genericize atomic config changes

irc/utils/config.go

@@ -0,0 +1,32 @@
+// Copyright (c) 2022 Shivaram Lingamneni
+// released under the MIT license
+
+package utils
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+/*
+This can be used to implement the following pattern:
+
+1. Load and munge a config (this can be arbitrarily expensive)
+2. Use Set() to install the config
+3. Use Get() to access the config
+4. As long as any individual config is not modified (by any goroutine)
+   after the initial call to Set(), this is free of data races, and Get()
+   is extremely cheap (on amd64 it compiles down to plain MOV instructions).
+*/
+
+type ConfigStore[T any] struct {
+	ptr unsafe.Pointer
+}
+
+func (c *ConfigStore[T]) Get() *T {
+	return (*T)(atomic.LoadPointer(&c.ptr))
+}
+
+func (c *ConfigStore[T]) Set(ptr *T) {
+	atomic.StorePointer(&c.ptr, unsafe.Pointer(ptr))
+}