[LLDB][Value] Require type size when reading a scalar (#153386)

When reading a value as a scalar, the type size is required. It's
returned as a `std::optional`. This optional isn't checked for scalar
values, where it is unconditionally accessed.

This came up in the
[Shell/Process/Windows/msstl_smoke.cpp](https://github.com/llvm/llvm-project/blob/4e10b62442e9edf1769b98406b0559f515d9791f/lldb/test/Shell/Process/Windows/msstl_smoke.cpp)
test. There, LLDB breaks at the function entry, so all locals aren't
initialized yet. Most values will contain garbage. The [`std::list`
synthetic
provider](https://github.com/llvm/llvm-project/blob/4e10b62442e9edf1769b98406b0559f515d9791f/lldb/source/Plugins/Language/CPlusPlus/GenericList.cpp#L517)
tries to read the value using `GetData`. However, in
[`ValueObject::GetData`](https://github.com/llvm/llvm-project/blob/4e10b62442e9edf1769b98406b0559f515d9791f/lldb/source/ValueObject/ValueObject.cpp#L766),
[`ValueObjectChild::UpdateValue`](https://github.com/llvm/llvm-project/blob/88c993fbc5b87030b082aeb99d4db94cc885ed1d/lldb/source/ValueObject/ValueObjectChild.cpp#L102)
fails because the parent already failed to read its data, so `m_value`
won't have a compiler type, thus the size can't be read.

Author: Nerixyz

lldb/source/Core/Value.cpp

@@ -347,6 +347,9 @@ Status Value::GetValueAsData(ExecutionContext *exe_ctx, DataExtractor &data,
     else
       data.SetAddressByteSize(sizeof(void *));
 
+    if (!type_size)
+      return Status::FromErrorString("type does not have a size");
+
     uint32_t result_byte_size = *type_size;
     if (m_value.GetData(data, result_byte_size))
       return error; // Success;

lldb/unittests/Core/CMakeLists.txt

@@ -15,6 +15,7 @@ add_lldb_unittest(LLDBCoreTests
   SourceManagerTest.cpp
   TelemetryTest.cpp
   UniqueCStringMapTest.cpp
+  Value.cpp
 
   LINK_COMPONENTS
     Support

lldb/unittests/Core/Value.cpp

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/Core/Value.h"
+#include "Plugins/Platform/MacOSX/PlatformMacOSX.h"
+#include "Plugins/TypeSystem/Clang/TypeSystemClang.h"
+#include "TestingSupport/SubsystemRAII.h"
+#include "TestingSupport/Symbol/ClangTestUtils.h"
+
+#include "lldb/Utility/DataExtractor.h"
+
+#include "gtest/gtest.h"
+
+using namespace lldb_private;
+using namespace lldb_private::clang_utils;
+
+TEST(ValueTest, GetValueAsData) {
+  SubsystemRAII<FileSystem, HostInfo, PlatformMacOSX> subsystems;
+  auto holder = std::make_unique<clang_utils::TypeSystemClangHolder>("test");
+  auto *clang = holder->GetAST();
+
+  Value v(Scalar(42));
+  DataExtractor extractor;
+
+  // no compiler type
+  Status status = v.GetValueAsData(nullptr, extractor, nullptr);
+  ASSERT_TRUE(status.Fail());
+
+  // with compiler type
+  v.SetCompilerType(clang->GetBasicType(lldb::BasicType::eBasicTypeChar));
+
+  status = v.GetValueAsData(nullptr, extractor, nullptr);
+  ASSERT_TRUE(status.Success());
+}