Share connection between curl requests

[rvdgracht]

Using a curl_share between curl requests allows sharing an open connection and re-use cached DNS, PSL and TLS session id.

This change allows performing multiple requests without having to do re-perform the (full) TLS handshake.

For reference, on a stm32mp151c with OPTEE + pkcs11 TA a full TLS handshake takes ~8 seconds. Mostly due to small pager pool (internal sram) available for OPTEE.

With this change a mTLS curl request take around 60ms after the initial connection has been established.

[rvdgracht]

I have some concerns about sharing the connection (CURL_LOCK_DATA_CONNECT) between the mainloop (poller) and the download thread. The documentation states: "It is not supported to share connections between multiple concurrent threads." I did place locking on the shared data including CURL_LOCK_DATA_CONNECT though.
I have succesfully tested this. So the mainloop can poll for status while the download thread is streaming data over the
same connection. Maybe someone with more extensive libcurl knowledge can verify this.

[Bastian-Krause]

I think inter-thread connection sharing as it's implemented here should be okay. Maybe we should add -fsanitize=thread here, so we stumble upon any issues:

rauc-hawkbit-updater/.github/workflows/tests.yml

Line 16 in 76f5a1d

- CFLAGS="-fsanitize=address -fsanitize=leak -g" LDFLAGS="-fsanitize=address -fsanitize=leak"

[rvdgracht]

Maybe we should add -fsanitize=thread here

Added.

[Bastian-Krause]

Maybe we should add -fsanitize=thread here

Added.

TSAN works by inserting instrumentation into compiled code to detect race conditions. We use glib from Debian in CI, meaning those libraries are not built with TSAN instrumentation, so memory accesses cannot be tracked. This leads to false positives. So suggesting to enable the sanitizer was not a good idea after all.

I'll drop the commit.

Additional changes required.

[Bastian-Krause]

I've changed the PR slightly:

• use GMutexes instead of pthread_mutex_t, add mutex init/clear
• moved curl_share_setopt() calls to set_default_curl_opts() guarded with g_once_init_enter()/g_once_init_leave()
• add g_return_if_fail() for return value of curl_share_init()
• added asserts in curl lock/unlock functions

[ejoerns]

Looks good to me overall. Just a few notes left about possible enhancements I see.