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, 2 insertions, 2 deletions

diff --git a/consensus/ethash/algorithm_go1.7.go b/consensus/ethash/algorithm_go1.7.go
index c34d041c3..c7f7f48e4 100644
--- a/consensus/ethash/algorithm_go1.7.go
+++ b/consensus/ethash/algorithm_go1.7.go
@@ -25,7 +25,7 @@ package ethash
 func cacheSize(block uint64) uint64 {
 	// If we have a pre-generated value, use that
 	epoch := int(block / epochLength)
-	if epoch < len(cacheSizes) {
+	if epoch < maxEpoch {
 		return cacheSizes[epoch]
 	}
 	// We don't have a way to verify primes fast before Go 1.8
@@ -39,7 +39,7 @@ func cacheSize(block uint64) uint64 {
 func datasetSize(block uint64) uint64 {
 	// If we have a pre-generated value, use that
 	epoch := int(block / epochLength)
-	if epoch < len(datasetSizes) {
+	if epoch < maxEpoch {
 		return datasetSizes[epoch]
 	}
 	// We don't have a way to verify primes fast before Go 1.8