linting: Consider pre-allocating sliceVar (prealloc)

While updating, also addressed some redundant fmt.Sprintf()

    opts/throttledevice.go:86:2: Consider pre-allocating `out` (prealloc)
        var out []string
        ^
    opts/ulimit.go:37:2: Consider pre-allocating `out` (prealloc)
        var out []string
        ^
    opts/ulimit.go:47:2: Consider pre-allocating `ulimits` (prealloc)
        var ulimits []*units.Ulimit
        ^
    opts/weightdevice.go:68:2: Consider pre-allocating `out` (prealloc)
        var out []string
        ^
    cli/context/store/metadatastore.go:96:2: Consider pre-allocating `res` (prealloc)
        var res []Metadata
        ^
    cli/context/store/store.go:127:2: Consider pre-allocating `names` (prealloc)
        var names []string
        ^
    cli/compose/loader/loader.go:223:2: Consider pre-allocating `keys` (prealloc)
        var keys []string
        ^
    cli/compose/loader/loader.go:397:2: Consider pre-allocating `services` (prealloc)
        var services []types.ServiceConfig
        ^
    cli/command/stack/loader/loader.go:63:2: Consider pre-allocating `msgs` (prealloc)
        var msgs []string
        ^
    cli/command/stack/loader/loader.go:118:2: Consider pre-allocating `configFiles` (prealloc)
        var configFiles []composetypes.ConfigFile
        ^
    cli/command/formatter/container.go:245:2: Consider pre-allocating `joinLabels` (prealloc)
        var joinLabels []string
        ^
    cli/command/formatter/container.go:265:2: Consider pre-allocating `mounts` (prealloc)
        var mounts []string
        ^
    cli/command/formatter/container.go:316:2: Consider pre-allocating `result` (prealloc)
        var result []string
        ^
    cli/command/formatter/displayutils.go:43:2: Consider pre-allocating `display` (prealloc)
        var (
        ^
    cli/command/formatter/volume.go:103:2: Consider pre-allocating `joinLabels` (prealloc)
        var joinLabels []string
        ^
    cli-plugins/manager/manager_test.go:49:2: Consider pre-allocating `dirs` (prealloc)
        var dirs []string
        ^
    cli/command/swarm/init.go:69:2: Consider pre-allocating `defaultAddrPool` (prealloc)
        var defaultAddrPool []string
        ^
    cli/command/manifest/push.go:195:2: Consider pre-allocating `blobReqs` (prealloc)
        var blobReqs []manifestBlob
        ^
    cli/command/secret/formatter.go:111:2: Consider pre-allocating `joinLabels` (prealloc)
        var joinLabels []string
        ^
    cli/command/network/formatter.go:104:2: Consider pre-allocating `joinLabels` (prealloc)
        var joinLabels []string
        ^
    cli/command/context/list.go:52:2: Consider pre-allocating `contexts` (prealloc)
        var contexts []*formatter.ClientContext
        ^
    cli/command/config/formatter.go:104:2: Consider pre-allocating `joinLabels` (prealloc)
        var joinLabels []string
        ^
    cli/command/trust/common_test.go:23:2: Consider pre-allocating `targetNames` (prealloc)
        var targetNames []string
        ^
    cli/command/service/generic_resource_opts.go:55:2: Consider pre-allocating `generic` (prealloc)
        var generic []swarm.GenericResource
        ^
    cli/command/service/generic_resource_opts.go:98:2: Consider pre-allocating `l` (prealloc)
        var l []swarm.GenericResource
        ^
    cli/command/service/opts.go:378:2: Consider pre-allocating `netAttach` (prealloc)
        var netAttach []swarm.NetworkAttachmentConfig
        ^
    cli/command/service/update.go:731:2: Consider pre-allocating `limits` (prealloc)
        var limits []*units.Ulimit
        ^
    cli/command/service/update.go:1315:2: Consider pre-allocating `newNetworks` (prealloc)
        var newNetworks []swarm.NetworkAttachmentConfig
        ^
    cli/command/service/update.go:1514:2: Consider pre-allocating `out` (prealloc)
        var out []string
        ^
    cli/compose/convert/service.go:713:2: Consider pre-allocating `ulimits` (prealloc)
        var ulimits []*units.Ulimit
        ^
    cli/compose/convert/volume.go:13:2: Consider pre-allocating `mounts` (prealloc)
        var mounts []mount.Mount
        ^
    cli/command/stack/swarm/list.go:39:2: Consider pre-allocating `stacks` (prealloc)
        var stacks []*formatter.Stack
        ^

