refactor(appsec): move _stacktrace from _iast to _shared package (#17329)

## Summary
- Move the native `_stacktrace` C extension from `ddtrace/appsec/_iast/` to `ddtrace/appsec/_shared/` so it can be reused by both IAST and SCA without creating a dependency from SCA into IAST internals.
- Update all imports, build configuration (`setup.py`), and test references to use the new `ddtrace.appsec._shared._stacktrace` path.

Split out from #17156 to keep PRs incremental and reviewable.

> **Important:** Before merging this PR, DataDog/datadog-lambda-python#761 must be merged first, since `datadog-lambda-python` imports `ddtrace.appsec._iast._stacktrace` and needs to be updated to the new path.

## Test plan
- [ ] Existing IAST stacktrace tests pass (`tests/appsec/iast/test_stacktrace.py`)
- [ ] IAST memcheck tests pass (`tests/appsec/iast_memcheck/test_iast_mem_check.py`)
- [ ] Architecture loading module test passes (`tests/appsec/architectures/test_appsec_loading_modules.py`)
- [ ] Serverless import test passes (`tests/internal/test_serverless.py`)
- [ ] Native C extension builds correctly from new path

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: alberto.vara <alberto.vara@datadoghq.com>

Author: avara1986

ddtrace/appsec/_iast/__init__.py

@@ -199,6 +199,7 @@ def enable_iast_propagation():
         from ddtrace.appsec._iast._ast.ast_patching import _should_iast_patch
         from ddtrace.appsec._iast._loader import _exec_iast_patched_module
         from ddtrace.appsec._iast._taint_tracking import initialize_native_state
+        from ddtrace.appsec._shared._stacktrace import get_info_frame  # noqa: F401
 
         global _iast_propagation_enabled
         if _iast_propagation_enabled:

ddtrace/appsec/_iast/_stacktrace.pyi

@@ -15,6 +15,10 @@
 
 log = get_logger(__name__)
 
+# Lazy-cached reference to avoid loading _stacktrace (a C extension used only
+# by IAST) when _patch_utils is imported by non-IAST code paths.
+_get_info_frame = None
+
 # Cached paths for relativizing file paths (computed once at import time).
 _CWD = os.path.abspath(os.getcwd())
 _PURELIB_PATH = sysconfig.get_path("purelib") or ""
@@ -52,12 +56,12 @@ def get_caller_frame_info() -> tuple:
 
     Returns (None, None, None, None) when no relevant frame is found.
     """
-    try:
-        from ddtrace.appsec._iast._stacktrace import get_info_frame
-    except ImportError:
-        return None, None, None, None
+    global _get_info_frame
+    if _get_info_frame is None:
+        from ddtrace.appsec._shared._stacktrace import get_info_frame
 
-    frame_info = get_info_frame()
+        _get_info_frame = get_info_frame
+    frame_info = _get_info_frame()
     if not frame_info or frame_info[0] in ("", -1, None):
         return None, None, None, None
 

ddtrace/appsec/_patch_utils.py

@@ -359,7 +359,7 @@ static PyMethodDef StacktraceMethods[] = { { "get_info_frame",
                                            { NULL, NULL, 0, NULL } };
 
 static struct PyModuleDef stacktrace = { PyModuleDef_HEAD_INIT,
-                                         "ddtrace.appsec._iast._stacktrace",
+                                         "ddtrace.appsec._shared._stacktrace",
                                          "stacktrace module",
                                          -1,
                                          StacktraceMethods };

ddtrace/appsec/_shared/__init__.py

@@ -0,0 +1,3 @@
+from typing import Optional
+
+def get_info_frame() -> tuple[Optional[str], Optional[int], Optional[str], Optional[str]]: ...

ddtrace/appsec/_iast/_stacktrace.c ddtrace/appsec/_shared/_stacktrace.cddtrace/appsec/_iast/_stacktrace.c renamed to ddtrace/appsec/_shared/_stacktrace.c

@@ -1275,9 +1275,9 @@ def get_exts_for(name):
     if platform.system() not in ("Windows", ""):
         ext_modules.append(
             Extension(
-                "ddtrace.appsec._iast._stacktrace",
+                "ddtrace.appsec._shared._stacktrace",
                 sources=[
-                    "ddtrace/appsec/_iast/_stacktrace.c",
+                    "ddtrace/appsec/_shared/_stacktrace.c",
                 ],
                 extra_compile_args=extra_compile_args + debug_compile_args + fast_build_args,
             )

ddtrace/appsec/_shared/_stacktrace.pyi

@@ -17,6 +17,7 @@
 MODULE_IAST_ONLY = [
     "ddtrace.appsec._iast",
     "ddtrace.appsec._iast._taint_tracking._native",
+    "ddtrace.appsec._shared._stacktrace",
 ]
 
 

setup.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 
-from ddtrace.appsec._iast._stacktrace import get_info_frame
+from ddtrace.appsec._shared._stacktrace import get_info_frame
 
 
 def test_stacktrace():

tests/appsec/architectures/test_appsec_loading_modules.py

@@ -1,6 +1,6 @@
 import os
 
-from ddtrace.appsec._iast._stacktrace import get_info_frame
+from ddtrace.appsec._shared._stacktrace import get_info_frame
 
 
 CWD = os.path.abspath(os.getcwd())