[lldb] Upgrade `GetIndexOfChildWithName` to use `llvm::Expected` (#136693)

This patch replaces the use of `UINT32_MAX` as the error return value of
`GetIndexOfChildWithName` with `llvm::Expected`.


# Tasks to do in another PR

1. Replace `CalculateNumChildrenIgnoringErrors` with
`CalculateNumChildren`. See [this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2056319358).
2. Update `lldb_private::formatters::ExtractIndexFromString` to use
`llvm::Expected`. See [this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2054217536).
3. Create a new class which carries both user and internal errors. See
[this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2056439608).

2 files changed, 10 insertions, 7 deletions

diff --git a/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp b/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp
index 553ee7e..16c3226 100644
--- a/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp
+++ b/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp
@@ -2037,19 +2037,19 @@ lldb::ValueObjectSP ScriptInterpreterPythonImpl::GetChildAtIndex(
   return ret_val;
 }
 
-int ScriptInterpreterPythonImpl::GetIndexOfChildWithName(
+llvm::Expected<int> ScriptInterpreterPythonImpl::GetIndexOfChildWithName(
     const StructuredData::ObjectSP &implementor_sp, const char *child_name) {
   if (!implementor_sp)
-    return UINT32_MAX;
+    return llvm::createStringError("Type has no child named '%s'", child_name);
 
   StructuredData::Generic *generic = implementor_sp->GetAsGeneric();
   if (!generic)
-    return UINT32_MAX;
+    return llvm::createStringError("Type has no child named '%s'", child_name);
   auto *implementor = static_cast<PyObject *>(generic->GetValue());
   if (!implementor)
-    return UINT32_MAX;
+    return llvm::createStringError("Type has no child named '%s'", child_name);
 
-  int ret_val = UINT32_MAX;
+  int ret_val = INT32_MAX;
 
   {
     Locker py_lock(this,
@@ -2058,6 +2058,8 @@ int ScriptInterpreterPythonImpl::GetIndexOfChildWithName(
                                                                  child_name);
   }
 
+  if (ret_val == INT32_MAX)
+    return llvm::createStringError("Type has no child named '%s'", child_name);
   return ret_val;
 }
 
diff --git a/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h b/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h
index 0f290281..5d77608 100644
--- a/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h
+++ b/lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPythonImpl.h
@@ -128,8 +128,9 @@ public:
   GetChildAtIndex(const StructuredData::ObjectSP &implementor,
                   uint32_t idx) override;
 
-  int GetIndexOfChildWithName(const StructuredData::ObjectSP &implementor,
-                              const char *child_name) override;
+  llvm::Expected<int>
+  GetIndexOfChildWithName(const StructuredData::ObjectSP &implementor,
+                          const char *child_name) override;
 
   bool UpdateSynthProviderInstance(
       const StructuredData::ObjectSP &implementor) override;