From 774689062b4ac1921434a6c7a2ac78b8f29ac85a Mon Sep 17 00:00:00 2001
From: Luke Shumaker <lukeshu@lukeshu.com>
Date: Thu, 5 Jan 2023 19:20:25 -0700
Subject: containers: Add SyncValue and SyncPool types

---
 lib/containers/syncpool.go  | 33 ++++++++++++++++++++++
 lib/containers/syncvalue.go | 67 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 100 insertions(+)
 create mode 100644 lib/containers/syncpool.go
 create mode 100644 lib/containers/syncvalue.go

diff --git a/lib/containers/syncpool.go b/lib/containers/syncpool.go
new file mode 100644
index 0000000..cb5398d
--- /dev/null
+++ b/lib/containers/syncpool.go
@@ -0,0 +1,33 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package containers
+
+import (
+	"sync"
+)
+
+type SyncPool[T any] struct {
+	New func() T
+
+	inner sync.Pool
+}
+
+func (p *SyncPool[T]) Get() (val T, ok bool) {
+	_val := p.inner.Get()
+	switch {
+	case _val != nil:
+		//nolint:forcetypeassert // Typed wrapper around untyped lib.
+		return _val.(T), true
+	case p.New != nil:
+		return p.New(), true
+	default:
+		var zero T
+		return zero, false
+	}
+}
+
+func (p *SyncPool[T]) Put(val T) {
+	p.inner.Put(val)
+}
diff --git a/lib/containers/syncvalue.go b/lib/containers/syncvalue.go
new file mode 100644
index 0000000..160db3c
--- /dev/null
+++ b/lib/containers/syncvalue.go
@@ -0,0 +1,67 @@
+// Copyright (C) 2022-2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package containers
+
+import (
+	"sync"
+)
+
+// SyncValue is a typed equivalent of sync/atomic.Value.
+//
+// It is not actually a wrapper around sync/atomic.Value for
+// allocation-performance reasons.
+type SyncValue[T comparable] struct {
+	mu  sync.Mutex
+	ok  bool
+	val T
+}
+
+// This uses a dumb mutex-based solution because
+//
+//  1. Performance is good enough, because in the fast-path mutexes
+//     use the same compare-and-swap as sync/atomic.Value; and because
+//     all of these methods are short we're unlikely to hit the
+//     mutex's slow path.
+//
+//  2. We could use sync/atomic.Pointer[T], which by itself would have
+//     the same performance characteristics as sync/atomic.Value but
+//     without the benefit of runtime_procPin()/runtime_procUnpin().
+//     We want to avoid that because it means we're doing an
+//     allocation for every store/swap; avoiding that is our whole
+//     reason for not just wraping sync/atomic.Value.  So then we'd
+//     want to use a SyncPool to reuse allocations; but (1) that adds
+//     more sync-overhead, and (2) it also gets trickier because we'd
+//     have to be careful about not adding a pointer back to the pool
+//     when load has grabbed the pointer but not yet dereferenced it.
+
+func (v *SyncValue[T]) Load() (val T, ok bool) {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+	return v.val, v.ok
+}
+
+func (v *SyncValue[T]) Store(val T) {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+	v.val, v.ok = val, true
+}
+
+func (v *SyncValue[T]) Swap(newV T) (oldV T, oldOK bool) {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+	oldV, oldOK = v.val, v.ok
+	v.val, v.ok = newV, true
+	return
+}
+
+func (v *SyncValue[T]) CompareAndSwap(oldV, newV T) (swapped bool) {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+	if !v.ok || v.val != oldV {
+		return false
+	}
+	v.val = newV
+	return true
+}
-- 
cgit v1.1-4-g5e80