functions: common concurrency stream for sync and async (#314)

* functions: add bounded concurrency

* functions: plug runners to sync and async interfaces

* functions: update documentation about the new env var

* functions: fix test flakiness

* functions: the runner is self-regulated, no need to set a number of runners

* functions: push the execution to the background on incoming requests

* functions: ensure async tasks are always on

* functions: add prioritization to tasks consumption

Ensure that Sync tasks are consumed before Async tasks. Also, fixes
termination races problems for free.

* functions: remove stale comments

* functions: improve mem availability calculation

* functions: parallel run for async tasks

* functions: check for memory availability before pulling async task

* functions: comment about rnr.hasAvailableMemory and sync.Cond

* functions: implement memory check for async runners using Cond vars

* functions: code grooming

- remove unnecessary goroutines
- fix stale docs
- reorganize import group

* Revert "functions: implement memory check for async runners using Cond vars"

This reverts commit 922e64032201a177c03ce6a46240925e3d35430d.

* Revert "functions: comment about rnr.hasAvailableMemory and sync.Cond"

This reverts commit 49ad7d52d341f12da9603b1a1df9d145871f0e0a.

* functions: set a minimum memory availability for sync

* functions: simplify the implementation by removing the priority queue

* functions: code grooming

- code deduplication
- review waitgroups Waits

api/server/runner_test.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"bytes"
+	"context"
 	"net/http"
 	"strings"
 	"testing"
@@ -22,11 +23,13 @@ func testRunner(t *testing.T) *runner.Runner {
 
 func TestRouteRunnerGet(t *testing.T) {
 	buf := setLogBuffer()
+	tasks := mockTasksConduit()
+
 	s := New(&datastore.Mock{
 		FakeApps: []*models.App{
 			{Name: "myapp", Config: models.Config{}},
 		},
-	}, &mqs.Mock{}, testRunner(t))
+	}, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {
@@ -61,11 +64,13 @@ func TestRouteRunnerGet(t *testing.T) {
 
 func TestRouteRunnerPost(t *testing.T) {
 	buf := setLogBuffer()
+	tasks := mockTasksConduit()
+
 	s := New(&datastore.Mock{
 		FakeApps: []*models.App{
 			{Name: "myapp", Config: models.Config{}},
 		},
-	}, &mqs.Mock{}, testRunner(t))
+	}, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {
@@ -102,6 +107,13 @@ func TestRouteRunnerPost(t *testing.T) {
 
 func TestRouteRunnerExecution(t *testing.T) {
 	buf := setLogBuffer()
+
+	tasks := make(chan runner.TaskRequest)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go runner.StartWorkers(ctx, testRunner(t), tasks)
+
 	s := New(&datastore.Mock{
 		FakeApps: []*models.App{
 			{Name: "myapp", Config: models.Config{}},
@@ -110,7 +122,7 @@ func TestRouteRunnerExecution(t *testing.T) {
 			{Path: "/myroute", AppName: "myapp", Image: "iron/hello", Headers: map[string][]string{"X-Function": {"Test"}}},
 			{Path: "/myerror", AppName: "myapp", Image: "iron/error", Headers: map[string][]string{"X-Function": {"Test"}}},
 		},
-	}, &mqs.Mock{}, testRunner(t))
+	}, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {