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// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause
// Package race contains a helper to "race" two functions, returning the first
// successful result. It also allows explicitly triggering the
// (possibly-waiting) second function when the first function returns an error
// or indicates that it should be retried.
package race
import (
"context"
"errors"
"time"
)
type resultType int
const (
first resultType = iota
second
)
// queryResult is an internal type for storing the result of a function call
type queryResult[T any] struct {
ty resultType
res T
err error
}
// Func is the signature of a function to be called.
type Func[T any] func(context.Context) (T, error)
// Race allows running two functions concurrently and returning the first
// non-error result returned.
type Race[T any] struct {
func1, func2 Func[T]
d time.Duration
results chan queryResult[T]
startFallback chan struct{}
}
// New creates a new Race that, when Start is called, will immediately call
// func1 to obtain a result. After the timeout d or if triggered by an error
// response from func1, func2 will be called.
func New[T any](d time.Duration, func1, func2 Func[T]) *Race[T] {
ret := &Race[T]{
func1: func1,
func2: func2,
d: d,
results: make(chan queryResult[T], 2),
startFallback: make(chan struct{}),
}
return ret
}
// Start will start the "race" process, returning the first non-error result or
// the errors that occurred when calling func1 and/or func2.
func (rh *Race[T]) Start(ctx context.Context) (T, error) {
ctx, cancel := context.WithCancel(ctx)
defer cancel()
// func1 is started immediately
go func() {
ret, err := rh.func1(ctx)
rh.results <- queryResult[T]{first, ret, err}
}()
// func2 is started after a timeout
go func() {
wait := time.NewTimer(rh.d)
defer wait.Stop()
// Wait for our timeout, trigger, or context to finish.
select {
case <-ctx.Done():
// Nothing to do; we're done
var zero T
rh.results <- queryResult[T]{second, zero, ctx.Err()}
return
case <-rh.startFallback:
case <-wait.C:
}
ret, err := rh.func2(ctx)
rh.results <- queryResult[T]{second, ret, err}
}()
// For each possible result, get it off the channel.
var errs []error
for range 2 {
res := <-rh.results
// If this was an error, store it and hope that the other
// result gives us something.
if res.err != nil {
errs = append(errs, res.err)
// Start the fallback function immediately if this is
// the first function's error, to avoid having
// to wait.
if res.ty == first {
close(rh.startFallback)
}
continue
}
// Got a valid response! Return it.
return res.res, nil
}
// If we get here, both raced functions failed. Return whatever errors
// we have, joined together.
var zero T
return zero, errors.Join(errs...)
}
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