fix(asio): memory leaks over ssl::stream lifetime (IDFGH-16977)

[rtyle]

Description

asio::ssl::stream leaks a significant amount (>20KB) of memory over its lifetime.

asio::ssl::mbedtls::engine::impl has these members ...

mbedtls_ssl_context ssl_{};
mbedtls_entropy_context entropy_{};
mbedtls_ctr_drbg_context ctr_drbg_{};
mbedtls_ssl_config conf_{};
mbedtls_x509_crt public_cert_{};
mbedtls_pk_context pk_key_{};
mbedtls_x509_crt ca_cert_{};

... which are properly init'ed in its constructor ...

mbedtls_ssl_init(&ssl_);
mbedtls_ssl_config_init(&conf_);
mbedtls_ctr_drbg_init(&ctr_drbg_);
mbedtls_entropy_init(&entropy_);
mbedtls_x509_crt_init(&public_cert_);
mbedtls_pk_init(&pk_key_);
mbedtls_x509_crt_init(&ca_cert_);

... but are never free'd ... until now

~impl()
{
    mbedtls_x509_crt_free(&ca_cert_);
    mbedtls_pk_free(&pk_key_);
    mbedtls_x509_crt_free(&public_cert_);
    mbedtls_entropy_free(&entropy_);
    mbedtls_ctr_drbg_free(&ctr_drbg_);
    mbedtls_ssl_config_free(&conf_);
    mbedtls_ssl_free(&ssl_);
}

asio::ssl::mbedtls::engine::impl::configure calls ...

mbedtls_x509_crt_init(&public_cert_);
mbedtls_pk_init(&pk_key_);
mbedtls_x509_crt_init(&ca_cert_);

... again without first free'ing any resources that might be held by such. now this is done first:

mbedtls_x509_crt_free(&ca_cert_);
mbedtls_pk_free(&pk_key_);
mbedtls_x509_crt_free(&public_cert_);

asio::ssl::engine has this member

std::pair<std::shared_ptr<bio>, std::shared_ptr<bio>> bio_;

which is made in its constructor

explicit engine(std::shared_ptr<context> ctx): ctx_(std::move(ctx)),
    bio_(bio::new_pair("mbedtls-engine")), state_(IDLE), verify_mode_(0) {}

asio::ssl::mbedtls::bio::new_pair creates a cyclic reference between its paired elements

static std::pair<std::shared_ptr<bio>, std::shared_ptr<bio>>  new_pair(const char *error_location)
{
    auto b1 = std::shared_ptr<bio>(new (std::nothrow) bio);
    auto b2 = std::shared_ptr<bio>(new (std::nothrow) bio);
    if (b1 == nullptr || b2 == nullptr) {
        throw_alloc_failure(error_location);
    } else {
        b1->peer_ = b2;
        b2->peer_ = b1;
    }
    return std::make_pair(b1, b2);
}

there is no asio::ssl::engine destructor to untie this cycle so when the pair member is destroyed, its elements will leak.
a destructor is needed to fix this ...

~engine()
{
    bio::untie_pair(bio_);
}

... along with untie_pair ...

// untie cyclic shared_ptr references made by new_pair in preparation for destruction
static void untie_pair(std::pair<std::shared_ptr<bio>, std::shared_ptr<bio>>& pair)
{
    if (pair.first) {
        pair.first->peer_.reset();
    }
    if (pair.second) {
        pair.second->peer_.reset();
    }
}

Related

Testing

I am writing an esphome smtp component that uses asio to send email through smtp.gmai.com.
My smtp component calls on esphome for asio support

async def to_code(config):
add_idf_component(name="espressif/asio", ref=">=1.32.0", submodules=["asio/asio"])

Without this fix, my esp cores3 would run out of memory after a few sessions.
Using esphome debug memory monitoring over the device's web interface,
it was apparent the huge memory drops were due to may smtp component's activity.

The first fix was obvious: mbedtls init'ed resources must be free'd.
After that fix, things improved considerably but there was still a leak of ~2KB/stream lifetime.
This was due to the cyclic shared_ptr references that allowed bio pair elements to be leaked.

My project is running comfortably now.
The heap memory available graph in the esphome web interface shows a steady horizontal line.

---

Note

Adds destructors and cleanup to mbedtls engine and unties BIO pair to eliminate memory leaks during ssl::stream lifecycle.

• Memory management fixes:
  • mbedtls_engine.hpp:
    • Add engine::~engine() to call bio::untie_pair(bio_) and break cyclic shared_ptr references.
    • Add impl::~impl() to free mbedtls resources: ssl_, conf_, ctr_drbg_, entropy_, ca_cert_, pk_key_, public_cert_.
    • In impl::configure(...), free ca_cert_, pk_key_, and public_cert_ before re-initializing them.
  • mbedtls_bio.hpp:
    • Add bio::untie_pair(...) to reset peer_ pointers created by bio::new_pair(...).

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