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli-plugins/manager/manager_test.go

@@ -46,7 +46,7 @@ func TestListPluginCandidates(t *testing.T) {
 	)
 	defer dir.Remove()
 
-	var dirs []string
+	dirs := make([]string, 0, 6)
 	for _, d := range []string{"plugins1", "nonexistent", "plugins2", "plugins3", "plugins4", "plugins5"} {
 		dirs = append(dirs, dir.Join(d))
 	}

cli/command/config/formatter.go

@@ -101,7 +101,7 @@ func (c *configContext) Labels() string {
 	if mapLabels == nil {
 		return ""
 	}
-	var joinLabels []string
+	joinLabels := make([]string, 0, len(mapLabels))
 	for k, v := range mapLabels {
 		joinLabels = append(joinLabels, k+"="+v)
 	}

cli/command/context/list.go

@@ -51,7 +51,7 @@ func runList(dockerCli command.Cli, opts *listOptions) error {
 	var (
 		curContext = dockerCli.CurrentContext()
 		curFound   bool
-		contexts   []*formatter.ClientContext
+		contexts   = make([]*formatter.ClientContext, 0, len(contextMap))
 	)
 	for _, rawMeta := range contextMap {
 		isCurrent := rawMeta.Name == curContext

cli/command/formatter/container.go

@@ -245,7 +245,7 @@ func (c *ContainerContext) Labels() string {
 		return ""
 	}
 
-	var joinLabels []string
+	joinLabels := make([]string, 0, len(c.c.Labels))
 	for k, v := range c.c.Labels {
 		joinLabels = append(joinLabels, k+"="+v)
 	}
@@ -265,7 +265,7 @@ func (c *ContainerContext) Label(name string) string {
 // If the trunc option is set, names can be truncated (ellipsized).
 func (c *ContainerContext) Mounts() string {
 	var name string
-	var mounts []string
+	mounts := make([]string, 0, len(c.c.Mounts))
 	for _, m := range c.c.Mounts {
 		if m.Name == "" {
 			name = m.Source
@@ -289,7 +289,7 @@ func (c *ContainerContext) LocalVolumes() string {
 		}
 	}
 
-	return fmt.Sprintf("%d", count)
+	return strconv.Itoa(count)
 }
 
 // Networks returns a comma-separated string of networks that the container is
@@ -299,7 +299,7 @@ func (c *ContainerContext) Networks() string {
 		return ""
 	}
 
-	networks := []string{}
+	networks := make([]string, 0, len(c.c.NetworkSettings.Networks))
 	for k := range c.c.NetworkSettings.Networks {
 		networks = append(networks, k)
 	}
@@ -316,7 +316,7 @@ func DisplayablePorts(ports []types.Port) string {
 		last  uint16
 	}
 	groupMap := make(map[string]*portGroup)
-	var result []string
+	var result []string //nolint:prealloc
 	var hostMappings []string
 	var groupMapKeys []string
 	sort.Slice(ports, func(i, j int) bool {
@@ -331,7 +331,7 @@ func DisplayablePorts(ports []types.Port) string {
 				hostMappings = append(hostMappings, fmt.Sprintf("%s:%d->%d/%s", port.IP, port.PublicPort, port.PrivatePort, port.Type))
 				continue
 			}
-			portKey = fmt.Sprintf("%s/%s", port.IP, port.Type)
+			portKey = port.IP + "/" + port.Type
 		}
 		group := groupMap[portKey]
 
