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tailscale/types/lazy/unsync.go

100 lines
2.4 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package lazy
// GValue is a lazily computed value.
//
// Use either Get or GetErr, depending on whether your fill function returns an
// error.
//
// Recursive use of a GValue from its own fill function will panic.
//
// GValue is not safe for concurrent use. (Mnemonic: G is for one Goroutine,
// which isn't strictly true if you provide your own synchronization between
// goroutines, but in practice most of our callers have been using it within
// a single goroutine.)
type GValue[T any] struct {
done bool
calling bool
V T
err error
}
// Set attempts to set z's value to val, and reports whether it succeeded.
// Set only succeeds if none of Get/GetErr/Set have been called before.
func (z *GValue[T]) Set(v T) bool {
if z.done {
return false
}
if z.calling {
panic("Set while Get fill is running")
}
z.V = v
z.done = true
return true
}
// MustSet sets z's value to val, or panics if z already has a value.
func (z *GValue[T]) MustSet(val T) {
if !z.Set(val) {
panic("Set after already filled")
}
}
// Get returns z's value, calling fill to compute it if necessary.
// f is called at most once.
func (z *GValue[T]) Get(fill func() T) T {
if !z.done {
if z.calling {
panic("recursive lazy fill")
}
z.calling = true
z.V = fill()
z.done = true
z.calling = false
}
return z.V
}
// GetErr returns z's value, calling fill to compute it if necessary.
// f is called at most once, and z remembers both of fill's outputs.
func (z *GValue[T]) GetErr(fill func() (T, error)) (T, error) {
if !z.done {
if z.calling {
panic("recursive lazy fill")
}
z.calling = true
z.V, z.err = fill()
z.done = true
z.calling = false
}
return z.V, z.err
}
// GFunc wraps a function to make it lazy.
//
// The returned function calls fill the first time it's called, and returns
// fill's result on every subsequent call.
//
// The returned function is not safe for concurrent use.
func GFunc[T any](fill func() T) func() T {
var v GValue[T]
return func() T {
return v.Get(fill)
}
}
// SyncFuncErr wraps a function to make it lazy.
//
// The returned function calls fill the first time it's called, and returns
// fill's results on every subsequent call.
//
// The returned function is not safe for concurrent use.
func GFuncErr[T any](fill func() (T, error)) func() (T, error) {
var v GValue[T]
return func() (T, error) {
return v.GetErr(fill)
}
}