PEP 768: Add some clarifications and minor edits

peps/pep-0768.rst

@@ -141,8 +141,10 @@ A new structure is added to PyThreadState to support remote debugging:
 
 This structure is appended to ``PyThreadState``, adding only a few fields that
 are **never accessed during normal execution**. The ``debugger_pending_call`` field
-indicates when a debugger has requested execution, while ``debugger_script``
-provides Python code to be executed when the interpreter reaches a safe point.
+indicates when a debugger has requested execution, while ``debugger_script_path``
+provides a filesystem path to a Python source file (.py) that will be executed when
+the interpreter reaches a safe point. The path must point to a Python source file,
+not compiled Python code (.pyc) or any other format.
 
 The value for ``MAX_SCRIPT_PATH_SIZE`` will be a trade-off between binary size
 and how big debugging scripts' paths can be. To limit the memory overhead per
@@ -177,7 +179,7 @@ debugger support:
 These offsets allow debuggers to locate critical debugging control structures in
 the target process's memory space. The ``eval_breaker`` and ``remote_debugger_support``
 offsets are relative to each ``PyThreadState``, while the ``debugger_pending_call``
-and ``debugger_script`` offsets are relative to each ``_PyRemoteDebuggerSupport``
+and ``debugger_script_path`` offsets are relative to each ``_PyRemoteDebuggerSupport``
 structure, allowing the new structure and its fields to be found regardless of
 where they are in memory. ``debugger_script_path_size`` informs the attaching
 tool of the size of the buffer.
@@ -200,13 +202,19 @@ When a debugger wants to attach to a Python process, it follows these steps:
 
 5. Write control information:
 
-   - Write a filename containing Python code to be executed into the
-     ``debugger_script`` field in ``_PyRemoteDebuggerSupport``.
+   - Most debuggers will pause the process before writing to its memory. This is
+     standard practice for tools like GDB, which use SIGSTOP or ptrace to pause the process.
+     This approach prevents races when writing to process memory. Profilers and other tools
+     that don't wish to stop the process can still use this interface, but they need to
+     handle possible races, which is a normal consideration for profilers in general.
+
+   - Write a file path to a Python source file (.py) into the
+     ``debugger_script_path`` field in ``_PyRemoteDebuggerSupport``.
    - Set ``debugger_pending_call`` flag in ``_PyRemoteDebuggerSupport`` to 1
    - Set ``_PY_EVAL_PLEASE_STOP_BIT`` in the ``eval_breaker`` field
 
-Once the interpreter reaches the next safe point, it will execute the script
-provided by the debugger.
+Once the interpreter reaches the next safe point, it will execute the Python code
+contained in the file specified by the debugger.
 
 Interpreter Integration
 -----------------------
@@ -237,7 +245,7 @@ to be audited or disabled if desired by a system's administrator.
     if (tstate->eval_breaker) {
         if (tstate->remote_debugger_support.debugger_pending_call) {
             tstate->remote_debugger_support.debugger_pending_call = 0;
-            const char *path = tstate->remote_debugger_support.debugger_script;
+            const char *path = tstate->remote_debugger_support.debugger_script_path;
             if (*path) {
                 if (0 != PySys_Audit("debugger_script", "%s", path)) {
                     PyErr_Clear();
@@ -273,16 +281,17 @@ arbitrary Python code within the context of a specified Python process:
 
 .. code-block:: python
 
-  def remote_exec(pid: int, code: str, timeout: int = 0) -> None:
+  def remote_exec(pid: int, code: str) -> None:
       """
       Executes a block of Python code in a given remote Python process.
 
+      This function returns immediately, and the code will be executed at the next
+      available opportunity in the target process, similar to how signals are handled.
+      There is no way to determine when or if the code has been executed.
+
       Args:
            pid (int): The process ID of the target Python process.
            code (str): A string containing the Python code to be executed.
-           timeout (int): An optional timeout for waiting for the remote
-              process to execute the code. If the timeout is exceeded a
-              ``TimeoutError`` will be raised.
       """
 
 An example usage of the API would look like:
@@ -292,9 +301,7 @@ An example usage of the API would look like:
     import sys
     # Execute a print statement in a remote Python process with PID 12345
     try:
-        sys.remote_exec(12345, "print('Hello from remote execution!')", timeout=3)
-    except TimeoutError:
-        print(f"The remote process took too long to execute the code")
+        sys.remote_exec(12345, "print('Hello from remote execution!')")
     except Exception as e:
         print(f"Failed to execute code: {e}")
 
@@ -322,6 +329,37 @@ feature. This way, tools can offer a useful error message explaining why they
 won't work, instead of believing that they have attached and then never having
 their script run.
 
+Multi-threading Considerations
+------------------------------
+
+Debugging code injected through this interface executes opportunistically in
+whichever thread first encounters a safe evaluation point after the request is
+made. This behavior mirrors how Python handles signals, providing a reliable
+execution model without adding overhead. For developers needing to target
+specific threads, the debug script can be installed only on the desired thread
+structure or in all of them if needed.
+
+The Global Interpreter Lock (GIL) continues to govern execution as normal when
+debug code runs. This means if a target thread is currently executing a C
+extension that holds the GIL without releasing it, the debug code will wait
+until that operation completes and the GIL becomes available. However, the
+interface introduces no additional GIL contention beyond what the debugging
+code itself requires. Importantly, the interface remains fully compatible with
+Python's free-threaded mode, where the GIL is not held, allowing debugger code
+to execute in any available thread.
+
+In situations where all threads in the target process are blocked—waiting on I/O
+operations, sleep states, or external resources—the debugging code might not
+execute immediately. In these cases, debuggers can send a pre-registered signal
+to the process, which will interrupt the sleep state and force thread scheduling,
+creating an opportunity for the debug code to run or leverage any other mechanism
+that can force the target process to resume execution.
+
+The execution pattern closely resembles how Python handles signals internally.
+The interpreter guarantees that debug code only runs at safe points, never
+interrupting atomic operations within the interpreter itself. This approach
+ensures that debugging operations cannot corrupt the interpreter state while
+still providing timely execution in most real-world scenarios.
 
 Backwards Compatibility
 =======================
@@ -454,8 +492,8 @@ Rejected Ideas
 Writing Python code into the buffer
 -----------------------------------
 
-We have chosen to have debuggers write the code to be executed into a file
-whose path is written into a buffer in the remote process. This has been deemed
+We have chosen to have debuggers write the path to a file containing Python code
+into a buffer in the remote process. This has been deemed
 more secure than writing the Python code to be executed itself into a buffer in
 the remote process, because it means that an attacker who has gained arbitrary
 writes in a process but not arbitrary code execution or file system