Load runtimes dynamically via go1.8 plugins

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

Add registration for more subsystems via plugins

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

Move content service to separate package

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

services/execution/collector.go

@@ -0,0 +1,89 @@
+package execution
+
+import (
+	"sync"
+
+	"github.com/docker/containerd"
+
+	"golang.org/x/net/context"
+)
+
+func newCollector(ctx context.Context, runtimes map[string]containerd.Runtime) (*collector, error) {
+	c := &collector{
+		context:      ctx,
+		ch:           make(chan *containerd.Event, 2048),
+		eventClients: make(map[*eventClient]struct{}),
+	}
+	for _, r := range runtimes {
+		if err := c.collect(r); err != nil {
+			return nil, err
+		}
+	}
+	// run the publisher
+	go c.publisher()
+	// run a goroutine that waits for the context to be done
+	// and closes the channel after all writes have finished
+	go c.waitDone()
+	return c, nil
+}
+
+type eventClient struct {
+	eCh chan error
+	w   *grpcEventWriter
+}
+
+type collector struct {
+	mu sync.Mutex
+	wg sync.WaitGroup
+
+	context      context.Context
+	ch           chan *containerd.Event
+	eventClients map[*eventClient]struct{}
+}
+
+// collect collects events from the provided runtime
+func (c *collector) collect(r containerd.Runtime) error {
+	c.wg.Add(1)
+	go func() {
+		defer c.wg.Done()
+		for e := range r.Events(c.context) {
+			c.ch <- e
+		}
+	}()
+	return nil
+}
+
+func (c *collector) forward(w *grpcEventWriter) error {
+	client := &eventClient{
+		w:   w,
+		eCh: make(chan error, 1),
+	}
+	c.mu.Lock()
+	c.eventClients[client] = struct{}{}
+	c.mu.Unlock()
+	err := <-client.eCh
+	c.mu.Lock()
+	delete(c.eventClients, client)
+	c.mu.Unlock()
+	return err
+}
+
+func (c *collector) publisher() {
+	for e := range c.ch {
+		c.mu.Lock()
+		for client := range c.eventClients {
+			if err := client.w.Write(e); err != nil {
+				client.eCh <- err
+			}
+		}
+		c.mu.Unlock()
+	}
+}
+
+// waitDone waits for the context to finish, waits for all the goroutines to finish
+// collecting grpc events from the shim, and closes the output channel
+func (c *collector) waitDone() {
+	<-c.context.Done()
+	c.wg.Wait()
+	close(c.ch)
+}