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