Detect against invalid variant index for LibStdC++ std::variant data formatters

[jeffreytan81]

https://github.com/llvm/llvm-project/pull/68012/files added new data formatters for LibStdC++ std::variant.

However, this formatter can crash if std::variant's index field has invalid value (exceeds the number of template arguments).
This can happen if the current IP stops at a place std::variant is not initialized yet.

This patch fixes the crash by ensuring the index is a valid value.

[llvmbot]

@llvm/pr-subscribers-lldb

Author: None (jeffreytan81)

Changes

https://github.com/llvm/llvm-project/pull/68012/files added new data formatters for LibStdC++ std::variant.

However, this formatter can crash if std::variant's index field has invalid value (exceeds the number of template arguments).
This can happen if the current IP stops at a place std::variant is not initialized yet.

This patch fixes the crash by ensuring the index is a valid value.

---

Full diff: https://github.com/llvm/llvm-project/pull/69253.diff

1 Files Affected:

• (modified) lldb/examples/synthetic/gnu_libstdcpp.py (+5)

diff --git a/lldb/examples/synthetic/gnu_libstdcpp.py b/lldb/examples/synthetic/gnu_libstdcpp.py
index 29c926167fb440c..f778065aaca3771 100644
--- a/lldb/examples/synthetic/gnu_libstdcpp.py
+++ b/lldb/examples/synthetic/gnu_libstdcpp.py
@@ -914,6 +914,11 @@ def get_variant_npos_value(index_byte_size):
     if index == npos_value:
         return " No Value"
 
+    # Invalid index can happen when the variant is not initialized yet.
+    template_arg_count = data_obj.GetType().GetNumberOfTemplateArguments()
+    if index >= template_arg_count:
+        return " <Invalid>"
+
     active_type = data_obj.GetType().GetTemplateArgumentType(index)
     return f" Active Type = {active_type.GetDisplayTypeName()} "
 

[clayborg]

We should add a test. Should be easy to access the "_M_index" child and set its value to something invalid and then make sure the summary is "".

[jeffreytan81]

We should add a test. Should be easy to access the "_M_index" child and set its value to something invalid and then make sure the summary is "".

Sure, I can do that.

[clayborg]

If this is what was crashing, we should modify the GetTemplateArgumentType() to not crash with an inalid index as part of this fix.

[rupprecht]

This seems to cause TestTemplatePackArgs.py to fail, e.g. https://lab.llvm.org/buildbot/#/builders/68/builds/61819

  File "/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/lldb/test/API/lang/cpp/class-template-parameter-pack/TestTemplatePackArgs.py", line 42, in test_template_argument_pack
    self.assertEqual(nested_template.GetTemplateArgumentType(2).GetName(), "bool")
AssertionError: '' != 'bool'

[DavidSpickett]

Same for AArch64 Ubuntu (https://lab.llvm.org/buildbot/#/builders/96/builds/47090) and Windows (https://lab.llvm.org/buildbot/#/builders/219/builds/6372) (32 bit ARM is out of action right now).

Same symptom as the above comment.

[DavidSpickett]

I've reverted it as the failure reason wasn't immediately obvious to me. Looks like it will reproduce quite easily.