Use conc (#52805)

## Test plan

<!-- All pull requests REQUIRE a test plan:
https://docs.sourcegraph.com/dev/background-information/testing_principles
-->

Author: camdencheek

internal/goroutine/BUILD.bazel

@@ -193,7 +193,7 @@ func (s *gcsStore) create(ctx context.Context, bucket gcsBucketHandle) error {
 }
 
 func (s *gcsStore) deleteSources(ctx context.Context, bucket gcsBucketHandle, sources []string) error {
-	return RunWorkersOverStrings(sources, func(index int, source string) error {
+	return ForEachString(sources, func(index int, source string) error {
 		if err := bucket.Object(source).Delete(ctx); err != nil {
 			return errors.Wrap(err, "failed to delete source object")
 		}

internal/uploadstore/BUILD.bazel

@@ -2,81 +2,24 @@ package uploadstore
 
 import (
 	"runtime"
-	"sync"
 
-	"github.com/sourcegraph/sourcegraph/lib/errors"
+	"github.com/sourcegraph/conc/pool"
 )
 
-// poolWorker is a function invoked by RunWorkers that sends
-// any errors that occur during execution down a shared channel.
-type poolWorker func(errs chan<- error)
-
-// runWorkersN invokes the given worker n times and collects the
-// errors from each invocation.
-func runWorkersN(n int, worker poolWorker) (err error) {
-	errs := make(chan error, n)
-
-	var wg sync.WaitGroup
-	for i := 0; i < n; i++ {
-		wg.Add(1)
-		go func() { worker(errs); wg.Done() }()
-	}
-
-	go func() {
-		wg.Wait()
-		close(errs)
-	}()
-
-	for e := range errs {
-		if err == nil {
-			err = e
-		} else {
-			err = errors.Append(err, e)
-		}
-	}
-
-	return err
-}
-
-// RunWorkersOverStrings invokes the given worker once for each of the
-// given string values. The worker function will receive the index as well
-// as the string value as parameters. Workers will be invoked in a number
+// ForEachString invokes the given callback once for each of the
+// given string values. The callback function will receive the index as well
+// as the string value as parameters. Callbacks will be invoked in a number
 // of concurrent routines proportional to the maximum number of CPUs that
 // can be executing simultaneously.
-func RunWorkersOverStrings(values []string, worker func(index int, value string) error) error {
-	return runWorkersOverStringsN(runtime.GOMAXPROCS(0), values, worker)
-}
-
-// RunWorkersOverStrings invokes the given worker once for each of the
-// given string values. The worker function will receive the index as well
-// as the string value as parameters. Workers will be invoked in n concurrent
-// routines.
-func runWorkersOverStringsN(n int, values []string, worker func(index int, value string) error) error {
-	return runWorkersN(n, indexedStringWorker(loadIndexedStringChannel(values), worker))
-}
-
-type indexedString struct {
-	index int
-	value string
-}
-
-func loadIndexedStringChannel(values []string) <-chan indexedString {
-	ch := make(chan indexedString, len(values))
-	defer close(ch)
-
+func ForEachString(values []string, f func(index int, value string) error) error {
+	p := pool.New().
+		WithErrors().
+		WithMaxGoroutines(runtime.GOMAXPROCS(0))
 	for i, value := range values {
-		ch <- indexedString{index: i, value: value}
-	}
-
-	return ch
-}
-
-func indexedStringWorker(ch <-chan indexedString, worker func(index int, value string) error) poolWorker {
-	return func(errs chan<- error) {
-		for value := range ch {
-			if err := worker(value.index, value.value); err != nil {
-				errs <- err
-			}
-		}
+		i, value := i, value
+		p.Go(func() error {
+			return f(i, value)
+		})
 	}
+	return p.Wait()
 }

internal/uploadstore/gcs_client.go

@@ -200,7 +200,7 @@ func (s *s3Store) Compose(ctx context.Context, destination string, sources ...st
 	var m sync.Mutex
 	etags := map[int]*string{}
 
-	if err := RunWorkersOverStrings(sources, func(index int, source string) error {
+	if err := ForEachString(sources, func(index int, source string) error {
 		partNumber := index + 1
 
 		copyResult, err := s.client.UploadPartCopy(ctx, &s3.UploadPartCopyInput{
@@ -331,7 +331,7 @@ func (s *s3Store) create(ctx context.Context) error {
 }
 
 func (s *s3Store) deleteSources(ctx context.Context, bucket string, sources []string) error {
-	return RunWorkersOverStrings(sources, func(index int, source string) error {
+	return ForEachString(sources, func(index int, source string) error {
 		if _, err := s.client.DeleteObject(ctx, &s3.DeleteObjectInput{
 			Bucket: aws.String(bucket),
 			Key:    aws.String(source),