Moves out node pool manager behind an extension using runner pool abstraction (Part 2) (#862)

* Move out node-pool manager and replace it with RunnerPool extension

* adds extension points for runner pools in load-balanced mode

* adds error to return values in RunnerPool and Runner interfaces

* Implements runner pool contract with context-aware shutdown

* fixes issue with range

* fixes tests to use runner abstraction

* adds empty test file as a workaround for build requiring go source files in top-level package

* removes flappy timeout test

* update docs to reflect runner pool setup

* refactors system tests to use runner abstraction

* removes poolmanager

* moves runner interfaces from models to api/runnerpool package

* Adds a second runner to pool docs example

* explicitly check for request spillover to second runner in test

* moves runner pool package name for system tests

* renames runner pool pointer variable for consistency

* pass model json to runner

* automatically cast to http.ResponseWriter in load-balanced call case

* allow overriding of server RunnerPool via a programmatic ServerOption

* fixes return type of ResponseWriter in test

* move Placer interface to runnerpool package

* moves hash-based placer out of open source project

* removes siphash from Gopkg.lock

api/agent/static_pool_test.go

@@ -0,0 +1,84 @@
+package agent
+
+import (
+	"context"
+	"testing"
+
+	pool "github.com/fnproject/fn/api/runnerpool"
+)
+
+func setupStaticPool(runners []string) pool.RunnerPool {
+	return newStaticRunnerPool(runners, mockRunnerFactory)
+}
+
+type mockStaticRunner struct {
+	address string
+}
+
+func (r *mockStaticRunner) TryExec(ctx context.Context, call pool.RunnerCall) (bool, error) {
+	return true, nil
+}
+
+func (r *mockStaticRunner) Close(ctx context.Context) error {
+	return nil
+}
+
+func (r *mockStaticRunner) Address() string {
+	return r.address
+}
+
+func mockRunnerFactory(addr string) (pool.Runner, error) {
+	return &mockStaticRunner{address: addr}, nil
+}
+
+func TestNewStaticPool(t *testing.T) {
+	addrs := []string{"127.0.0.1:8080", "127.0.0.1:8081"}
+	np := setupStaticPool(addrs)
+
+	runners, err := np.Runners(nil)
+	if err != nil {
+		t.Fatalf("Failed to list runners %v", err)
+	}
+	if len(runners) != len(addrs) {
+		t.Fatalf("Invalid number of runners %v", len(runners))
+	}
+}
+
+func TestAddNodeToPool(t *testing.T) {
+	addrs := []string{"127.0.0.1:8080", "127.0.0.1:8081"}
+	np := setupStaticPool(addrs).(*staticRunnerPool)
+	np.AddRunner("127.0.0.1:8082")
+
+	runners, err := np.Runners(nil)
+	if err != nil {
+		t.Fatalf("Failed to list runners %v", err)
+	}
+	if len(runners) != 3 {
+		t.Fatalf("Invalid number of runners %v", len(runners))
+	}
+}
+
+func TestRemoveNodeFromPool(t *testing.T) {
+	addrs := []string{"127.0.0.1:8080", "127.0.0.1:8081"}
+	np := setupStaticPool(addrs).(*staticRunnerPool)
+	np.RemoveRunner("127.0.0.1:8081")
+
+	runners, err := np.Runners(nil)
+	if err != nil {
+		t.Fatalf("Failed to list runners %v", err)
+	}
+
+	if len(runners) != 1 {
+		t.Fatalf("Invalid number of runners %v", len(runners))
+	}
+
+	np.RemoveRunner("127.0.0.1:8081")
+
+	runners, err = np.Runners(nil)
+	if err != nil {
+		t.Fatalf("Failed to list runners %v", err)
+	}
+	if len(runners) != 1 {
+		t.Fatalf("Invalid number of runners %v", len(runners))
+	}
+}