consensus/ethash: improve cache/dataset handling (#15864)

* consensus/ethash: add maxEpoch constant

* consensus/ethash: improve cache/dataset handling

There are two fixes in this commit:

Unmap the memory through a finalizer like the libethash wrapper did. The
release logic was incorrect and freed the memory while it was being
used, leading to crashes like in #14495 or #14943.

Track caches and datasets using simplelru instead of reinventing LRU
logic. This should make it easier to see whether it's correct.

* consensus/ethash: restore 'future item' logic in lru

* consensus/ethash: use mmap even in test mode

This makes it possible to shorten the time taken for TestCacheFileEvict.

* consensus/ethash: shuffle func calc*Size comments around

* consensus/ethash: ensure future cache/dataset is in the lru cache

* consensus/ethash: add issue link to the new test

* consensus/ethash: fix vet

* consensus/ethash: fix test

* consensus: tiny issue + nitpick fixes

1 files changed, 7 insertions, 3 deletions

diff --git a/consensus/ethash/consensus.go b/consensus/ethash/consensus.go
index 82d23c92b..92a23d4a4 100644
--- a/consensus/ethash/consensus.go
+++ b/consensus/ethash/consensus.go
@@ -476,7 +476,7 @@ func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Head
 	}
 	// Sanity check that the block number is below the lookup table size (60M blocks)
 	number := header.Number.Uint64()
-	if number/epochLength >= uint64(len(cacheSizes)) {
+	if number/epochLength >= maxEpoch {
 		// Go < 1.7 cannot calculate new cache/dataset sizes (no fast prime check)
 		return errNonceOutOfRange
 	}
@@ -484,14 +484,18 @@ func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Head
 	if header.Difficulty.Sign() <= 0 {
 		return errInvalidDifficulty
 	}
+
 	// Recompute the digest and PoW value and verify against the header
 	cache := ethash.cache(number)
-
 	size := datasetSize(number)
 	if ethash.config.PowMode == ModeTest {
 		size = 32 * 1024
 	}
-	digest, result := hashimotoLight(size, cache, header.HashNoNonce().Bytes(), header.Nonce.Uint64())
+	digest, result := hashimotoLight(size, cache.cache, header.HashNoNonce().Bytes(), header.Nonce.Uint64())
+	// Caches are unmapped in a finalizer. Ensure that the cache stays live
+	// until after the call to hashimotoLight so it's not unmapped while being used.
+	runtime.KeepAlive(cache)
+
 	if !bytes.Equal(header.MixDigest[:], digest) {
 		return errInvalidMixDigest
 	}