Thread safety not documented

[gotthardp]

I believe the use of the functions from multiple threads should be better documented.
For example, the statement The purpose and intend of tpm2-tss's contexts is to have 1 context per thread. from #2914 (comment)

Is the same true for the Tcti contexts?

[joholl]

Good question. I'd say the design goal is being able to have multiple tcti contexts for a given thread. I'm not aware of any showstoppers here (global variables etc.). Let me make clear, however, that this is not the focus of our tests. So I would not bet money on it.

In other words - we should write tests for using multiple tctis in the same thread before documenting this.

---

I remember the tcti-libtpms had a challenge for multi-threading, but we did our best to make it work. See also this comment. Again, not the focus of our tests.

tpm2-tss/src/tss2-tcti/tcti-libtpms.c

Lines 33 to 40 in 5bcdeb7

	/*
	* libtpms API calls need to be wrapped. We set the current active TCTI module
	* for this thread. This is needed because libtpms may call callbacks and these
	* need to know which TCTI context they have to operate on.
	*
	* This macro assumes that int ret is declared. Jumps to fail_label on error. In
	* this case, rc contains the respective error code.
	*/