Report back errors in GetNumChildren()

This is a proof-of-concept patch that illustrates how to use the
Expected return values to surface rich error messages all the way up
to the ValueObjectPrinter.

Author: adrian-prantl

lldb/include/lldb/DataFormatters/ValueObjectPrinter.h

@@ -127,7 +127,7 @@ class ValueObjectPrinter {
   void PrintChild(lldb::ValueObjectSP child_sp,
                   const DumpValueObjectOptions::PointerDepth &curr_ptr_depth);
 
-  uint32_t GetMaxNumChildrenToPrint(bool &print_dotdotdot);
+  llvm::Expected<uint32_t> GetMaxNumChildrenToPrint(bool &print_dotdotdot);
 
   void
   PrintChildren(bool value_printed, bool summary_printed,

lldb/source/Core/ValueObjectVariable.cpp

@@ -99,7 +99,8 @@ ValueObjectVariable::CalculateNumChildren(uint32_t max) {
   CompilerType type(GetCompilerType());
 
   if (!type.IsValid())
-    return 0;
+    return llvm::make_error<llvm::StringError>("invalid type",
+                                               llvm::inconvertibleErrorCode());
 
   ExecutionContext exe_ctx(GetExecutionContextRef());
   const bool omit_empty_base_classes = true;

lldb/source/DataFormatters/ValueObjectPrinter.cpp

@@ -621,13 +621,17 @@ void ValueObjectPrinter::PrintChild(
   }
 }
 
-uint32_t ValueObjectPrinter::GetMaxNumChildrenToPrint(bool &print_dotdotdot) {
+llvm::Expected<uint32_t>
+ValueObjectPrinter::GetMaxNumChildrenToPrint(bool &print_dotdotdot) {
   ValueObject &synth_valobj = GetValueObjectForChildrenGeneration();
 
   if (m_options.m_pointer_as_array)
     return m_options.m_pointer_as_array.m_element_count;
 
-  uint32_t num_children = synth_valobj.GetNumChildrenIgnoringErrors();
+  auto num_children_or_err = synth_valobj.GetNumChildren();
+  if (!num_children_or_err)
+    return num_children_or_err;
+  uint32_t num_children = *num_children_or_err;
   print_dotdotdot = false;
   if (num_children) {
     const size_t max_num_children = GetMostSpecializedValue()
@@ -704,7 +708,12 @@ void ValueObjectPrinter::PrintChildren(
   ValueObject &synth_valobj = GetValueObjectForChildrenGeneration();
 
   bool print_dotdotdot = false;
-  size_t num_children = GetMaxNumChildrenToPrint(print_dotdotdot);
+  auto num_children_or_err = GetMaxNumChildrenToPrint(print_dotdotdot);
+  if (!num_children_or_err) {
+    *m_stream << " <" << llvm::toString(num_children_or_err.takeError()) << '>';
+    return;
+  }
+  uint32_t num_children = *num_children_or_err;
   if (num_children) {
     bool any_children_printed = false;
 
@@ -753,13 +762,19 @@ bool ValueObjectPrinter::PrintChildrenOneLiner(bool hide_names) {
   ValueObject &synth_valobj = GetValueObjectForChildrenGeneration();
 
   bool print_dotdotdot = false;
-  size_t num_children = GetMaxNumChildrenToPrint(print_dotdotdot);
+  auto num_children_or_err = GetMaxNumChildrenToPrint(print_dotdotdot);
+  if (!num_children_or_err) {
+    *m_stream << '<' << llvm::toString(num_children_or_err.takeError()) << '>';
+    return true;
+  }
+  uint32_t num_children = *num_children_or_err;
 
   if (num_children) {
     m_stream->PutChar('(');
 
     bool did_print_children = false;
-    for (uint32_t idx = 0; idx < num_children; ++idx) {
+    for (uint32_t idx = 0; idx < num_children; ++
+           idx) {
       lldb::ValueObjectSP child_sp(synth_valobj.GetChildAtIndex(idx));
       if (child_sp)
         child_sp = child_sp->GetQualifiedRepresentationIfAvailable(

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -5268,7 +5268,8 @@ TypeSystemClang::GetNumChildren(lldb::opaque_compiler_type_t type,
                                 bool omit_empty_base_classes,
                                 const ExecutionContext *exe_ctx) {
   if (!type)
-    return 0;
+    return llvm::make_error<llvm::StringError>("invalid clang type",
+                                               llvm::inconvertibleErrorCode());
 
   uint32_t num_children = 0;
   clang::QualType qual_type(RemoveWrappingTypes(GetQualType(type)));
@@ -5326,8 +5327,9 @@ TypeSystemClang::GetNumChildren(lldb::opaque_compiler_type_t type,
       num_children += std::distance(record_decl->field_begin(),
                                record_decl->field_end());
     }
-    break;
-
+    return llvm::make_error<llvm::StringError>(
+        "incomplete type \"" + GetDisplayTypeName(type).GetString() + "\"",
+        llvm::inconvertibleErrorCode());
   case clang::Type::ObjCObject:
   case clang::Type::ObjCInterface:
     if (GetCompleteQualType(&getASTContext(), qual_type)) {

lldb/source/Symbol/CompilerType.cpp

@@ -777,7 +777,8 @@ CompilerType::GetNumChildren(bool omit_empty_base_classes,
     if (auto type_system_sp = GetTypeSystem())
       return type_system_sp->GetNumChildren(m_type, omit_empty_base_classes,
                                        exe_ctx);
-  return 0;
+  return llvm::make_error<llvm::StringError>("invalid type",
+                                             llvm::inconvertibleErrorCode());
 }
 
 lldb::BasicType CompilerType::GetBasicTypeEnumeration() const {

lldb/test/API/functionalities/valobj_errors/Makefile

@@ -0,0 +1,9 @@
+C_SOURCES := main.c
+LD_EXTRAS = hidden.o
+
+a.out: hidden.o
+
+hidden.o: hidden.c
+	$(CC) -g0 -c -o $@ $<
+
+include Makefile.rules

lldb/test/API/functionalities/valobj_errors/TestValueObjectErrors.py

@@ -0,0 +1,15 @@
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class ValueObjectErrorsTestCase(TestBase):
+    def test(self):
+        """Test that the error message for a missing type
+           is visible when printing an object"""
+        self.build()
+        lldbutil.run_to_source_breakpoint(
+            self, "break here", lldb.SBFileSpec('main.c'))
+        self.expect('v -ptr-depth 1 x',
+                    substrs=['<incomplete type "Opaque">'])

lldb/test/API/functionalities/valobj_errors/hidden.c

@@ -0,0 +1,3 @@
+struct Opaque { int i, j, k; } *global;
+struct Opaque *getOpaque() { return &global; }
+  

lldb/test/API/functionalities/valobj_errors/main.c

@@ -0,0 +1,9 @@
+struct Opaque;
+struct Opaque *getOpaque();
+void puts(const char*);
+
+int main() {
+  struct Opaque *x = getOpaque();
+  puts("break here\n");
+  return (int)x;
+}