1 files changed, 88 insertions, 0 deletions
diff --git a/src/cmd/go/internal/par/queue.go b/src/cmd/go/internal/par/queue.go
new file mode 100644
index 0000000..180bc75
--- /dev/null
+++ b/src/cmd/go/internal/par/queue.go
@@ -0,0 +1,88 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package par
+
+import "fmt"
+
+// Queue manages a set of work items to be executed in parallel. The number of
+// active work items is limited, and excess items are queued sequentially.
+type Queue struct {
+	maxActive int
+	st        chan queueState
+}
+
+type queueState struct {
+	active  int // number of goroutines processing work; always nonzero when len(backlog) > 0
+	backlog []func()
+	idle    chan struct{} // if non-nil, closed when active becomes 0
+}
+
+// NewQueue returns a Queue that executes up to maxActive items in parallel.
+//
+// maxActive must be positive.
+func NewQueue(maxActive int) *Queue {
+	if maxActive < 1 {
+		panic(fmt.Sprintf("par.NewQueue called with nonpositive limit (%d)", maxActive))
+	}
+
+	q := &Queue{
+		maxActive: maxActive,
+		st:        make(chan queueState, 1),
+	}
+	q.st <- queueState{}
+	return q
+}
+
+// Add adds f as a work item in the queue.
+//
+// Add returns immediately, but the queue will be marked as non-idle until after
+// f (and any subsequently-added work) has completed.
+func (q *Queue) Add(f func()) {
+	st := <-q.st
+	if st.active == q.maxActive {
+		st.backlog = append(st.backlog, f)
+		q.st <- st
+		return
+	}
+	if st.active == 0 {
+		// Mark q as non-idle.
+		st.idle = nil
+	}
+	st.active++
+	q.st <- st
+
+	go func() {
+		for {
+			f()
+
+			st := <-q.st
+			if len(st.backlog) == 0 {
+				if st.active--; st.active == 0 && st.idle != nil {
+					close(st.idle)
+				}
+				q.st <- st
+				return
+			}
+			f, st.backlog = st.backlog[0], st.backlog[1:]
+			q.st <- st
+		}
+	}()
+}
+
+// Idle returns a channel that will be closed when q has no (active or enqueued)
+// work outstanding.
+func (q *Queue) Idle() <-chan struct{} {
+	st := <-q.st
+	defer func() { q.st <- st }()
+
+	if st.idle == nil {
+		st.idle = make(chan struct{})
+		if st.active == 0 {
+			close(st.idle)
+		}
+	}
+
+	return st.idle
+}