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

consensus/ethash/ethash_test.go

@@ -17,7 +17,11 @@
 package ethash
 
 import (
+	"io/ioutil"
 	"math/big"
+	"math/rand"
+	"os"
+	"sync"
 	"testing"
 
 	"github.com/ethereum/go-ethereum/core/types"
@@ -38,3 +42,38 @@ func TestTestMode(t *testing.T) {
 		t.Fatalf("unexpected verification error: %v", err)
 	}
 }
+
+// This test checks that cache lru logic doesn't crash under load.
+// It reproduces https://github.com/ethereum/go-ethereum/issues/14943
+func TestCacheFileEvict(t *testing.T) {
+	tmpdir, err := ioutil.TempDir("", "ethash-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+	e := New(Config{CachesInMem: 3, CachesOnDisk: 10, CacheDir: tmpdir, PowMode: ModeTest})
+
+	workers := 8
+	epochs := 100
+	var wg sync.WaitGroup
+	wg.Add(workers)
+	for i := 0; i < workers; i++ {
+		go verifyTest(&wg, e, i, epochs)
+	}
+	wg.Wait()
+}
+
+func verifyTest(wg *sync.WaitGroup, e *Ethash, workerIndex, epochs int) {
+	defer wg.Done()
+
+	const wiggle = 4 * epochLength
+	r := rand.New(rand.NewSource(int64(workerIndex)))
+	for epoch := 0; epoch < epochs; epoch++ {
+		block := int64(epoch)*epochLength - wiggle/2 + r.Int63n(wiggle)
+		if block < 0 {
+			block = 0
+		}
+		head := &types.Header{Number: big.NewInt(block), Difficulty: big.NewInt(100)}
+		e.VerifySeal(nil, head)
+	}
+}