[lldb] Introduce ScriptedFrameProvider for real threads

This patch introduces a new scripting affordance: `ScriptedFrameProvider`.

This allows users to provide custom stack frames for real native threads,
augmenting or replacing the standard unwinding mechanism. This is useful
for:
- Providing frames for custom calling conventions or languages
- Reconstructing missing frames from crash dumps or core files
- Adding diagnostic or synthetic frames for debugging

The frame provider supports four merge strategies:
- Replace: Replace entire stack with scripted frames
- Prepend: Add scripted frames before real stack
- Append: Add scripted frames after real stack
- ReplaceByIndex: Replace specific frames by index

With this change, frames can be synthesized from different sources:
- Either from a dictionary containing a PC address and frame index
- Or by creating a ScriptedFrame python object for full control

To use it, first register the scripted frame provider then use existing
commands:
   (lldb) frame provider register -C my_module.MyFrameProvider

      or

   (lldb) script thread.RegisterFrameProvider("my_module.MyFrameProvider", lldb.SBStructuredData())

      then

   (lldb) bt

See examples/python/templates/scripted_frame_provider.py for details.

Architecture changes:
- Moved ScriptedFrame from `Plugins` to `Interpreter` to avoid
   layering violations
- Moved `RegisterContextMemory` from `Plugins` to `Target` as it only
   depends on Target and Utility layers
- Added `ScriptedFrameProvider` C++ wrapper and Python interface
- Updated `Thread::GetStackFrameList` to apply merge strategies

rdar://161834688

Signed-off-by: Med Ismail Bennani <[email protected]>

Author: medismailben

lldb/bindings/python/CMakeLists.txt

@@ -107,6 +107,7 @@ function(finish_swig_python swig_target lldb_python_bindings_dir lldb_python_tar
     "plugins"
     FILES
     "${LLDB_SOURCE_DIR}/examples/python/templates/parsed_cmd.py"
+    "${LLDB_SOURCE_DIR}/examples/python/templates/scripted_frame_provider.py"
     "${LLDB_SOURCE_DIR}/examples/python/templates/scripted_process.py"
     "${LLDB_SOURCE_DIR}/examples/python/templates/scripted_platform.py"
     "${LLDB_SOURCE_DIR}/examples/python/templates/operating_system.py"

lldb/examples/python/templates/scripted_frame_provider.py

@@ -0,0 +1,147 @@
+from abc import ABCMeta, abstractmethod
+
+import lldb
+
+
+class ScriptedFrameProvider(metaclass=ABCMeta):
+    """
+    The base class for a scripted frame provider.
+
+    A scripted frame provider allows you to provide custom stack frames for a
+    thread, which can be used to augment or replace the standard unwinding
+    mechanism. This is useful for:
+
+    - Providing frames for custom calling conventions or languages
+    - Reconstructing missing frames from crash dumps or core files
+    - Adding diagnostic or synthetic frames for debugging
+    - Visualizing state machines or async execution contexts
+
+    Most of the base class methods are `@abstractmethod` that need to be
+    overwritten by the inheriting class.
+
+    Example usage:
+
+    .. code-block:: python
+
+        # Attach a frame provider to a thread
+        thread = process.GetSelectedThread()
+        error = lldb.SBError()
+        thread.SetScriptedFrameProvider(
+            "my_module.MyFrameProvider",
+            lldb.SBStructuredData()
+        )
+    """
+
+    @abstractmethod
+    def __init__(self, thread, args):
+        """Construct a scripted frame provider.
+
+        Args:
+            thread (lldb.SBThread): The thread for which to provide frames.
+            args (lldb.SBStructuredData): A Dictionary holding arbitrary
+                key/value pairs used by the scripted frame provider.
+        """
+        self.thread = None
+        self.args = None
+        self.target = None
+        self.process = None
+
+        if isinstance(thread, lldb.SBThread) and thread.IsValid():
+            self.thread = thread
+            self.process = thread.GetProcess()
+            if self.process and self.process.IsValid():
+                self.target = self.process.GetTarget()
+
+        if isinstance(args, lldb.SBStructuredData) and args.IsValid():
+            self.args = args
+
+    def get_merge_strategy(self):
+        """Get the merge strategy for how scripted frames should be integrated.
+
+        The merge strategy determines how the scripted frames are combined with the
+        real unwound frames from the thread's normal unwinder.
+
+        Returns:
+            int: One of the following lldb.ScriptedFrameProviderMergeStrategy values:
+
+            - lldb.eScriptedFrameProviderMergeStrategyReplace: Replace the entire stack
+              with scripted frames. The thread will only show frames provided
+              by this provider.
+
+            - lldb.eScriptedFrameProviderMergeStrategyPrepend: Prepend scripted frames
+              before the real unwound frames. Useful for adding synthetic frames
+              at the top of the stack while preserving the actual callstack below.
+
+            - lldb.eScriptedFrameProviderMergeStrategyAppend: Append scripted frames
+              after the real unwound frames. Useful for showing additional context
+              after the actual callstack ends.
+
+            - lldb.eScriptedFrameProviderMergeStrategyReplaceByIndex: Replace specific
+              frames at given indices with scripted frames, keeping other real frames
+              intact. The idx field in each frame dictionary determines which real
+              frame to replace (e.g., idx=0 replaces frame 0, idx=2 replaces frame 2).
+
+        The default implementation returns Replace strategy.
+
+        Example:
+
+        .. code-block:: python
+
+            def get_merge_strategy(self):
+                # Only show our custom frames
+                return lldb.eScriptedFrameProviderMergeStrategyReplace
+
+            def get_merge_strategy(self):
+                # Add diagnostic frames on top of real stack
+                return lldb.eScriptedFrameProviderMergeStrategyPrepend
+
+            def get_merge_strategy(self):
+                # Replace frame 0 and frame 2 with custom frames, keep others
+                return lldb.eScriptedFrameProviderMergeStrategyReplaceByIndex
+        """
+        return lldb.eScriptedFrameProviderMergeStrategyReplace
+
+    @abstractmethod
+    def get_stackframes(self):
+        """Get the list of stack frames to provide.
+
+        This method is called when the thread's backtrace is requested
+        (e.g., via the 'bt' command). The returned frames will be integrated
+        with the real frames according to the mode returned by get_mode().
+
+        Returns:
+            List[Dict]: A list of frame dictionaries, where each dictionary
+                describes a single stack frame. Each dictionary should contain:
+
+            Required fields:
+            - idx (int): The frame index (0 for innermost/top frame)
+            - pc (int): The program counter address for this frame
+
+            Alternatively, you can return a list of ScriptedFrame objects
+            for more control over frame behavior.
+
+        Example:
+
+        .. code-block:: python
+
+            def get_stackframes(self):
+                frames = []
+
+                # Frame 0: Current function
+                frames.append({
+                    "idx": 0,
+                    "pc": 0x100001234,
+                })
+
+                # Frame 1: Caller
+                frames.append({
+                    "idx": 1,
+                    "pc": 0x100001000,
+                })
+
+                return frames
+
+        Note:
+            The frames are indexed from 0 (innermost/newest) to N (outermost/oldest).
+        """
+        pass