@@ -372,7 +372,7 @@ func formGroup(key string, start, last uint16) string {
 	if ip != "" {
 		group = fmt.Sprintf("%s:%s->%s", ip, group, group)
 	}
-	return fmt.Sprintf("%s/%s", group, groupType)
+	return group + "/" + groupType
 }
 
 func comparePorts(i, j types.Port) bool {

cli/command/formatter/displayutils.go

@@ -41,7 +41,7 @@ func Ellipsis(s string, maxDisplayWidth int) string {
 	}
 
 	var (
-		display      []int
+		display      = make([]int, 0, len(rs))
 		displayWidth int
 	)
 	for _, r := range rs {

cli/command/formatter/volume.go

@@ -100,7 +100,7 @@ func (c *volumeContext) Labels() string {
 		return ""
 	}
 
-	var joinLabels []string
+	joinLabels := make([]string, 0, len(c.v.Labels))
 	for k, v := range c.v.Labels {
 		joinLabels = append(joinLabels, k+"="+v)
 	}

cli/command/manifest/push.go

@@ -192,9 +192,9 @@ func buildManifestDescriptor(targetRepo *registry.RepositoryInfo, imageManifest
 }
 
 func buildBlobRequestList(imageManifest types.ImageManifest, repoName reference.Named) ([]manifestBlob, error) {
-	var blobReqs []manifestBlob
-
-	for _, blobDigest := range imageManifest.Blobs() {
+	blobs := imageManifest.Blobs()
+	blobReqs := make([]manifestBlob, 0, len(blobs))
+	for _, blobDigest := range blobs {
 		canonical, err := reference.WithDigest(repoName, blobDigest)
 		if err != nil {
 			return nil, err

cli/command/network/formatter.go

@@ -101,7 +101,7 @@ func (c *networkContext) Labels() string {
 		return ""
 	}
 
-	var joinLabels []string
+	joinLabels := make([]string, 0, len(c.n.Labels))
 	for k, v := range c.n.Labels {
 		joinLabels = append(joinLabels, k+"="+v)
 	}

cli/command/secret/formatter.go

@@ -108,7 +108,7 @@ func (c *secretContext) Labels() string {
 	if mapLabels == nil {
 		return ""
 	}
-	var joinLabels []string
+	joinLabels := make([]string, 0, len(mapLabels))
 	for k, v := range mapLabels {
 		joinLabels = append(joinLabels, k+"="+v)
 	}

cli/command/service/generic_resource_opts.go

@@ -52,7 +52,7 @@ func ParseGenericResources(value []string) ([]swarm.GenericResource, error) {
 
 // genericResourcesFromGRPC converts a GRPC GenericResource to a GenericResource
 func genericResourcesFromGRPC(genericRes []*swarmapi.GenericResource) []swarm.GenericResource {
-	var generic []swarm.GenericResource
+	generic := make([]swarm.GenericResource, 0, len(genericRes))
 	for _, res := range genericRes {
 		var current swarm.GenericResource
 
@@ -95,7 +95,7 @@ func buildGenericResourceMap(genericRes []swarm.GenericResource) (map[string]swa
 }
 
 func buildGenericResourceList(genericRes map[string]swarm.GenericResource) []swarm.GenericResource {
-	var l []swarm.GenericResource
+	l := make([]swarm.GenericResource, 0, len(genericRes))
 
 	for _, res := range genericRes {
 		l = append(l, res)

cli/command/service/opts.go

@@ -378,8 +378,9 @@ func resolveNetworkID(ctx context.Context, apiClient client.NetworkAPIClient, ne
 }
 
 func convertNetworks(networks opts.NetworkOpt) []swarm.NetworkAttachmentConfig {
-	var netAttach []swarm.NetworkAttachmentConfig
-	for _, net := range networks.Value() {
+	nws := networks.Value()
+	netAttach := make([]swarm.NetworkAttachmentConfig, 0, len(nws))
+	for _, net := range nws {
 		netAttach = append(netAttach, swarm.NetworkAttachmentConfig{
 			Target:     net.Target,
 			Aliases:    net.Aliases,

