Make result order deterministic

pool/result_context_pool.go

@@ -20,11 +20,10 @@ type ResultContextPool[T any] struct {
 // Go submits a task to the pool. If all goroutines in the pool
 // are busy, a call to Go() will block until the task can be started.
 func (p *ResultContextPool[T]) Go(f func(context.Context) (T, error)) {
+	idx := p.agg.nextIndex()
 	p.contextPool.Go(func(ctx context.Context) error {
 		res, err := f(ctx)
-		if err == nil || p.collectErrored {
-			p.agg.add(res)
-		}
+		p.agg.save(idx, res, err != nil)
 		return err
 	})
 }
@@ -33,7 +32,7 @@ func (p *ResultContextPool[T]) Go(f func(context.Context) (T, error)) {
 // returns an error if any of the tasks errored.
 func (p *ResultContextPool[T]) Wait() ([]T, error) {
 	err := p.contextPool.Wait()
-	return p.agg.results, err
+	return p.agg.collect(p.collectErrored), err
 }
 
 // WithCollectErrored configures the pool to still collect the result of a task

pool/result_context_pool_test.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"sort"
 	"strconv"
 	"sync/atomic"
 	"testing"
@@ -223,7 +222,6 @@ func TestResultContextPool(t *testing.T) {
 					})
 				}
 				res, err := g.Wait()
-				sort.Ints(res)
 				require.Equal(t, expected, res)
 				require.NoError(t, err)
 				require.Equal(t, int64(0), currentConcurrent.Load())

pool/result_error_pool.go

@@ -8,9 +8,8 @@ import (
 // type and an error. Tasks are executed in the pool with Go(), then the
 // results of the tasks are returned by Wait().
 //
-// The order of the results is not guaranteed to be the same as the order the
-// tasks were submitted. If your use case requires consistent ordering,
-// consider using the `stream` package or `Map` from the `iter` package.
+// The order of the results is guaranteed to be the same as the order the
+// tasks were submitted.
 //
 // The configuration methods (With*) will panic if they are used after calling
 // Go() for the first time.
@@ -23,11 +22,10 @@ type ResultErrorPool[T any] struct {
 // Go submits a task to the pool. If all goroutines in the pool
 // are busy, a call to Go() will block until the task can be started.
 func (p *ResultErrorPool[T]) Go(f func() (T, error)) {
+	idx := p.agg.nextIndex()
 	p.errorPool.Go(func() error {
 		res, err := f()
-		if err == nil || p.collectErrored {
-			p.agg.add(res)
-		}
+		p.agg.save(idx, res, err != nil)
 		return err
 	})
 }
@@ -36,7 +34,7 @@ func (p *ResultErrorPool[T]) Go(f func() (T, error)) {
 // returning the results and any errors from tasks.
 func (p *ResultErrorPool[T]) Wait() ([]T, error) {
 	err := p.errorPool.Wait()
-	return p.agg.results, err
+	return p.agg.collect(p.collectErrored), err
 }
 
 // WithCollectErrored configures the pool to still collect the result of a task

pool/result_error_pool_test.go

@@ -13,7 +13,7 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestResultErrorGroup(t *testing.T) {
+func TestResultErrorPool(t *testing.T) {
 	t.Parallel()
 
 	err1 := errors.New("err1")

pool/result_pool.go

@@ -2,6 +2,7 @@ package pool
 
 import (
 	"context"
+	"sort"
 	"sync"
 )
 
@@ -19,9 +20,8 @@ func NewWithResults[T any]() *ResultPool[T] {
 // Tasks are executed in the pool with Go(), then the results of the tasks are
 // returned by Wait().
 //
-// The order of the results is not guaranteed to be the same as the order the
-// tasks were submitted. If your use case requires consistent ordering,
-// consider using the `stream` package or `Map` from the `iter` package.
+// The order of the results is guaranteed to be the same as the order the
+// tasks were submitted.
 type ResultPool[T any] struct {
 	pool Pool
 	agg  resultAggregator[T]
@@ -30,16 +30,17 @@ type ResultPool[T any] struct {
 // Go submits a task to the pool. If all goroutines in the pool
 // are busy, a call to Go() will block until the task can be started.
 func (p *ResultPool[T]) Go(f func() T) {
+	idx := p.agg.nextIndex()
 	p.pool.Go(func() {
-		p.agg.add(f())
+		p.agg.save(idx, f(), false)
 	})
 }
 
