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// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause
// Package feature tracks which features are linked into the binary.
package feature
import (
"errors"
"reflect"
"tailscale.com/util/testenv"
)
var ErrUnavailable = errors.New("feature not included in this build")
var in = map[string]bool{}
// Registered reports the set of registered features.
//
// The returned map should not be modified by the caller,
// not accessed concurrently with calls to Register.
func Registered() map[string]bool { return in }
// Register notes that the named feature is linked into the binary.
func Register(name string) {
if _, ok := in[name]; ok {
panic("duplicate feature registration for " + name)
}
in[name] = true
}
// Hook is a func that can only be set once.
//
// It is not safe for concurrent use.
type Hook[Func any] struct {
f Func
ok bool
}
// IsSet reports whether the hook has been set.
func (h *Hook[Func]) IsSet() bool {
return h.ok
}
// Set sets the hook function, panicking if it's already been set
// or f is the zero value.
//
// It's meant to be called in init.
func (h *Hook[Func]) Set(f Func) {
if h.ok {
panic("Set on already-set feature hook")
}
if reflect.ValueOf(f).IsZero() {
panic("Set with zero value")
}
h.f = f
h.ok = true
}
// SetForTest sets the hook function for tests, blowing
// away any previous value. It will panic if called from
// non-test code.
//
// It returns a restore function that resets the hook
// to its previous value.
func (h *Hook[Func]) SetForTest(f Func) (restore func()) {
testenv.AssertInTest()
old := *h
h.f, h.ok = f, true
return func() { *h = old }
}
// Get returns the hook function, or panics if it hasn't been set.
// Use IsSet to check if it's been set, or use GetOrNil if you're
// okay with a nil return value.
func (h *Hook[Func]) Get() Func {
if !h.ok {
panic("Get on unset feature hook, without IsSet")
}
return h.f
}
// GetOk returns the hook function and true if it has been set,
// otherwise its zero value and false.
func (h *Hook[Func]) GetOk() (f Func, ok bool) {
return h.f, h.ok
}
// GetOrNil returns the hook function or nil if it hasn't been set.
func (h *Hook[Func]) GetOrNil() Func {
return h.f
}
// Hooks is a slice of funcs.
//
// As opposed to a single Hook, this is meant to be used when
// multiple parties are able to install the same hook.
type Hooks[Func any] []Func
// Add adds a hook to the list of hooks.
//
// Add should only be called during early program
// startup before Tailscale has started.
// It is not safe for concurrent use.
func (h *Hooks[Func]) Add(f Func) {
if reflect.ValueOf(f).IsZero() {
panic("Add with zero value")
}
*h = append(*h, f)
}
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