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, 10 insertions, 7 deletions

diff --git a/consensus/ethash/sealer.go b/consensus/ethash/sealer.go
index c2447e473..b5e742d8b 100644
--- a/consensus/ethash/sealer.go
+++ b/consensus/ethash/sealer.go
@@ -97,10 +97,9 @@ func (ethash *Ethash) Seal(chain consensus.ChainReader, block *types.Block, stop
 func (ethash *Ethash) mine(block *types.Block, id int, seed uint64, abort chan struct{}, found chan *types.Block) {
 	// Extract some data from the header
 	var (
-		header = block.Header()
-		hash   = header.HashNoNonce().Bytes()
-		target = new(big.Int).Div(maxUint256, header.Difficulty)
-
+		header  = block.Header()
+		hash    = header.HashNoNonce().Bytes()
+		target  = new(big.Int).Div(maxUint256, header.Difficulty)
 		number  = header.Number.Uint64()
 		dataset = ethash.dataset(number)
 	)
@@ -111,13 +110,14 @@ func (ethash *Ethash) mine(block *types.Block, id int, seed uint64, abort chan s
 	)
 	logger := log.New("miner", id)
 	logger.Trace("Started ethash search for new nonces", "seed", seed)
+search:
 	for {
 		select {
 		case <-abort:
 			// Mining terminated, update stats and abort
 			logger.Trace("Ethash nonce search aborted", "attempts", nonce-seed)
 			ethash.hashrate.Mark(attempts)
-			return
+			break search
 
 		default:
 			// We don't have to update hash rate on every nonce, so update after after 2^X nonces
@@ -127,7 +127,7 @@ func (ethash *Ethash) mine(block *types.Block, id int, seed uint64, abort chan s
 				attempts = 0
 			}
 			// Compute the PoW value of this nonce
-			digest, result := hashimotoFull(dataset, hash, nonce)
+			digest, result := hashimotoFull(dataset.dataset, hash, nonce)
 			if new(big.Int).SetBytes(result).Cmp(target) <= 0 {
 				// Correct nonce found, create a new header with it
 				header = types.CopyHeader(header)
@@ -141,9 +141,12 @@ func (ethash *Ethash) mine(block *types.Block, id int, seed uint64, abort chan s
 				case <-abort:
 					logger.Trace("Ethash nonce found but discarded", "attempts", nonce-seed, "nonce", nonce)
 				}
-				return
+				break search
 			}
 			nonce++
 		}
 	}
+	// Datasets are unmapped in a finalizer. Ensure that the dataset stays live
+	// during sealing so it's not unmapped while being read.
+	runtime.KeepAlive(dataset)
 }