Enable prealloc linter

This will give us nice little performance gains in some code paths.

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

.golangci.yml

@@ -10,6 +10,7 @@ linters:
     - unused
     - misspell
     - bodyclose
+    - prealloc
   disable:
     - errcheck
 

errors.go

@@ -90,7 +90,7 @@ func (ErrManifestUnverified) Error() string {
 type ErrManifestVerification []error
 
 func (errs ErrManifestVerification) Error() string {
-	var parts []string
+	parts := make([]string, 0, len(errs))
 	for _, err := range errs {
 		parts = append(parts, err.Error())
 	}

registry/handlers/app.go

@@ -441,6 +441,7 @@ func (app *App) register(routeName string, dispatch dispatchFunc) {
 // configureEvents prepares the event sink for action.
 func (app *App) configureEvents(configuration *configuration.Configuration) {
 	// Configure all of the endpoint sinks.
+	// nolint:prealloc
 	var sinks []events.Sink
 	for _, endpoint := range configuration.Notifications.Endpoints {
 		if endpoint.Disabled {

registry/proxy/proxytagservice_test.go

@@ -49,7 +49,7 @@ func (m *mockTagStore) All(ctx context.Context) ([]string, error) {
 	m.Lock()
 	defer m.Unlock()
 
-	var tags []string
+	tags := make([]string, 0, len(m.mapping))
 	for tag := range m.mapping {
 		tags = append(tags, tag)
 	}

registry/storage/driver/azure/azure.go

@@ -416,7 +416,7 @@ func directDescendants(blobs []string, prefix string) []string {
 		}
 	}
 
-	var keys []string
+	keys := make([]string, 0, len(out))
 	for k := range out {
 		keys = append(keys, k)
 	}

registry/storage/driver/inmemory/mfs.go

@@ -1,6 +1,7 @@
 package inmemory
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"path"
@@ -97,8 +98,13 @@ func (d *dir) list(p string) ([]string, error) {
 		return nil, errIsNotDir
 	}
 
-	var children []string
-	for _, child := range n.(*dir).children {
+	dir, ok := n.(*dir)
+	if !ok {
+		return nil, errors.New("not a directory")
+	}
+
+	children := make([]string, 0, len(dir.children))
+	for _, child := range dir.children {
 		children = append(children, child.path())
 	}
 

registry/storage/driver/s3-aws/s3.go

@@ -1456,7 +1456,7 @@ func (w *writer) Commit() error {
 	}
 	w.committed = true
 
-	var completedUploadedParts completedParts
+	completedUploadedParts := make(completedParts, 0, len(w.parts))
 	for _, part := range w.parts {
 		completedUploadedParts = append(completedUploadedParts, &s3.CompletedPart{
 			ETag:       part.ETag,

registry/storage/driver/s3-aws/s3_test.go

@@ -500,6 +500,7 @@ func TestWalk(t *testing.T) {
 	}
 
 	// create file structure matching fileset above
+	// nolint:prealloc
 	var created []string
 	for _, p := range fileset {
 		err := drvr.PutContent(context.Background(), p, []byte("content "+p))

registry/storage/tagstore.go

@@ -24,25 +24,24 @@ type tagStore struct {
 
 // All returns all tags
 func (ts *tagStore) All(ctx context.Context) ([]string, error) {
-	var tags []string
-
 	pathSpec, err := pathFor(manifestTagPathSpec{
 		name: ts.repository.Named().Name(),
 	})
 	if err != nil {
-		return tags, err
+		return nil, err
 	}
 
 	entries, err := ts.blobStore.driver.List(ctx, pathSpec)
 	if err != nil {
 		switch err := err.(type) {
 		case storagedriver.PathNotFoundError:
-			return tags, distribution.ErrRepositoryUnknown{Name: ts.repository.Named().Name()}
+			return nil, distribution.ErrRepositoryUnknown{Name: ts.repository.Named().Name()}
 		default:
-			return tags, err
+			return nil, err
 		}
 	}
 
+	tags := make([]string, 0, len(entries))
 	for _, entry := range entries {
 		_, filename := path.Split(entry)
 		tags = append(tags, filename)

testutil/manifests.go

@@ -14,7 +14,7 @@ import (
 func MakeManifestList(blobstatter distribution.BlobStatter, manifestDigests []digest.Digest) (*manifestlist.DeserializedManifestList, error) {
 	ctx := context.Background()
 
-	var manifestDescriptors []manifestlist.ManifestDescriptor
+	manifestDescriptors := make([]manifestlist.ManifestDescriptor, 0, len(manifestDigests))
 	for _, manifestDigest := range manifestDigests {
 		descriptor, err := blobstatter.Stat(ctx, manifestDigest)
 		if err != nil {