
             except KeyError:
                 # global (fallback)
                 return self._parent_header_global

ipykernel/ipkernel.py

@@ -71,7 +71,8 @@ def _get_comm_manager(*args, **kwargs):
 
 import threading
 
-threading_start = threading.Thread.start
+_threading_Thread_run = threading.Thread.run
+_threading_Thread__init__ = threading.Thread.__init__
 
 
 class IPythonKernel(KernelBase):
@@ -158,6 +159,9 @@ def __init__(self, **kwargs):
 
             appnope.nope()
 
+        self._new_threads_parent_header = {}
+        self._initialize_thread_hooks()
+
         if hasattr(gc, "callbacks"):
             # while `gc.callbacks` exists since Python 3.3, pypy does not
             # implement it even as of 3.9.
@@ -356,7 +360,7 @@ def set_sigint_result():
     async def execute_request(self, stream, ident, parent):
         """Override for cell output - cell reconciliation."""
         parent_header = extract_header(parent)
-        self._associate_identity_of_new_threads_with(parent_header)
+        self._associate_new_top_level_threads_with(parent_header)
         await super().execute_request(stream, ident, parent)
 
     async def do_execute(
@@ -724,31 +728,47 @@ def do_clear(self):
             self.shell.reset(False)
         return dict(status="ok")
 
-    def _associate_identity_of_new_threads_with(self, parent_header):
-        """Intercept the identity of any thread started after this method finished,
-
-        and associate the thread's output with the parent header frame, which allows
-        to direct the outputs to the cell which started the thread.
+    def _associate_new_top_level_threads_with(self, parent_header):
+        """Store the parent header to associate it with new top-level threads"""
+        self._new_threads_parent_header = parent_header
 
-        This is a no-op if the `self._stdout` and `self._stderr` are not
-        sub-classes of `OutStream`.
-        """
+    def _initialize_thread_hooks(self):
+        """Store thread hierarchy and thread-parent_header associations."""
         stdout = self._stdout
         stderr = self._stderr
+        kernel_thread_ident = threading.get_ident()
+        kernel = self
 
-        def start_closure(self: threading.Thread):
+        def run_closure(self: threading.Thread):
             """Wrap the `threading.Thread.start` to intercept thread identity.
 
             This is needed because there is no "start" hook yet, but there
             might be one in the future: https://bugs.python.org/issue14073
+
+            This is a no-op if the `self._stdout` and `self._stderr` are not
+            sub-classes of `OutStream`.
             """
 
-            threading_start(self)
+            try:
+                parent = self._ipykernel_parent_thread_ident  # type:ignore[attr-defined]
+            except AttributeError:
+                return
             for stream in [stdout, stderr]:
                 if isinstance(stream, OutStream):
-                    stream._thread_parents[self.ident] = parent_header
+                    if parent == kernel_thread_ident:
+                        stream._thread_to_parent_header[
+                            self.ident
+                        ] = kernel._new_threads_parent_header
+                    else:
+                        stream._thread_to_parent[self.ident] = parent
+            _threading_Thread_run(self)
+
+        def init_closure(self: threading.Thread, *args, **kwargs):
+            _threading_Thread__init__(self, *args, **kwargs)
+            self._ipykernel_parent_thread_ident = threading.get_ident()  # type:ignore[attr-defined]
 
-        threading.Thread.start = start_closure  # type:ignore[method-assign]
+        threading.Thread.__init__ = init_closure  # type:ignore[method-assign]
+        threading.Thread.run = run_closure  # type:ignore[method-assign]
 
     def _clean_thread_parent_frames(
         self, phase: t.Literal["start", "stop"], info: t.Dict[str, t.Any]
@@ -768,11 +788,18 @@ def _clean_thread_parent_frames(
         active_threads = {thread.ident for thread in threading.enumerate()}
         for stream in [self._stdout, self._stderr]:
             if isinstance(stream, OutStream):
-                thread_parents = stream._thread_parents
-                for identity in list(thread_parents.keys()):
+                thread_to_parent_header = stream._thread_to_parent_header
+                for identity in list(thread_to_parent_header.keys()):
+                    if identity not in active_threads:
+                        try:
+                            del thread_to_parent_header[identity]
+                        except KeyError:
+                            pass
+                thread_to_parent = stream._thread_to_parent
+                for identity in list(thread_to_parent.keys()):
                     if identity not in active_threads:
                         try:
-                            del thread_parents[identity]
+                            del thread_to_parent[identity]
                         except KeyError:
                             pass