[lldb][Target] RunThreadPlan to save/restore the ExecutionContext's frame if one exists

lldb/source/Target/Process.cpp

@@ -5080,7 +5080,13 @@ Process::RunThreadPlan(ExecutionContext &exe_ctx,
     return eExpressionSetupError;
   }
 
-  StackID ctx_frame_id = selected_frame_sp->GetStackID();
+  // If the ExecutionContext has a frame, we want to make sure to save/restore
+  // that frame into exe_ctx. This can happen when we run expressions from a
+  // non-selected SBFrame, in which case we don't want some thread-plan
+  // to overwrite the ExecutionContext frame.
+  StackID ctx_frame_id = exe_ctx.HasFrameScope()
+                             ? exe_ctx.GetFrameRef().GetStackID()
+                             : selected_frame_sp->GetStackID();
 
   // N.B. Running the target may unset the currently selected thread and frame.
   // We don't want to do that either, so we should arrange to reset them as

lldb/test/API/commands/expression/expr-from-non-zero-frame/Makefile

@@ -0,0 +1,3 @@
+C_SOURCES := main.c
+
+include Makefile.rules

lldb/test/API/commands/expression/expr-from-non-zero-frame/TestExprFromNonZeroFrame.py

@@ -0,0 +1,30 @@
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class ExprFromNonZeroFrame(TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
+
+    def test(self):
+        """
+        Tests that we can use SBFrame::EvaluateExpression on a frame
+        that we're not stopped in, even if thread-plans run as part of
+        parsing the expression (e.g., when running static initializers).
+        """
+        self.build()
+
+        (_, _, thread, _) = lldbutil.run_to_source_breakpoint(
+            self, "Break here", lldb.SBFileSpec("main.c")
+        )
+        frame = thread.GetFrameAtIndex(1)
+
+        # Using a function pointer inside the expression ensures we
+        # emit a ptrauth static initializer on arm64e into the JITted
+        # expression. The thread-plan that runs for this static
+        # initializer should save/restore the current execution context
+        # frame (which in this test is frame #1).
+        result = frame.EvaluateExpression("int (*fptr)() = &func; fptr()")
+        self.assertTrue(result.GetError().Success())
+        self.assertEqual(result.GetValueAsSigned(), 5)

lldb/test/API/commands/expression/expr-from-non-zero-frame/main.c

@@ -0,0 +1,6 @@
+int func(void) {
+  __builtin_printf("Break here");
+  return 5;
+}
+
+int main(int argc, const char *argv[]) { return func(); }