[LLDB] Fix for libc++ atomic allowing modification of contained value

[clayborg]

Sorry for delay, I was on recharge for all of December.

lldb/source/Plugins/Language/CPlusPlus/LibCxxAtomic.cpp

lldb_private::formatters::LibcxxStdAtomicSyntheticFrontEnd::GetChildAtIndex(
     size_t idx) {
   if (idx == 0)
     return m_real_child->GetSP()->Clone(ConstString("Value"));
   return nullptr;
 }
 
 size_t lldb_private::formatters::LibcxxStdAtomicSyntheticFrontEnd::
     GetIndexOfChildWithName(ConstString name) {
-  return formatters::ExtractIndexFromString(name.GetCString());
+  return name == "Value" ? 0 : UINT32_MAX;
 }
 
 SyntheticChildrenFrontEnd *
 lldb_private::formatters::LibcxxAtomicSyntheticFrontEndCreator(
     CXXSyntheticChildren *, lldb::ValueObjectSP valobj_sp) {
   if (valobj_sp)
     return new LibcxxStdAtomicSyntheticFrontEnd(valobj_sp);
   return nullptr;
 }

lldb/test/API/python_api/value/change_values/libcxx/atomic/Makefile

+CXX_SOURCES := main.cpp
+
+USE_LIBCPP := 1
+
+CXXFLAGS_EXTRAS := -O0
+include Makefile.rules

lldb/test/API/python_api/value/change_values/libcxx/atomic/TestChangeValue.py

+"""
+Test change libc++ std::atomic values.
+"""
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+class LibcxxChangeValueTestCase(TestBase):
+
+    def setUp(self):
+        # Call super's setUp().
+        TestBase.setUp(self)
+
+    @add_test_categories(["libc++"])
+    @expectedFailureAll(oslist=["windows"], bugnumber="llvm.org/pr24772")
+    def test(self):
+        """Test that we can change values of libc++ std::atomic."""
+        self.build()
+        self.runCmd("file " + self.getBuildArtifact("a.out"), CURRENT_EXECUTABLE_SET)
+
+        bkpt = self.target().FindBreakpointByID(
+            lldbutil.run_break_set_by_source_regexp(
+                self, "Set break point at this line."))
+
+        self.runCmd("run", RUN_SUCCEEDED)
+
+        # Get Frame #0.
+        target = self.dbg.GetSelectedTarget()
+        process = target.GetProcess()
+        self.assertState(process.GetState(), lldb.eStateStopped)
+        thread = lldbutil.get_stopped_thread(
+            process, lldb.eStopReasonBreakpoint)
+        self.assertTrue(
+            thread.IsValid(),
+            "There should be a thread stopped due to breakpoint condition")
+        frame0 = thread.GetFrameAtIndex(0)
+        self.assertTrue(frame0.IsValid(), "Got a valid frame.")
+
+        q_value = frame0.FindVariable("Q")
+        self.assertTrue(q_value.IsValid(), "Got the SBValue for val")
+        inner_val = q_value.GetChildAtIndex(0)
+        self.assertTrue(inner_val.IsValid(), "Got the SBValue for inner atomic val")
+        result = inner_val.SetValueFromCString("42")
+        self.assertTrue(result, "Setting val returned True.")
+        result = inner_val.GetValueAsUnsigned()
+        self.assertTrue(result == 42, "Got correct value (42)")

lldb/test/API/python_api/value/change_values/libcxx/atomic/main.cpp

+#include <atomic>
+
+int main()
+{
+    std::atomic<int> Q(1);
+    return Q; // Set break point at this line.
+}