types/lazy: add DeferredInit type

It is sometimes necessary to defer initialization steps until the first actual usage
or until certain prerequisites have been met. For example, policy setting and
policy source registration should not occur during package initialization.
Instead, they should be deferred until the syspolicy package is actually used.
Additionally, any errors should be properly handled and reported, rather than
causing a panic within the package's init function.

In this PR, we add DeferredInit, to facilitate the registration and invocation
of deferred initialization functions.

Updates #12687

Signed-off-by: Nick Hill <mykola.khyl@gmail.com>

types/lazy/deferred.go

@@ -0,0 +1,98 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lazy
+
+import (
+	"sync"
+	"sync/atomic"
+
+	"tailscale.com/types/ptr"
+)
+
+// DeferredInit allows one or more funcs to be deferred
+// until [DeferredInit.Do] is called for the first time.
+//
+// DeferredInit is safe for concurrent use.
+type DeferredInit struct {
+	DeferredFuncs
+}
+
+// DeferredFuncs allows one or more funcs to be deferred
+// until the owner's [DeferredInit.Do] method is called
+// for the first time.
+//
+// DeferredFuncs is safe for concurrent use.
+type DeferredFuncs struct {
+	m     sync.Mutex
+	funcs []func() error
+
+	// err is either:
+	//    * nil, if deferred init has not yet been completed
+	//    * nilErrPtr, if initialization completed successfully
+	//    * non-nil and not nilErrPtr, if there was an error
+	//
+	// It is an atomic.Pointer so it can be read without m held.
+	err atomic.Pointer[error]
+}
+
+// Defer adds a function to be called when [DeferredInit.Do]
+// is called for the first time. It returns true on success,
+// or false if [DeferredInit.Do] has already been called.
+func (d *DeferredFuncs) Defer(f func() error) bool {
+	d.m.Lock()
+	defer d.m.Unlock()
+	if d.err.Load() != nil {
+		return false
+	}
+	d.funcs = append(d.funcs, f)
+	return true
+}
+
+// MustDefer is like [DeferredFuncs.Defer], but panics
+// if [DeferredInit.Do] has already been called.
+func (d *DeferredFuncs) MustDefer(f func() error) {
+	if !d.Defer(f) {
+		panic("deferred init already completed")
+	}
+}
+
+// Do calls previously deferred init functions if it is being called
+// for the first time on this instance of [DeferredInit].
+// It stops and returns an error if any init function returns an error.
+//
+// It is safe for concurrent use, and the deferred init is guaranteed
+// to have been completed, either successfully or with an error,
+// when Do() returns.
+func (d *DeferredInit) Do() error {
+	err := d.err.Load()
+	if err == nil {
+		err = d.doSlow()
+	}
+	return *err
+}
+
+func (d *DeferredInit) doSlow() (err *error) {
+	d.m.Lock()
+	defer d.m.Unlock()
+	if err := d.err.Load(); err != nil {
+		return err
+	}
+	defer func() {
+		d.err.Store(err)
+		d.funcs = nil // do not keep funcs alive after invoking
+	}()
+	for _, f := range d.funcs {
+		if err := f(); err != nil {
+			return ptr.To(err)
+		}
+	}
+	return nilErrPtr
+}
+
+// Funcs is a shorthand for &d.DeferredFuncs.
+// The returned value can safely be passed to external code,
+// allowing to defer init funcs without also exposing [DeferredInit.Do].
+func (d *DeferredInit) Funcs() *DeferredFuncs {
+	return &d.DeferredFuncs
+}

