SocketMbedTLS::close() calls mbedtls_psa_crypto_free(), tearing down process-global PSA state while other sockets use it (heap corruption) #612

Summary

With mbedTLS 3.6+ (and 4.x), SocketMbedTLS calls psa_crypto_init() in its constructor and mbedtls_psa_crypto_free() in close() (ixwebsocket/IXSocketMbedTLS.cpp, currently lines 55 and 332).

mbedtls_psa_crypto_free() is not a per-socket cleanup. It tears down mbedTLS's process-global PSA state: all key slots and the global entropy/RNG. In any process with more than one TLS connection alive, closing one socket wipes state that the others are still using. The symptoms are glibc heap-corruption aborts and segfaults in other threads.

Observed

A process with ~6 concurrent ix::WebSocket TLS connections, each reconnecting on its own thread, aborted repeatedly. Two representative backtraces from core dumps:

#9  __libc_free
#10 psa_wipe_all_key_slots
#11 mbedtls_psa_crypto_free
#12 ix::SocketMbedTLS::close()
#13 ix::SocketMbedTLS::~SocketMbedTLS()
#14 ix::WebSocketTransport::connectToUrl(...)
#15 ix::WebSocket::connect(int)
#16 ix::WebSocket::checkConnection(bool)
#17 ix::WebSocket::run()

#9  __libc_free
#10 mbedtls_md_free
#11 mbedtls_entropy_free
#12 mbedtls_psa_crypto_free
#13 ix::SocketMbedTLS::close()
#14 ix::WebSocketTransport::closeSocket()
#15 ix::WebSocketTransport::receiveFromSocket()
#16 ix::WebSocketTransport::poll()
#17 ix::WebSocket::run()

Minimal reproduction (deterministic, no server needed):

1. Construct socket A, then confirm PSA works (psa_generate_random succeeds).
2. Construct and close() socket B.
3. Call psa_generate_random again on behalf of socket A. It now returns PSA_ERROR_BAD_STATE (-137).

Suggested fix

• Remove mbedtls_psa_crypto_free() from SocketMbedTLS::close(). PSA is process-wide, and freeing it belongs to process teardown (or to the application), not to any one socket. psa_crypto_init() is idempotent, so keeping it in the constructor is fine.
• Related: initMBedTLS() runs from both the constructor and init(). With MBEDTLS_THREADING_C enabled, that re-initialises the entropy/ctr_drbg/ssl context mutexes without freeing them first. A per-socket "already initialised" guard avoids that.
• Worth documenting: concurrent use of PSA from multiple threads also requires mbedTLS to be built with MBEDTLS_THREADING_C. The stock mbedtls_config.h leaves it off.

I can open a PR with the change and a small regression test if that's useful.