[lldb-dev] Too many ModuleSP references

[Jim Ingham via lldb-dev]

Thanks for digging into this.

Maybe a better option would be to have IRExecutionUnit hold a SymbolContextScope, rather than a SymbolContext.  We started out with SymbolContext's because they were convenient but they hold too many things alive so when we run into SymbolContext's we've been converting them to the SymbolContextScope's when possible.  I can't see any need for the IRExecutionUnit to hold the module alive, so long as it handles failure to get the SymbolContext gracefully, which shouldn't be hard.

Jim

> On Feb 21, 2017, at 9:03 AM, Pavel Labath via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Hello all,
> 
> I've been debugging the newly added TestStepOverBreakpoint.py, which
> has been failing on windows, for a very prosaic reason: after the test
> completes, we are unable to run "make clean" because lldb still holds
> the file open.
> 
> After some debugging, I've found that this happens because the test
> case stores the SBBreakpoint object in a member variable of the python
> test case class. The breakpoint class ends up transitively holding a
> reference to the main executable module, which prevents the module
> from being garbage-collected when the target is destroyed.
> 
> For reference, the full ownership chain is something like:
> StepOverBreakpointTestCase(python) => SBBreakpoint => Breakpoint =>
> BreakpointLocation => LLVMUserExpression => IRExecutionUnit =>
> SymbolContext => Module.
> 
> To get the test working, we need to break this chain somewhere. A
> couple of places I see are:
> - BreakpointLocation: Remove the compiled expression reference when
> the target is destroyed (AFAICS, it is used as a cache to avoid
> recomputing the expression every time. It can be theoretically
> recomputed if needed, but that shouldn't be necessary as the target is
> destroyed anyway)
> 
> - SBBreakpoint: make SBBreakpoint hold a weak_ptr to the Breakpoint
> object. When the target is destroyed, the SBBreakpoint object becomes
> invalid (One doesn't cannot do anything useful with the breakpoint
> once the target has been deleted anyway).
> 
> - StepOverBreakpointTestCase: Have the test not store the breakpoints
> in the test case object. Basically, declare that this is not a bug,
> and it's the users responsibility to clean up necessary objects.
> 
> Any thoughts on what is the appropriate solution here?
> 
> cheers,
> pavel
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