cli/command/service/update.go

@@ -727,8 +727,10 @@ func updateUlimits(flags *pflag.FlagSet, ulimits []*units.Ulimit) []*units.Ulimi
 			newUlimits[ulimit.Name] = ulimit
 		}
 	}
-
-	var limits []*units.Ulimit
+	if len(newUlimits) == 0 {
+		return nil
+	}
+	limits := make([]*units.Ulimit, 0, len(newUlimits))
 	for _, ulimit := range newUlimits {
 		limits = append(limits, ulimit)
 	}
@@ -1307,7 +1309,7 @@ func updateNetworks(ctx context.Context, apiClient client.NetworkAPIClient, flag
 	}
 
 	existingNetworks := make(map[string]struct{})
-	var newNetworks []swarm.NetworkAttachmentConfig
+	var newNetworks []swarm.NetworkAttachmentConfig //nolint:prealloc
 	for _, network := range specNetworks {
 		if _, exists := idsToRemove[network.Target]; exists {
 			continue
@@ -1503,10 +1505,13 @@ func updateCapabilities(flags *pflag.FlagSet, containerSpec *swarm.ContainerSpec
 }
 
 func capsList(caps map[string]bool) []string {
+	if len(caps) == 0 {
+		return nil
+	}
 	if caps[opts.AllCapabilities] {
 		return []string{opts.AllCapabilities}
 	}
-	var out []string
+	out := make([]string, 0, len(caps))
 	for c := range caps {
 		out = append(out, c)
 	}

cli/command/stack/loader/loader.go

@@ -61,7 +61,7 @@ func getDictsFrom(configFiles []composetypes.ConfigFile) []map[string]interface{
 }
 
 func propertyWarnings(properties map[string]string) string {
-	var msgs []string
+	msgs := make([]string, 0, len(properties))
 	for name, description := range properties {
 		msgs = append(msgs, fmt.Sprintf("%s: %s", name, description))
 	}
@@ -129,7 +129,7 @@ func buildEnvironment(env []string) (map[string]string, error) {
 }
 
 func loadConfigFiles(filenames []string, stdin io.Reader) ([]composetypes.ConfigFile, error) {
-	var configFiles []composetypes.ConfigFile
+	configFiles := make([]composetypes.ConfigFile, 0, len(filenames))
 
 	for _, filename := range filenames {
 		configFile, err := loadConfigFile(filename, stdin)

cli/command/stack/swarm/list.go

@@ -36,7 +36,7 @@ func GetStacks(dockerCli command.Cli) ([]*formatter.Stack, error) {
 			ztack.Services++
 		}
 	}
-	var stacks []*formatter.Stack
+	stacks := make([]*formatter.Stack, 0, len(m))
 	for _, stack := range m {
 		stacks = append(stacks, stack)
 	}

cli/command/swarm/init.go

@@ -66,11 +66,10 @@ func newInitCommand(dockerCli command.Cli) *cobra.Command {
 }
 
 func runInit(dockerCli command.Cli, flags *pflag.FlagSet, opts initOptions) error {
-	var defaultAddrPool []string
-
 	client := dockerCli.Client()
 	ctx := context.Background()
 
+	defaultAddrPool := make([]string, 0, len(opts.defaultAddrPools))
 	for _, p := range opts.defaultAddrPools {
 		defaultAddrPool = append(defaultAddrPool, p.String())
 	}

cli/command/system/events_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 func TestEventsFormat(t *testing.T) {
-	var evts []events.Message
+	var evts []events.Message //nolint:prealloc
 	for i, action := range []events.Action{events.ActionCreate, events.ActionStart, events.ActionAttach, events.ActionDie} {
 		evts = append(evts, events.Message{
 			Status: string(action),

cli/command/trust/common_test.go

@@ -20,7 +20,7 @@ func TestMatchReleasedSignaturesSortOrder(t *testing.T) {
 
 	rows := matchReleasedSignatures(targets)
 
