Move ch ring placement back from old FnLB. (#930)

* fn: bring back CH ring placer into FN repo based on original FnLB
* fn: move placement code into runnerpool directory

api/runnerpool/naive_placer.go

@@ -0,0 +1,65 @@
+package runnerpool
+
+import (
+	"context"
+	"time"
+
+	"github.com/fnproject/fn/api/common"
+	"github.com/fnproject/fn/api/models"
+
+	"github.com/sirupsen/logrus"
+)
+
+const (
+	// sleep time to attempt placement across all runners before retrying
+	retryWaitInterval = 10 * time.Millisecond
+)
+
+type naivePlacer struct{}
+
+func NewNaivePlacer() Placer {
+	logrus.Info("Creating new naive runnerpool placer")
+	return &naivePlacer{}
+}
+
+func (sp *naivePlacer) PlaceCall(rp RunnerPool, ctx context.Context, call RunnerCall) error {
+	timeout := time.After(call.SlotDeadline().Sub(time.Now()))
+
+	for {
+		select {
+		case <-ctx.Done():
+			return models.ErrCallTimeoutServerBusy
+		case <-timeout:
+			return models.ErrCallTimeoutServerBusy
+		default:
+			runners, err := rp.Runners(call)
+			if err != nil {
+				logrus.WithError(err).Error("Failed to find runners for call")
+			} else {
+				for _, r := range runners {
+					placed, err := r.TryExec(ctx, call)
+					if err != nil {
+						logrus.WithError(err).Error("Failed during call placement")
+					}
+					if placed {
+						return err
+					}
+				}
+			}
+
+			remaining := call.SlotDeadline().Sub(time.Now())
+			if remaining <= 0 {
+				return models.ErrCallTimeoutServerBusy
+			}
+
+			// backoff
+			select {
+			case <-ctx.Done():
+				return models.ErrCallTimeoutServerBusy
+			case <-timeout:
+				return models.ErrCallTimeoutServerBusy
+			case <-time.After(common.MinDuration(retryWaitInterval, remaining)):
+			}
+		}
+	}
+}