refactor: CBlockHeaderAndShortTxIDs constructor now always takes an explicit nonce.
test: Make blockencodings_tests deterministic using fixed seed providing deterministic
CBlockHeaderAndShortTxID nonces and dummy transaction IDs.
Fixes very rare flaky test failures, where the ShortIDs of test transactions collide, leading to
`READ_STATUS_FAILED` from PartiallyDownloadedBlock::InitData and/or `IsTxAvailable` giving `false`
when the transaction should actually be available.
* Use a new `FastRandomContext` with a fixed seed in each test, to ensure 'random' uint256s
used as fake prevouts are deterministic, so in-turn test txids and short IDs are deterministic
and don't collide causing very rare but flaky test failures.
* Add new test-only/internal initializer for `CBlockHeaderAndShortTxIDs` that takes a specified
nonce to further ensure determinism and avoid rare but undesireable short ID collisions.
In a test context this nonce is set to a fixed known-good value. Normally it is random, as
previously.
Flaky test failures can be reproduced with:
```patch
diff --git a/src/blockencodings.cpp b/src/blockencodings.cpp
index 695e8d806a..64d635a97a 100644
--- a/src/blockencodings.cpp
+++ b/src/blockencodings.cpp
@@ -44,7 +44,8 @@ void CBlockHeaderAndShortTxIDs::FillShortTxIDSelector() const {
uint64_t CBlockHeaderAndShortTxIDs::GetShortID(const Wtxid& wtxid) const {
static_assert(SHORTTXIDS_LENGTH == 6, "shorttxids calculation assumes 6-byte shorttxids");
- return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0xffffffffffffL;
+ // return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0xffffffffffffL;
+ return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0x0f;
}
```
to increase the likelihood of a short ID collision; and running
```shell
set -e;
n=0;
while (( n++ < 5000 )); do
src/test/test_bitcoin --run_test=blockencodings_tests;
done
```
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