Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/pkg/healthcheck/policy.go

@@ -0,0 +1,120 @@
+package healthcheck
+
+import (
+	"log"
+	"math/rand"
+	"sync/atomic"
+)
+
+var (
+	// SupportedPolicies is the collection of policies registered
+	SupportedPolicies = make(map[string]func() Policy)
+)
+
+// RegisterPolicy adds a custom policy to the proxy.
+func RegisterPolicy(name string, policy func() Policy) {
+	SupportedPolicies[name] = policy
+}
+
+// Policy decides how a host will be selected from a pool. When all hosts are unhealthy, it is assumed the
+// healthchecking failed. In this case each policy will *randomly* return a host from the pool to prevent
+// no traffic to go through at all.
+type Policy interface {
+	Select(pool HostPool) *UpstreamHost
+}
+
+func init() {
+	RegisterPolicy("random", func() Policy { return &Random{} })
+	RegisterPolicy("least_conn", func() Policy { return &LeastConn{} })
+	RegisterPolicy("round_robin", func() Policy { return &RoundRobin{} })
+}
+
+// Random is a policy that selects up hosts from a pool at random.
+type Random struct{}
+
+// Select selects an up host at random from the specified pool.
+func (r *Random) Select(pool HostPool) *UpstreamHost {
+	// instead of just generating a random index
+	// this is done to prevent selecting a down host
+	var randHost *UpstreamHost
+	count := 0
+	for _, host := range pool {
+		if host.Down() {
+			continue
+		}
+		count++
+		if count == 1 {
+			randHost = host
+		} else {
+			r := rand.Int() % count
+			if r == (count - 1) {
+				randHost = host
+			}
+		}
+	}
+	return randHost
+}
+
+// Spray is a policy that selects a host from a pool at random. This should be used as a last ditch
+// attempt to get a host when all hosts are reporting unhealthy.
+type Spray struct{}
+
+// Select selects an up host at random from the specified pool.
+func (r *Spray) Select(pool HostPool) *UpstreamHost {
+	rnd := rand.Int() % len(pool)
+	randHost := pool[rnd]
+	log.Printf("[WARNING] All hosts reported as down, spraying to target: %s", randHost.Name)
+	return randHost
+}
+
+// LeastConn is a policy that selects the host with the least connections.
+type LeastConn struct{}
+
+// Select selects the up host with the least number of connections in the
+// pool.  If more than one host has the same least number of connections,
+// one of the hosts is chosen at random.
+func (r *LeastConn) Select(pool HostPool) *UpstreamHost {
+	var bestHost *UpstreamHost
+	count := 0
+	leastConn := int64(1<<63 - 1)
+	for _, host := range pool {
+		if host.Down() {
+			continue
+		}
+		hostConns := host.Conns
+		if hostConns < leastConn {
+			bestHost = host
+			leastConn = hostConns
+			count = 1
+		} else if hostConns == leastConn {
+			// randomly select host among hosts with least connections
+			count++
+			if count == 1 {
+				bestHost = host
+			} else {
+				r := rand.Int() % count
+				if r == (count - 1) {
+					bestHost = host
+				}
+			}
+		}
+	}
+	return bestHost
+}
+
+// RoundRobin is a policy that selects hosts based on round robin ordering.
+type RoundRobin struct {
+	Robin uint32
+}
+
+// Select selects an up host from the pool using a round robin ordering scheme.
+func (r *RoundRobin) Select(pool HostPool) *UpstreamHost {
+	poolLen := uint32(len(pool))
+	selection := atomic.AddUint32(&r.Robin, 1) % poolLen
+	host := pool[selection]
+	// if the currently selected host is down, just ffwd to up host
+	for i := uint32(1); host.Down() && i < poolLen; i++ {
+		host = pool[(selection+i)%poolLen]
+	}
+	return host
+}