types/lazy/deferred_test.go

@@ -0,0 +1,277 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lazy
+
+import (
+	"errors"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+func ExampleDeferredInit() {
+	// DeferredInit allows both registration and invocation of the
+	// deferred funcs. It should remain internal to the code that "owns" it.
+	var di DeferredInit
+	// Deferred funcs will not be executed until [DeferredInit.Do] is called.
+	deferred := di.Defer(func() error {
+		fmt.Println("Internal init")
+		return nil
+	})
+	// [DeferredInit.Defer] reports whether the function was successfully deferred.
+	// A func can only fail to defer if [DeferredInit.Do] has already been called.
+	if deferred {
+		fmt.Printf("Internal init has been deferred\n\n")
+	}
+
+	// If necessary, the value returned by [DeferredInit.Funcs]
+	// can be shared with external code to facilitate deferring
+	// funcs without allowing it to call [DeferredInit.Do].
+	df := di.Funcs()
+	// If a certain init step must be completed for the program
+	// to function correctly, and failure to defer it indicates
+	// a coding error, use [DeferredFuncs.MustDefer] instead of
+	// [DeferredFuncs.Defer]. It panics if Do() has already been called.
+	df.MustDefer(func() error {
+		fmt.Println("External init - 1")
+		return nil
+	})
+	// A deferred func may return an error to indicate a failed init.
+	// If a deferred func returns an error, execution stops
+	// and the error is propagated to the caller.
+	df.Defer(func() error {
+		fmt.Println("External init - 2")
+		return errors.New("bang!")
+	})
+	// The deferred function below won't be executed.
+	df.Defer(func() error {
+		fmt.Println("Unreachable")
+		return nil
+	})
+
+	// When [DeferredInit]'s owner needs initialization to be completed,
+	// it can call [DeferredInit.Do]. When called for the first time,
+	// it invokes the deferred funcs.
+	err := di.Do()
+	if err != nil {
+		fmt.Printf("Deferred init failed: %v\n", err)
+	}
+	// [DeferredInit.Do] is safe for concurrent use and can be called
+	// multiple times by the same or different goroutines.
+	// However, the deferred functions are never invoked more than once.
+	// If the deferred init fails on the first attempt, all subsequent
+	// [DeferredInit.Do] calls will return the same error.
+	if err = di.Do(); err != nil {
+		fmt.Printf("Deferred init failed: %v\n\n", err)
+	}
+
+	// Additionally, all subsequent attempts to defer a function will fail
+	// after [DeferredInit.Do] has been called.
+	deferred = di.Defer(func() error {
+		fmt.Println("Unreachable")
+		return nil
+	})
+	if !deferred {
+		fmt.Println("Cannot defer a func once init has been completed")
+	}
+
+	// Output:
+	// Internal init has been deferred
+	//
+	// Internal init
+	// External init - 1
+	// External init - 2
+	// Deferred init failed: bang!
+	// Deferred init failed: bang!
+	//
+	// Cannot defer a func once init has been completed
+}
+
+func TestDeferredInit(t *testing.T) {
+	tests := []struct {
+		name     string
+		numFuncs int
+	}{
+		{
+			name:     "no-funcs",
+			numFuncs: 0,
+		},
+		{
+			name:     "one-func",
+			numFuncs: 1,
+		},
+		{
+			name:     "many-funcs",
+			numFuncs: 1000,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var di DeferredInit
+
+			calls := make([]atomic.Bool, tt.numFuncs) // whether N-th func has been called
+			checkCalls := func() {
+				t.Helper()
+				for i := range calls {
+					if !calls[i].Load() {
+						t.Errorf("Func #%d has never been called", i)
+					}
+				}
+			}
+
+			// Defer funcs concurrently across multiple goroutines.
+			var wg sync.WaitGroup
+			wg.Add(tt.numFuncs)
+			for i := range tt.numFuncs {
