Clear stale last_profiled_frame values when initializing RemoteUnwinder

pablogsal · claude · pablogsal · commit 180daaa82672 · 2025-12-01T15:08:36.000Z
When a new RemoteUnwinder is created to profile a process that was previously profiled by another unwinder, the last_profiled_frame values in thread states may be stale. This caused the new unwinder to stop frame walking early (when hitting the stale marker) but fail to find cached frames (since the cache is empty in the new unwinder), resulting in empty frame lists. Fix by clearing last_profiled_frame for all threads when a new RemoteUnwinder with cache_frames=True is initialized. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
diff --git a/Modules/_remote_debugging/_remote_debugging.h b/Modules/_remote_debugging/_remote_debugging.h
@@ -374,6 +374,7 @@ extern int process_frame_chain(
 /* Frame cache functions */
 extern int frame_cache_init(RemoteUnwinderObject *unwinder);
 extern void frame_cache_cleanup(RemoteUnwinderObject *unwinder);
+extern int clear_last_profiled_frames(RemoteUnwinderObject *unwinder);
 extern void frame_cache_invalidate_stale(RemoteUnwinderObject *unwinder, PyObject *result);
 extern int frame_cache_lookup_and_extend(
     RemoteUnwinderObject *unwinder,
diff --git a/Modules/_remote_debugging/frames.c b/Modules/_remote_debugging/frames.c
@@ -405,6 +405,63 @@ frame_cache_cleanup(RemoteUnwinderObject *unwinder)
     Py_CLEAR(unwinder->frame_cache);
 }
 
+// Clear last_profiled_frame for all threads in the target process.
+// This must be called at the start of profiling to avoid stale values
+// from previous profilers causing us to stop frame walking early.
+int
+clear_last_profiled_frames(RemoteUnwinderObject *unwinder)
+{
+    uintptr_t current_interp = unwinder->interpreter_addr;
+    uintptr_t zero = 0;
+
+    while (current_interp != 0) {
+        // Get first thread in this interpreter
+        uintptr_t tstate_addr;
+        if (_Py_RemoteDebug_PagedReadRemoteMemory(
+                &unwinder->handle,
+                current_interp + unwinder->debug_offsets.interpreter_state.threads_head,
+                sizeof(void*),
+                &tstate_addr) < 0) {
+            // Non-fatal: just skip clearing
+            PyErr_Clear();
+            return 0;
+        }
+
+        // Iterate all threads in this interpreter
+        while (tstate_addr != 0) {
+            // Clear last_profiled_frame
+            uintptr_t lpf_addr = tstate_addr + unwinder->debug_offsets.thread_state.last_profiled_frame;
+            if (_Py_RemoteDebug_WriteRemoteMemory(&unwinder->handle, lpf_addr,
+                                                  sizeof(uintptr_t), &zero) < 0) {
+                // Non-fatal: just continue
+                PyErr_Clear();
+            }
+
+            // Move to next thread
+            if (_Py_RemoteDebug_PagedReadRemoteMemory(
+                    &unwinder->handle,
+                    tstate_addr + unwinder->debug_offsets.thread_state.next,
+                    sizeof(void*),
+                    &tstate_addr) < 0) {
+                PyErr_Clear();
+                break;
+            }
+        }
+
+        // Move to next interpreter
+        if (_Py_RemoteDebug_PagedReadRemoteMemory(
+                &unwinder->handle,
+                current_interp + unwinder->debug_offsets.interpreter_state.next,
+                sizeof(void*),
+                &current_interp) < 0) {
+            PyErr_Clear();
+            break;
+        }
+    }
+
+    return 0;
+}
+
 // Remove cache entries for threads not seen in the result
 // result structure: list of InterpreterInfo, where InterpreterInfo[1] is threads list,
 // and ThreadInfo[0] is the thread_id
diff --git a/Modules/_remote_debugging/module.c b/Modules/_remote_debugging/module.c
@@ -386,6 +386,12 @@ _remote_debugging_RemoteUnwinder___init___impl(RemoteUnwinderObject *self,
         return -1;
     }
 
+    // Clear stale last_profiled_frame values from previous profilers
+    // This prevents us from stopping frame walking early due to stale values
+    if (cache_frames) {
+        clear_last_profiled_frames(self);
+    }
+
     return 0;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -386,6 +386,12 @@ _remote_debugging_RemoteUnwinder___init___impl(RemoteUnwinderObject *self,`
`386`	`386`	`return -1;`
`387`	`387`	`}`
`388`	`388`
	`389`	`+ // Clear stale last_profiled_frame values from previous profilers`
	`390`	`+ // This prevents us from stopping frame walking early due to stale values`
	`391`	`+ if (cache_frames) {`
	`392`	`+ clear_last_profiled_frames(self);`
	`393`	`+ }`
	`394`	`+`
`389`	`395`	`return 0;`
`390`	`396`	`}`
`391`	`397`