[Lldb-commits] [lldb] [lldb] Fix libstdc++ std::string formatter after #147835 (PR #152993)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lldb

Author: David Spickett (DavidSpickett)

<details>
<summary>Changes</summary>

https://github.com/llvm/llvm-project/pull/147835 made changes that caused libstdc++ std::string tests to fail:
```
======================================================================
FAIL: test_unavailable_summary_libstdcxx_dwo (TestDataFormatterStdString.StdStringDataFormatterTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1805, in test_method
    return attrvalue(self)
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/string/TestDataFormatterStdString.py", line 223, in test_unavailable_summary_libstdcxx
    self.do_test_summary_unavailable()
  File "/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/string/TestDataFormatterStdString.py", line 213, in do_test_summary_unavailable
    self.assertEqual(summary, "Summary Unavailable", "No summary for bad value")
AssertionError: '(null)' != 'Summary Unavailable'
- (null)
+ Summary Unavailable
 : No summary for bad value
Config=aarch64-/home/tcwg-buildbot/worker/lldb-aarch64-ubuntu/build/bin/clang
----------------------------------------------------------------------
```

This test constructs an invalid std::string by starting with a null pointer.

Somehow this improvement in Clang uncovered a bug in the formatter. The formatter tries to access `_M_p` and checked whether the resulting ValueObjectSP was null, but not that it did not contain an error value.

I think that error value can be there if you are able to access one part of the path, `_M_dataplus`, but another part fails. Since the layout looks like this:
```
      struct _Alloc_hider :
      {
	      pointer _M_p; // The actual data.
      };

      _Alloc_hider	_M_dataplus;

      void
      _M_data(pointer __p)
      { _M_dataplus._M_p = __p; }
```
So I think we were able to read `_M_dataplus` just by offset, but then failed because it contains, or points to something containing nulls or a null pointer.

Or perhaps an error value means we know what the class member is, but could not read from it.

I found this by comparing with the libcxx formatter, so I've copied the same handling from there to fix the issue.

---
Full diff: https://github.com/llvm/llvm-project/pull/152993.diff


1 Files Affected:

- (modified) lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp (+4-3) 


``````````diff
diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
index 595e835b37df9..f4a695e036999 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
@@ -241,10 +241,11 @@ VectorIteratorSyntheticFrontEnd::GetIndexOfChildWithName(ConstString name) {
 bool lldb_private::formatters::LibStdcppStringSummaryProvider(
     ValueObject &valobj, Stream &stream, const TypeSummaryOptions &options) {
   ValueObjectSP ptr = valobj.GetChildAtNamePath({"_M_dataplus", "_M_p"});
-  if (!ptr)
-    return false;
+  if (!ptr || !ptr->GetError().Success())
+    stream << "Summary Unavailable";
+  else
+    stream << ptr->GetSummaryAsCString();
 
-  stream << ptr->GetSummaryAsCString();
   return true;
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/152993