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, 16 deletions

diff --git a/consensus/ethash/algorithm_go1.8_test.go b/consensus/ethash/algorithm_go1.8_test.go
index a822944a6..6648bd6a9 100644
--- a/consensus/ethash/algorithm_go1.8_test.go
+++ b/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)
 		}
 	}
 }