threads: qa: Add lock order annotation for `TxMempool::cs` #34813

I added this to see if static analysis would catch any lock order inversions while investigating #34731, it did not catch any but it is a worthwhile defense against statically-analyzable inversions of cs_main->TxMempool::cs.

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Concept ACK

I was surprised initially that triggering this warning seems to still require -Wthread-safety-beta. Should we have been setting that flag at least in the CI somewhere? Looks like it is moved out of beta in the upcoming 23 release: https://github.com/llvm/llvm-project/pull/152853.

Looks like it is moved out of beta in the upcoming 23 release: https://github.com/llvm/llvm-project/pull/152853.

That change was part of LLVM 22, which is currently being used in the CI (#34660).

Concept ACK.

That change was part of LLVM 22, which is currently being used in the CI (https://github.com/bitcoin/bitcoin/pull/34660).

Oh, should have checked the release notes properly, it is right there: https://releases.llvm.org/22.1.0/tools/clang/docs/ReleaseNotes.html#improvements-to-clang-s-diagnostics . Checked it manually in the meantime.

tested ACK 9085dee476d21daab786e88848597c755beecee4

ACK 9085dee476d21daab786e88848597c755beecee4.

Tested on Ubuntu 25.10 using Clang 22.1.1 by applying the following patch:

--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -257,7 +257,7 @@ public:
      * changing the chain tip. It's necessary to keep both mutexes locked until
      * the mempool is consistent with the new chain tip and fully populated.
      */
-    mutable RecursiveMutex cs ACQUIRED_AFTER(::cs_main);
+    mutable RecursiveMutex cs ACQUIRED_BEFORE(::cs_main);
     std::unique_ptr<TxGraph> m_txgraph GUARDED_BY(cs);
     mutable std::unique_ptr<TxGraph::BlockBuilder> m_builder GUARDED_BY(cs);
     indexed_transaction_set mapTx GUARDED_BY(cs);

When using Clang 17.0.6, the annotation is ignored, as expected.

ACK 9085dee476d21daab786e88848597c755beecee4

Built with cmake -B build -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DAPPEND_CXXFLAGS="-Wthread-safety-beta" (Clang 18.1.3).

Clean build, and verified the annotation catches wrong lock order when introducing a deliberate violation.