lldb/examples/python/templates/scripted_process.py

@@ -245,6 +245,7 @@ def __init__(self, process, args):
                 key/value pairs used by the scripted thread.
         """
         self.target = None
+        self.arch = None
         self.originating_process = None
         self.process = None
         self.args = None
@@ -266,6 +267,9 @@ def __init__(self, process, args):
             and process.IsValid()
         ):
             self.target = process.target
+            triple = self.target.triple
+            if triple:
+                self.arch = triple.split("-")[0]
             self.originating_process = process
             self.process = self.target.GetProcess()
             self.get_register_info()
@@ -352,17 +356,14 @@ def get_stackframes(self):
     def get_register_info(self):
         if self.register_info is None:
             self.register_info = dict()
-            if "x86_64" in self.originating_process.arch:
+            if "x86_64" in self.arch:
                 self.register_info["sets"] = ["General Purpose Registers"]
                 self.register_info["registers"] = INTEL64_GPR
-            elif (
-                "arm64" in self.originating_process.arch
-                or self.originating_process.arch == "aarch64"
-            ):
+            elif "arm64" in self.arch or self.arch == "aarch64":
                 self.register_info["sets"] = ["General Purpose Registers"]
                 self.register_info["registers"] = ARM64_GPR
             else:
-                raise ValueError("Unknown architecture", self.originating_process.arch)
+                raise ValueError("Unknown architecture", self.arch)
         return self.register_info
 
     @abstractmethod
@@ -405,11 +406,12 @@ def __init__(self, thread, args):
         """Construct a scripted frame.
 
         Args:
-            thread (ScriptedThread): The thread owning this frame.
+            thread (ScriptedThread/lldb.SBThread): The thread owning this frame.
             args (lldb.SBStructuredData): A Dictionary holding arbitrary
                 key/value pairs used by the scripted frame.
         """
         self.target = None
+        self.arch = None
         self.originating_thread = None
         self.thread = None
         self.args = None
@@ -424,10 +426,14 @@ def __init__(self, thread, args):
             or isinstance(thread, lldb.SBThread)
             and thread.IsValid()
         ):
-            self.target = thread.target
             self.process = thread.process
+            self.target = self.process.target
+            triple = self.target.triple
+            if triple:
+                self.arch = triple.split("-")[0]
+            tid = thread.tid if isinstance(thread, ScriptedThread) else thread.id
             self.originating_thread = thread
-            self.thread = self.process.GetThreadByIndexID(thread.tid)
+            self.thread = self.process.GetThreadByIndexID(tid)
             self.get_register_info()
 