+				go func() {
+					f := func() error {
+						if calls[i].Swap(true) {
+							t.Errorf("Func #%d has already been called", i)
+						}
+						return nil
+					}
+					if !di.Defer(f) {
+						t.Errorf("Func #%d cannot be deferred", i)
+						return
+					}
+					wg.Done()
+				}()
+			}
+			// Wait for all funcs to be deferred.
+			wg.Wait()
+
+			// Call [DeferredInit.Do] concurrently.
+			const N = 10000
+			for range N {
+				wg.Add(1)
+				go func() {
+					gotErr := di.Do()
+					checkError(t, gotErr, nil, false)
+					checkCalls()
+					wg.Done()
+				}()
+			}
+			wg.Wait()
+		})
+	}
+}
+
+func TestDeferredErr(t *testing.T) {
+	tests := []struct {
+		name    string
+		funcs   []func() error
+		wantErr error
+	}{
+		{
+			name:    "no-funcs",
+			wantErr: nil,
+		},
+		{
+			name:    "no-error",
+			funcs:   []func() error{func() error { return nil }},
+			wantErr: nil,
+		},
+		{
+			name: "error",
+			funcs: []func() error{
+				func() error { return nil },
+				func() error { return errors.New("bang!") },
+				func() error { return errors.New("unreachable") },
+			},
+			wantErr: errors.New("bang!"),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var di DeferredInit
+			for _, f := range tt.funcs {
+				di.MustDefer(f)
+			}
+
+			var wg sync.WaitGroup
+			N := 10000
+			for range N {
+				wg.Add(1)
+				go func() {
+					gotErr := di.Do()
+					checkError(t, gotErr, tt.wantErr, false)
+					wg.Done()
+				}()
+			}
+			wg.Wait()
+		})
+	}
+}
+
+func TestDeferAfterDo(t *testing.T) {
+	var di DeferredInit
+	var deferred, called atomic.Int32
+
+	deferOnce := func() bool {
+		ok := di.Defer(func() error {
+			called.Add(1)
+			return nil
+		})
+		if ok {
+			deferred.Add(1)
+		}
+		return ok
+	}
+
+	// Deferring a func before calling [DeferredInit.Do] should always succeed.
+	if !deferOnce() {
+		t.Fatal("Failed to defer a func")
+	}
+
+	// Defer up to N funcs concurrently while [DeferredInit.Do] is being called by the main goroutine.
+	// Since we'll likely attempt to defer some funcs after [DeferredInit.Do] has been called,
+	// we expect these late defers to fail, and the funcs will not be deferred or executed.
+	// However, the number of the deferred and called funcs should always be equal when [DeferredInit.Do] exits.
+	const N = 10000
+	var wg sync.WaitGroup
+	for range N {
+		wg.Add(1)
+		go func() {
+			deferOnce()
+			wg.Done()
+		}()
+	}
+
+	if err := di.Do(); err != nil {
+		t.Fatalf("DeferredInit.Do() failed: %v", err)
+	}
+	wantDeferred, wantCalled := deferred.Load(), called.Load()
+
+	if deferOnce() {
+		t.Error("An init func was deferred after DeferredInit.Do() returned")
+	}
+
+	// Wait for the goroutines deferring init funcs to exit.
+	// No funcs should be deferred after DeferredInit.Do() has returned,
+	// so the deferred and called counters should remain unchanged.
+	wg.Wait()
+	if gotDeferred := deferred.Load(); gotDeferred != wantDeferred {
+		t.Errorf("An init func was deferred after DeferredInit.Do() returned. Got %d, want %d", gotDeferred, wantDeferred)
+	}
+	if gotCalled := called.Load(); gotCalled != wantCalled {
+		t.Errorf("An init func was called after DeferredInit.Do() returned. Got %d, want %d", gotCalled, wantCalled)
+	}
+	if deferred, called := deferred.Load(), called.Load(); deferred != called {
+		t.Errorf("Deferred: %d; Called: %d", deferred, called)
+	}
+}
+
+func checkError(tb testing.TB, got, want error, fatal bool) {
+	tb.Helper()
+	f := tb.Errorf
+	if fatal {
+		f = tb.Fatalf
+	}
+	if (want == nil && got != nil) ||
+		(want != nil && got == nil) ||
+		(want != nil && got != nil && want.Error() != got.Error()) {
+		f("gotErr: %v; wantErr: %v", got, want)
+	}
+}