[libunwind] [libunwind][AIX] Call dlclose only when dlsym() fails (PR #112768)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libunwind

Author: Xing Xue (xingxue-ibm)

<details>
<summary>Changes</summary>

The personality routine `__xlcxx_personality_v0` in `libc++abi` is hard-coded in the unwinder as the handler for EH in applications generated by the legacy IBM C++ compiler. The symbol is resolved dynamically using `dlopen` to avoid a hard dependency of `libunwind` on `libc++abi` for cases such as non-C++ applications. However, `dlclose` was incorrectly called after `dlsym` succeeded, potentially invalidating the function pointer obtained from `dlsym` when the memory allocated for the `dlopen` is reclaimed. This PR changes to call `dlclose` only when `dlsym` fails.

---
Full diff: https://github.com/llvm/llvm-project/pull/112768.diff


1 Files Affected:

- (modified) libunwind/src/UnwindCursor.hpp (+1-1) 


``````````diff
diff --git a/libunwind/src/UnwindCursor.hpp b/libunwind/src/UnwindCursor.hpp
index 2a3aba28fb6ca5..9f3ffd19e1746a 100644
--- a/libunwind/src/UnwindCursor.hpp
+++ b/libunwind/src/UnwindCursor.hpp
@@ -2151,8 +2151,8 @@ bool UnwindCursor<A, R>::getInfoFromTBTable(pint_t pc, R &registers) {
         if (xlcPersonalityV0 == NULL) {
           _LIBUNWIND_TRACE_UNWINDING("dlsym() failed with errno=%d\n", errno);
           assert(0 && "dlsym() failed");
+          dlclose(libHandle);
         }
-        dlclose(libHandle);
         errno = saveErrno;
       }
       xlcPersonalityV0InitLock.unlock();

``````````

</details>


https://github.com/llvm/llvm-project/pull/112768