     @abstractmethod
@@ -508,7 +514,18 @@ def get_variables(self, filters):
 
     def get_register_info(self):
         if self.register_info is None:
-            self.register_info = self.originating_thread.get_register_info()
+            if isinstance(self.originating_thread, ScriptedThread):
+                self.register_info = self.originating_thread.get_register_info()
+            elif isinstance(self.originating_thread, lldb.SBThread):
+                self.register_info = dict()
+                if "x86_64" in self.arch:
+                    self.register_info["sets"] = ["General Purpose Registers"]
+                    self.register_info["registers"] = INTEL64_GPR
+                elif "arm64" in self.arch or self.arch == "aarch64":
+                    self.register_info["sets"] = ["General Purpose Registers"]
+                    self.register_info["registers"] = ARM64_GPR
+                else:
+                    raise ValueError("Unknown architecture", self.arch)
         return self.register_info
 
     @abstractmethod
@@ -642,12 +659,12 @@ def get_stop_reason(self):
 
             # TODO: Passthrough stop reason from driving process
             if self.driving_thread.GetStopReason() != lldb.eStopReasonNone:
-                if "arm64" in self.originating_process.arch:
+                if "arm64" in self.arch:
                     stop_reason["type"] = lldb.eStopReasonException
                     stop_reason["data"]["desc"] = (
                         self.driving_thread.GetStopDescription(100)
                     )
-                elif self.originating_process.arch == "x86_64":
+                elif self.arch == "x86_64":
                     stop_reason["type"] = lldb.eStopReasonSignal
                     stop_reason["data"]["signal"] = signal.SIGTRAP
                 else:

lldb/include/lldb/API/SBThread.h

@@ -229,6 +229,11 @@ class LLDB_API SBThread {
 
   SBValue GetSiginfo();
 
+  SBError RegisterFrameProvider(const char *class_name,
+                                SBStructuredData &args_data);
+
+  void ClearScriptedFrameProvider();
+
 private:
   friend class SBBreakpoint;
   friend class SBBreakpointLocation;

lldb/include/lldb/Interpreter/Interfaces/ScriptedFrameProviderInterface.h

@@ -0,0 +1,36 @@
+//===-- ScriptedFrameProviderInterface.h ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_INTERPRETER_INTERFACES_SCRIPTEDFRAMEPROVIDERINTERFACE_H
+#define LLDB_INTERPRETER_INTERFACES_SCRIPTEDFRAMEPROVIDERINTERFACE_H
+
+#include "lldb/lldb-private.h"
+
+#include "ScriptedInterface.h"
+
+namespace lldb_private {
+class ScriptedFrameProviderInterface : public ScriptedInterface {
+public:
+  virtual llvm::Expected<StructuredData::GenericSP>
+  CreatePluginObject(llvm::StringRef class_name, lldb::ThreadSP thread_sp,
+                     StructuredData::DictionarySP args_sp) = 0;
+
+  /// Get the merge strategy for how scripted frames should be integrated with
+  /// real frames
+  virtual lldb::ScriptedFrameProviderMergeStrategy GetMergeStrategy() {
+    return lldb::eScriptedFrameProviderMergeStrategyReplace;
+  }
+
+  virtual StructuredData::ArraySP
+  GetStackFrames(lldb::StackFrameListSP real_frames) {
+    return {};
+  }
+};
+} // namespace lldb_private
+
+#endif // LLDB_INTERPRETER_INTERFACES_SCRIPTEDFRAMEPROVIDERINTERFACE_H

lldb/include/lldb/Interpreter/ScriptInterpreter.h

@@ -27,6 +27,7 @@
 #include "lldb/Host/StreamFile.h"
 #include "lldb/Interpreter/Interfaces/OperatingSystemInterface.h"
 #include "lldb/Interpreter/Interfaces/ScriptedFrameInterface.h"
+#include "lldb/Interpreter/Interfaces/ScriptedFrameProviderInterface.h"
 #include "lldb/Interpreter/Interfaces/ScriptedPlatformInterface.h"
 #include "lldb/Interpreter/Interfaces/ScriptedProcessInterface.h"
 #include "lldb/Interpreter/Interfaces/ScriptedThreadInterface.h"
@@ -536,6 +537,11 @@ class ScriptInterpreter : public PluginInterface {
     return {};
   }
 
+  virtual lldb::ScriptedFrameProviderInterfaceSP
+  CreateScriptedFrameProviderInterface() {
+    return {};
+  }
+
   virtual lldb::ScriptedThreadPlanInterfaceSP
   CreateScriptedThreadPlanInterface() {
     return {};