Entropy pool implementation

[torben-hansen]

Issues:

CryptoAlg-1952

Description of changes:

Implements the backend for passive entropy. The integration as the actual entropy backend for RAND_bytes (for fips build), will occur in the next PR.

The entropy pool has one main action: RAND_entropy_pool_get. This function can be used to fetch entropy from the pool. If the pool is depleted, the "depleted workflow" is activated, that workflow requests a need for more entropy outside the module. New entropy is loaded into the module through RAND_load_entropy (the entropy sourcing part is not implemented yet).

Call-outs:

As noted in the source code:

	// Out-side module call
	// Could create a soft-lock in the struct here
	RAND_module_entropy_depleted();
	// And then unlock struct here

I could soft-lock the pool during the depleted workflow. This shouldn't really be necessary since the every operation is serialised in the thread.

Testing:

Added a bunch of tests. Can add more if needed.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[skmcgrail]

Should this abort if entropy_pool == NULL?. I know you gate some of the functions already, but not all of them.

[torben-hansen]

I changed it to only validate the pointer in "exported" functions, not the static ones.

Also changed such that the entropy pool doesn't abort. Callers can do that if they wish. But not at this layer.

[andrewhop]

NP: to me entropy_pool_cannot_satisfy_request seems like it was getting ready to abort, it's not clear what it does.

Suggested change

	static void entropy_pool_cannot_satisfy_request(void) {
	static void request_external_entropy(void) {

[torben-hansen]

if not entropy_pool_ensure_can_satisfy() then entropy_pool_cannot_satisfy_request() seems like the logically named next action.

Just tried to separate logic. Maybe it was too "smart". Just inlined the code instead.

[dkostic]

approved accidentally, taking it back for a moment :)

[torben-hansen]

Given #1112, this implementation is no longer needed. Closing.