middleware/cache: split cache in positive and negative and use lru (#298)

Make the cache memory bounded, by using a LRU cache. Also split the
cache in a positive and negative one - each with its own controls.

Extend the cache stanza to allow for this:

    cache {
       positive limit [ttl]
       negative limit [ttl]
    }

is now possible. This also add a cache_test.go in the toplevel test/
directory that exercises the caching path.

Fixes #260

middleware/cache/setup_test.go

@@ -0,0 +1,71 @@
+package cache
+
+import (
+	"testing"
+	"time"
+
+	"github.com/mholt/caddy"
+)
+
+func TestSetup(t *testing.T) {
+	tests := []struct {
+		input        string
+		shouldErr    bool
+		expectedNcap int
+		expectedPcap int
+		expectedNttl time.Duration
+		expectedPttl time.Duration
+	}{
+		{`cache`, false, defaultCap, defaultCap, maxNTTL, maxTTL},
+		{`cache {}`, false, defaultCap, defaultCap, maxNTTL, maxTTL},
+		{`cache example.nl {
+			positive 10
+		}`, false, defaultCap, 10, maxNTTL, maxTTL},
+		{`cache example.nl {
+			positive 10
+			negative 10 15
+		}`, false, 10, 10, 15 * time.Second, maxTTL},
+		{`cache 25 example.nl {
+			positive 10
+			negative 10 15
+		}`, false, 10, 10, 15 * time.Second, 25 * time.Second},
+		{`cache aaa example.nl`, false, defaultCap, defaultCap, maxNTTL, maxTTL},
+
+		// fails
+		{`cache example.nl {
+			positive
+			negative 10 15
+		}`, true, defaultCap, defaultCap, maxTTL, maxTTL},
+		{`cache example.nl {
+			positive 15
+			negative aaa
+		}`, true, defaultCap, defaultCap, maxTTL, maxTTL},
+	}
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		ca, err := cacheParse(c)
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %v: Expected error but found nil", i)
+			continue
+		} else if !test.shouldErr && err != nil {
+			t.Errorf("Test %v: Expected no error but found error: %v", i, err)
+			continue
+		}
+		if test.shouldErr && err != nil {
+			continue
+		}
+
+		if ca.ncap != test.expectedNcap {
+			t.Errorf("Test %v: Expected ncap %v but found: %v", i, test.expectedNcap, ca.ncap)
+		}
+		if ca.pcap != test.expectedPcap {
+			t.Errorf("Test %v: Expected pcap %v but found: %v", i, test.expectedPcap, ca.pcap)
+		}
+		if ca.nttl != test.expectedNttl {
+			t.Errorf("Test %v: Expected nttl %v but found: %v", i, test.expectedNttl, ca.nttl)
+		}
+		if ca.pttl != test.expectedPttl {
+			t.Errorf("Test %v: Expected pttl %v but found: %v", i, test.expectedPttl, ca.pttl)
+		}
+	}
+}