 // Wait cleans up all spawned goroutines, propagating any panics, and returning
 // a slice of results from tasks that did not panic.
 func (p *ResultPool[T]) Wait() []T {
 	p.pool.Wait()
-	return p.agg.results
+	return p.agg.collect(true)
 }
 
 // MaxGoroutines returns the maximum size of the pool.
@@ -83,11 +84,57 @@ func (p *ResultPool[T]) panicIfInitialized() {
 // goroutines. The zero value is valid and ready to use.
 type resultAggregator[T any] struct {
 	mu      sync.Mutex
+	len     int
 	results []T
+	errored []int
 }
 
-func (r *resultAggregator[T]) add(res T) {
+// nextIndex reserves a slot for a result. The returned value should be passed
+// to save() when adding a result to the aggregator.
+func (r *resultAggregator[T]) nextIndex() int {
 	r.mu.Lock()
-	r.results = append(r.results, res)
-	r.mu.Unlock()
+	defer r.mu.Unlock()
+
+	nextIdx := r.len
+	r.len += 1
+	return nextIdx
+}
+
+func (r *resultAggregator[T]) save(i int, res T, errored bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if i >= len(r.results) {
+		old := r.results
+		r.results = make([]T, r.len)
+		copy(r.results, old)
+	}
+
+	r.results[i] = res
+
+	if errored {
+		r.errored = append(r.errored, i)
+	}
+}
+
+// collect returns the set of aggregated results.
+func (r *resultAggregator[T]) collect(collectErrored bool) []T {
+	if !r.mu.TryLock() {
+		panic("collect should not be called until all goroutines have exited")
+	}
+
+	if collectErrored || len(r.errored) == 0 {
+		return r.results
+	}
+
+	filtered := r.results[:0]
+	sort.Ints(r.errored)
+	for i, e := range r.errored {
+		if i == 0 {
+			filtered = append(filtered, r.results[:e]...)
+		} else {
+			filtered = append(filtered, r.results[r.errored[i-1]+1:e]...)
+		}
+	}
+	return filtered
 }

pool/result_pool_test.go

@@ -2,7 +2,7 @@ package pool_test
 
 import (
 	"fmt"
-	"sort"
+	"math/rand"
 	"strconv"
 	"sync/atomic"
 	"testing"
@@ -22,8 +22,6 @@ func ExampleResultPool() {
 		})
 	}
 	res := p.Wait()
-	// Result order is nondeterministic, so sort them first
-	sort.Ints(res)
 	fmt.Println(res)
 
 	// Output:
@@ -62,10 +60,29 @@ func TestResultGroup(t *testing.T) {
 			})
 		}
 		res := g.Wait()
-		sort.Ints(res)
 		require.Equal(t, expected, res)
 	})
 
+	t.Run("deterministic order", func(t *testing.T) {
+		t.Parallel()
+		p := pool.NewWithResults[int]()
+		results := make([]int, 100)
+		for i := 0; i < 100; i++ {
+			results[i] = i
+		}
+		for _, result := range results {
+			result := result
+			p.Go(func() int {
+				// Add a random sleep to make it exceedingly unlikely that the
+				// results are returned in the order they are submitted.
+				time.Sleep(time.Duration(rand.Int()%100) * time.Millisecond)
+				return result
+			})
+		}
+		got := p.Wait()
+		require.Equal(t, results, got)
+	})
+
 	t.Run("limit", func(t *testing.T) {
 		t.Parallel()
 		for _, maxGoroutines := range []int{1, 10, 100} {
@@ -90,7 +107,6 @@ func TestResultGroup(t *testing.T) {
 					})
 				}
 				res := g.Wait()
-				sort.Ints(res)
 				require.Equal(t, expected, res)
 				require.Equal(t, int64(0), errCount.Load())
 				require.Equal(t, int64(0), currentConcurrent.Load())