consensus/ethash: improve cache/dataset handling (ethereum#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 ethereum#14495 or ethereum#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

consensus/ethash/algorithm.go

@@ -355,9 +355,11 @@ func hashimotoFull(dataset []uint32, hash []byte, nonce uint64) ([]byte, []byte)
 	return hashimoto(hash, nonce, uint64(len(dataset))*4, lookup)
 }
 
+const maxEpoch = 2048
+
 // datasetSizes is a lookup table for the ethash dataset size for the first 2048
 // epochs (i.e. 61440000 blocks).
-var datasetSizes = []uint64{
+var datasetSizes = [maxEpoch]uint64{
 	1073739904, 1082130304, 1090514816, 1098906752, 1107293056,
 	1115684224, 1124070016, 1132461952, 1140849536, 1149232768,
 	1157627776, 1166013824, 1174404736, 1182786944, 1191180416,
@@ -771,7 +773,7 @@ var datasetSizes = []uint64{
 
 // cacheSizes is a lookup table for the ethash verification cache size for the
 // first 2048 epochs (i.e. 61440000 blocks).
-var cacheSizes = []uint64{
+var cacheSizes = [maxEpoch]uint64{
 	16776896, 16907456, 17039296, 17170112, 17301056, 17432512, 17563072,
 	17693888, 17824192, 17955904, 18087488, 18218176, 18349504, 18481088,
 	18611392, 18742336, 18874304, 19004224, 19135936, 19267264, 19398208,

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

consensus/ethash/algorithm_go1.8.go

@@ -20,35 +20,41 @@ package ethash
 
 import "math/big"
 
-// cacheSize calculates and returns the size of the ethash verification cache that
-// belongs to a certain block number. The cache size grows linearly, however, we
-// always take the highest prime below the linearly growing threshold in order to
-// reduce the risk of accidental regularities leading to cyclic behavior.
+// cacheSize returns the size of the ethash verification cache that belongs to a certain
+// block number.
 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]
 	}
-	// No known cache size, calculate manually (sanity branch only)
+	return calcCacheSize(epoch)
+}
+
+// calcCacheSize calculates the cache size for epoch. The cache size grows linearly,
+// however, we always take the highest prime below the linearly growing threshold in order
+// to reduce the risk of accidental regularities leading to cyclic behavior.
+func calcCacheSize(epoch int) uint64 {
 	size := cacheInitBytes + cacheGrowthBytes*uint64(epoch) - hashBytes
 	for !new(big.Int).SetUint64(size / hashBytes).ProbablyPrime(1) { // Always accurate for n < 2^64
 		size -= 2 * hashBytes
 	}
 	return size
 }
 
-// datasetSize calculates and returns the size of the ethash mining dataset that
-// belongs to a certain block number. The dataset size grows linearly, however, we
-// always take the highest prime below the linearly growing threshold in order to
-// reduce the risk of accidental regularities leading to cyclic behavior.
+// datasetSize returns the size of the ethash mining dataset that belongs to a certain
+// block number.
 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]
 	}
-	// No known dataset size, calculate manually (sanity branch only)
+	return calcDatasetSize(epoch)
+}
+
+// calcDatasetSize calculates the dataset size for epoch. The dataset size grows linearly,
+// however, we always take the highest prime below the linearly growing threshold in order
+// to reduce the risk of accidental regularities leading to cyclic behavior.
+func calcDatasetSize(epoch int) uint64 {
 	size := datasetInitBytes + datasetGrowthBytes*uint64(epoch) - mixBytes
 	for !new(big.Int).SetUint64(size / mixBytes).ProbablyPrime(1) { // Always accurate for n < 2^64
 		size -= 2 * mixBytes

consensus/ethash/algorithm_go1.8_test.go

@@ -23,24 +23,15 @@ import "testing"
 // Tests whether the dataset size calculator works correctly by cross checking the
 // hard coded lookup table with the value generated by it.
 func TestSizeCalculations(t *testing.T) {
-	var tests []uint64
-
-	// Verify all the cache sizes from the lookup table
-	defer func(sizes []uint64) { cacheSizes = sizes }(cacheSizes)
-	tests, cacheSizes = cacheSizes, []uint64{}
-
-	for i, test := range tests {
-		if size := cacheSize(uint64(i*epochLength) + 1); size != test {
-			t.Errorf("cache %d: cache size mismatch: have %d, want %d", i, size, test)
+	// Verify all the cache and dataset sizes from the lookup table.
+	for epoch, want := range cacheSizes {
+		if size := calcCacheSize(epoch); size != want {
+			t.Errorf("cache %d: cache size mismatch: have %d, want %d", epoch, size, want)
 		}
 	}
-	// Verify all the dataset sizes from the lookup table
-	defer func(sizes []uint64) { datasetSizes = sizes }(datasetSizes)
-	tests, datasetSizes = datasetSizes, []uint64{}
-
-	for i, test := range tests {
-		if size := datasetSize(uint64(i*epochLength) + 1); size != test {
-			t.Errorf("dataset %d: dataset size mismatch: have %d, want %d", i, size, test)
+	for epoch, want := range datasetSizes {
+		if size := calcDatasetSize(epoch); size != want {
+			t.Errorf("dataset %d: dataset size mismatch: have %d, want %d", epoch, size, want)
 		}
 	}
 }

consensus/ethash/consensus.go

@@ -476,22 +476,26 @@ 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
 	}
 	// Ensure that we have a valid difficulty for the block
 	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
 	}