feat: add env var for setting max task concurrency

The GARDEN_TASK_CONCURRENCY_LIMIT env variable can now be used to
override the task graph's default concurrency limit. Values should be
integers.

Also set this env variable to 10 in our CircleCI config.

.circleci/config.yml

@@ -11,6 +11,7 @@ version: 2.1
       - image: circleci/node:10
         environment:
           GARDEN_DISABLE_VERSION_CHECK: "true"
+          GARDEN_TASK_CONCURRENCY_LIMIT: "10"
 
   # Attach's the current saved workspace
   attach-workspace: &attach-workspace

garden-service/src/task-graph.ts

@@ -41,6 +41,10 @@ export interface TaskResults {
 
 export const DEFAULT_CONCURRENCY = 6
 
+const concurrencyFromEnv = process.env.GARDEN_TASK_CONCURRENCY_LIMIT
+
+export const TASK_CONCURRENCY = (concurrencyFromEnv && parseInt(concurrencyFromEnv, 10)) || DEFAULT_CONCURRENCY
+
 export class TaskGraph {
   private roots: TaskNodeMap
   private index: TaskNodeMap
@@ -57,7 +61,7 @@ export class TaskGraph {
   private resultCache: ResultCache
   private opQueue: PQueue
 
-  constructor(private garden: Garden, private log: LogEntry, private concurrency: number = DEFAULT_CONCURRENCY) {
+  constructor(private garden: Garden, private log: LogEntry, private concurrency: number = TASK_CONCURRENCY) {
     this.roots = new TaskNodeMap()
     this.index = new TaskNodeMap()
     this.inProgress = new TaskNodeMap()