Fix a crasher in StackFrame::GetValueForVariableExpressionPath()

[clayborg]

Summary

There was a crash that would happen if an IDE would ask for a child of a shared pointer via any SB API call that ends up calling StackFrame::GetValueForVariableExpressionPath(). The previous code expects an error to be set describing why the synthetic child of a type was not able to be found, but we have some synthetic child providers that weren't setting the error and returning an empty value object shared pointer. This fixes that to ensure we don't lose our debug session by crashing, fully tests GetValueForVariableExpressionPath functionality, and ensures we don't crash on GetValueForVariableExpressionPath() in the future.

[aprantl]

packages/Python/lldbsuite/test/functionalities/var_path/TestVarPath.py

70

If I'm not mistaken, you might be able to replace the first 40 lines or so of this test with
lldbutil.run_to_source_breakpoint(self, "// Set a break point here", lldb.SBFileSpec("main.cpp"))

[aprantl]

packages/Python/lldbsuite/test/functionalities/var_path/TestVarPath.py

102

until here. run_to_source_breakpoint should always return a valid frame or fail.

[clayborg]

Adrian's suggestion was right, much simpler test case now!

[zturner]

source/Target/StackFrame.cpp

644–650

Not your code, but incidentally this looks wrong. formatv uses python style format strings where you don't specify the type code but instead specify something like {0}. So either the function is incorrectly named or the call is incorrect.

655–656

I think you can' just write this as SetErrorString, since there is no format happening.

packages/Python/lldbsuite/test/functionalities/var_path/Makefile

+LEVEL = ../../make
+
+CXX_SOURCES := main.cpp
+
+include $(LEVEL)/Makefile.rules

packages/Python/lldbsuite/test/functionalities/var_path/TestVarPath.py

+"""
+Make sure the getting a variable path works and doesn't crash.
+"""
+
+from __future__ import print_function
+
+
+import os
+import time
+import re
+import lldb
+import lldbsuite.test.lldbutil as lldbutil
+from lldbsuite.test.lldbtest import *
+
+
+class TestVarPath(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    # If your test case doesn't stress debug info, the
+    # set this to true.  That way it won't be run once for
+    # each debug info format.
+    NO_DEBUG_INFO_TESTCASE = True
+
+    def test_frame_var(self):
+        self.build()
+        self.do_test()
+
+    def setUp(self):
+        # Call super's setUp().
+        TestBase.setUp(self)
+
+    def verify_point(self, frame, var_name, var_typename, x_value, y_value):
+        v = frame.GetValueForVariablePath(var_name)
+        self.assertTrue(v.GetError().Success(), "Make sure we find '%s'" % (var_name))
+        self.assertTrue(v.GetType().GetName() == var_typename, 
+                        "Make sure '%s' has type '%s'" % (var_name, var_typename))
+
+        if '*' in var_typename:
+            valid_prefix = var_name + '->'
+            invalid_prefix = var_name + '.'
+        else:
+            valid_prefix = var_name + '.'
+            invalid_prefix = var_name + '->'
+
+        valid_x_path = valid_prefix + 'x'
+        valid_y_path = valid_prefix + 'y'
+        invalid_x_path = invalid_prefix + 'x'
+        invalid_y_path = invalid_prefix + 'y'
+        invalid_m_path = invalid_prefix + 'm'
+
+        v = frame.GetValueForVariablePath(valid_x_path)
+        self.assertTrue(v.GetError().Success(), "Make sure we find '%s'" % (valid_x_path))
+        self.assertTrue(v.GetValue() == str(x_value), "Make sure '%s' has a value of %i" % (valid_x_path, x_value))
+        self.assertTrue(v.GetType().GetName() == "int", "Make sure '%s' has type 'int'" % (valid_x_path))
+        v = frame.GetValueForVariablePath(invalid_x_path)
+        self.assertTrue(v.GetError().Fail(), "Make sure we don't find '%s'" % (invalid_x_path))
+
+        v = frame.GetValueForVariablePath(valid_y_path)
+        self.assertTrue(v.GetError().Success(), "Make sure we find '%s'" % (valid_y_path))
+        self.assertTrue(v.GetValue() == str(y_value), "Make sure '%s' has a value of %i" % (valid_y_path, y_value))
+        self.assertTrue(v.GetType().GetName() == "int", "Make sure '%s' has type 'int'" % (valid_y_path))
+        v = frame.GetValueForVariablePath(invalid_y_path)
+        self.assertTrue(v.GetError().Fail(), "Make sure we don't find '%s'" % (invalid_y_path))
+
+        v = frame.GetValueForVariablePath(invalid_m_path)
+        self.assertTrue(v.GetError().Fail(), "Make sure we don't find '%s'" % (invalid_m_path))
+
+    def do_test(self):
+        (target, process, thread, bkpt) = lldbutil.run_to_source_breakpoint(
+            self, "// Set a breakpoint here", lldb.SBFileSpec("main.cpp"))
+
+        frame = thread.GetFrameAtIndex(0)
+        v = frame.GetValueForVariablePath('no_such_variable')
+        self.assertTrue(v.GetError().Fail(), "Make sure we don't find 'no_such_variable'")
+
+        # Test an instance
+        self.verify_point(frame, 'pt', 'Point', 1, 2)
+        # Test a pointer
+        self.verify_point(frame, 'pt_ptr', 'Point *', 3030, 4040)
+        # Test using a pointer as an array
+        self.verify_point(frame, 'pt_ptr[-1]', 'Point', 1010, 2020)
+        self.verify_point(frame, 'pt_ptr[0]', 'Point', 3030, 4040)
+        self.verify_point(frame, 'pt_ptr[1]', 'Point', 5050, 6060)
+        # Test arrays
+        v = frame.GetValueForVariablePath('points')
+        self.assertTrue(v.GetError().Success(), 
+                        "Make sure we find 'points'")
+        self.verify_point(frame, 'points[0]', 'Point', 1010, 2020)
+        self.verify_point(frame, 'points[1]', 'Point', 3030, 4040)
+        self.verify_point(frame, 'points[2]', 'Point', 5050, 6060)
+        # Test a reference
+        self.verify_point(frame, 'pt_ref', 'Point &', 1, 2)
+        v = frame.GetValueForVariablePath('pt_sp')
+        self.assertTrue(v.GetError().Success(), "Make sure we find 'pt_sp'")
+        # Make sure we don't crash when looking for non existant child
+        # in type with synthetic children. This used to cause a crash.
+        v = frame.GetValueForVariablePath('pt_sp->not_valid_child')
+        self.assertTrue(v.GetError().Fail(), 
+                        "Make sure we don't find 'pt_sp->not_valid_child'")
+
+
+

packages/Python/lldbsuite/test/functionalities/var_path/main.cpp

+#include <memory>
+
+struct Point {
+  int x, y;
+};
+
+int main() {
+  Point pt = { 1, 2 };
+  Point points[] = {{1010,2020}, {3030,4040}, {5050,6060}};
+  Point *pt_ptr = &points[1];
+  Point &pt_ref = pt;
+  std::shared_ptr<Point> pt_sp(new Point{111,222});
+  return 0; // Set a breakpoint here
+}
+

source/Target/StackFrame.cpp

case '-':
             error.SetErrorStringWithFormatv(
                 "Failed to dereference reference type: %s", deref_error);
             return ValueObjectSP();
           }
         }
 
         valobj_sp = valobj_sp->Dereference(deref_error);
         if (error.Fail()) {
           error.SetErrorStringWithFormatv(
-              "Failed to dereference sythetic value: %s", deref_error);
+              "Failed to dereference sythetic value: {0}", deref_error);
+          return ValueObjectSP();
+        }
+        // Some synthetic plug-ins fail to set the error in Dereference
+        if (!valobj_sp) {
+          error.SetErrorString("Failed to dereference sythetic value");
           return ValueObjectSP();
         }
         expr_is_ptr = false;
       }
 
       var_expr = var_expr.drop_front(); // Remove the '-'
       LLVM_FALLTHROUGH;
     case '.': {
       var_expr = var_expr.drop_front(); // Remove the '.' or '>'
       separator_idx = var_expr.find_first_of(".-[");
       ConstString child_name(var_expr.substr(0, var_expr.find_first_of(".-[")));
 
       if (check_ptr_vs_member) {
         // We either have a pointer type and need to verify valobj_sp is a