initial release

internal/singleflight/singleflight.go

@@ -0,0 +1,75 @@
+// Original Copyright 2013 The Go Authors. All rights reserved.
+//
+// Modified by BuzzFeed to return duplicate counts.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package singleflight provides a duplicate function call suppression mechanism.
+package singleflight // import "github.com/pomerium/pomerium/internal/singleflight"
+
+import "sync"
+
+// call is an in-flight or completed singleflight.Do call
+type call struct {
+	wg sync.WaitGroup
+
+	// These fields are written once before the WaitGroup is done
+	// and are only read after the WaitGroup is done.
+	val interface{}
+	err error
+
+	// These fields are read and written with the singleflight
+	// mutex held before the WaitGroup is done, and are read but
+	// not written after the WaitGroup is done.
+	dups int
+}
+
+// Group represents a class of work and forms a namespace in
+// which units of work can be executed with duplicate suppression.
+type Group struct {
+	mu sync.Mutex       // protects m
+	m  map[string]*call // lazily initialized
+}
+
+// Result holds the results of Do, so they can be passed
+// on a channel.
+type Result struct {
+	Val   interface{}
+	Err   error
+	Count bool
+}
+
+// Do executes and returns the results of the given function, making
+// sure that only one execution is in-flight for a given key at a
+// time. If a duplicate comes in, the duplicate caller waits for the
+// original to complete and receives the same results.
+// The return value of Count indicates how many tiems v was given to multiple callers.
+// Count will be zero for requests are shared and only be non-zero for the originating request.
+func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, count int, err error) {
+	g.mu.Lock()
+
+	if g.m == nil {
+		g.m = make(map[string]*call)
+	}
+	if c, ok := g.m[key]; ok {
+		c.dups++
+		g.mu.Unlock()
+		c.wg.Wait()
+		return c.val, 0, c.err
+	}
+	c := new(call)
+	c.wg.Add(1)
+	g.m[key] = c
+
+	g.mu.Unlock()
+
+	c.val, c.err = fn()
+	c.wg.Done()
+
+	g.mu.Lock()
+	delete(g.m, key)
+	g.mu.Unlock()
+
+	return c.val, c.dups, c.err
+}