addUnchecked is (outside the tests) only called by ATMP, which already takes the tx pool read lock. So locking it twice more in both addUnchecked methods seems redundant.

Backport of Bitcoin Core PR13786
https://github.com/bitcoin/bitcoin/pull/13786

1. Build with Clang in Debug mode:

CXX=clang++ CC=clang cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja check

2. Verify that the compiler has not emitted a thread-safety warning.
3. Run the node: ./src/bitcoind -regtest
4. Verify that text similar to "Assertion failed: lock ... not held ..." is not printed on stderr.

Update:
To mitigate the risk of ignoring the search for warnings I suggest executing the subsequent test plans as follows:

CXX=clang++ CC=clang cmake .. -D CMAKE_CXX_FLAGS="-Werror=thread-safety-analysis" -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja check