Fix data races in sorting and Reverse

The custom sort functions used from the templates had some subtle data race- and related issues,
especially when used in the single page template.

This commit fixes this by making copies and protect the read and writes with a RWMutex.

The results are cached (it will typically be invoked *number of pages* times with exactly the same data).

This is, not surprisingly, also faster:

```
benchmark                           old ns/op     new ns/op     delta
BenchmarkSortByWeightAndReverse     14228         491           -96.55%

benchmark                           old allocs     new allocs     delta
BenchmarkSortByWeightAndReverse     1              0              -100.00%

benchmark                           old bytes     new bytes     delta
BenchmarkSortByWeightAndReverse     32            0             -100.00%
```

Fixes #1293

1 files changed, 60 insertions, 0 deletions

diff --git a/hugolib/pageCache_test.go b/hugolib/pageCache_test.go
new file mode 100644
index 000000000..6e48c63cf
--- /dev/null
+++ b/hugolib/pageCache_test.go
@@ -0,0 +1,60 @@
+package hugolib
+
+import (
+	"fmt"
+	"github.com/stretchr/testify/assert"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+func TestPageCache(t *testing.T) {
+	c1 := newPageCache()
+
+	changeFirst := func(p Pages) {
+		p[0].Description = "changed"
+	}
+
+	var o1 uint64 = 0
+	var o2 uint64 = 0
+
+	var wg sync.WaitGroup
+
+	var l1 sync.Mutex
+	var l2 sync.Mutex
+
+	var testPageSets []Pages
+
+	for j := 0; j < 50; j++ {
+		testPageSets = append(testPageSets, createSortTestPages(j+1))
+	}
+
+	for i := 0; i < 100; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j, pages := range testPageSets {
+				msg := fmt.Sprintf("Go %d %d %d %d", i, j, o1, o2)
+				l1.Lock()
+				p, c := c1.get("k1", pages, nil)
+				assert.Equal(t, !atomic.CompareAndSwapUint64(&o1, uint64(j), uint64(j+1)), c, "c1: "+msg)
+				l1.Unlock()
+				p2, c2 := c1.get("k1", p, nil)
+				assert.True(t, c2)
+				assert.True(t, probablyEqualPages(p, p2))
+				assert.True(t, probablyEqualPages(p, pages))
+				assert.NotNil(t, p, msg)
+
+				l2.Lock()
+				p3, c3 := c1.get("k2", pages, changeFirst)
+				assert.Equal(t, !atomic.CompareAndSwapUint64(&o2, uint64(j), uint64(j+1)), c3, "c3: "+msg)
+				l2.Unlock()
+				assert.NotNil(t, p3, msg)
+				assert.Equal(t, p3[0].Description, "changed", msg)
+			}
+		}()
+	}
+
+	wg.Wait()
+
+}