-	var targetNames []string
+	targetNames := make([]string, 0, len(rows))
 	for _, r := range rows {
 		targetNames = append(targetNames, r.SignedTag)
 	}

cli/compose/convert/service.go

@@ -710,7 +710,7 @@ func convertUlimits(origUlimits map[string]*composetypes.UlimitsConfig) []*units
 			}
 		}
 	}
-	var ulimits []*units.Ulimit
+	ulimits := make([]*units.Ulimit, 0, len(newUlimits))
 	for _, ulimit := range newUlimits {
 		ulimits = append(ulimits, ulimit)
 	}

cli/compose/convert/volume.go

@@ -12,14 +12,13 @@ type volumes map[string]composetypes.VolumeConfig
 
 // Volumes from compose-file types to engine api types
 func Volumes(serviceVolumes []composetypes.ServiceVolumeConfig, stackVolumes volumes, namespace Namespace) ([]mount.Mount, error) {
-	var mounts []mount.Mount
-
+	mounts := make([]mount.Mount, 0, len(serviceVolumes))
 	for _, volumeConfig := range serviceVolumes {
-		mount, err := convertVolumeToMount(volumeConfig, stackVolumes, namespace)
+		mnt, err := convertVolumeToMount(volumeConfig, stackVolumes, namespace)
 		if err != nil {
 			return nil, err
 		}
-		mounts = append(mounts, mount)
+		mounts = append(mounts, mnt)
 	}
 	return mounts, nil
 }

cli/compose/loader/loader.go

@@ -220,7 +220,7 @@ func GetUnsupportedProperties(configDicts ...map[string]interface{}) []string {
 }
 
 func sortedKeys(set map[string]bool) []string {
-	var keys []string
+	keys := make([]string, 0, len(set))
 	for key := range set {
 		keys = append(keys, key)
 	}
@@ -394,7 +394,7 @@ func formatInvalidKeyError(keyPrefix string, key interface{}) error {
 // LoadServices produces a ServiceConfig map from a compose file Dict
 // the servicesDict is not validated if directly used. Use Load() to enable validation
 func LoadServices(servicesDict map[string]interface{}, workingDir string, lookupEnv template.Mapping) ([]types.ServiceConfig, error) {
-	var services []types.ServiceConfig
+	services := make([]types.ServiceConfig, 0, len(servicesDict))
 
 	for name, serviceDef := range servicesDict {
 		serviceConfig, err := LoadService(name, serviceDef.(map[string]interface{}), workingDir, lookupEnv)

cli/context/store/metadatastore.go

@@ -109,7 +109,7 @@ func (s *metadataStore) list() ([]Metadata, error) {
 		}
 		return nil, err
 	}
-	var res []Metadata
+	res := make([]Metadata, 0, len(ctxDirs))
 	for _, dir := range ctxDirs {
 		c, err := s.getByID(contextdir(dir))
 		if err != nil {

cli/context/store/store.go

@@ -125,7 +125,7 @@ func Names(s Lister) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	var names []string
+	names := make([]string, 0, len(list))
 	for _, item := range list {
 		names = append(names, item.Name)
 	}

cli/manifest/types/types.go

@@ -55,14 +55,18 @@ func PlatformSpecFromOCI(p *ocispec.Platform) *manifestlist.PlatformSpec {
 
 // Blobs returns the digests for all the blobs referenced by this manifest
 func (i ImageManifest) Blobs() []digest.Digest {
-	digests := []digest.Digest{}
+	var digests []digest.Digest
 	switch {
 	case i.SchemaV2Manifest != nil:
-		for _, descriptor := range i.SchemaV2Manifest.References() {
+		refs := i.SchemaV2Manifest.References()
+		digests = make([]digest.Digest, 0, len(refs))
+		for _, descriptor := range refs {
 			digests = append(digests, descriptor.Digest)
 		}
 	case i.OCIManifest != nil:
-		for _, descriptor := range i.OCIManifest.References() {
+		refs := i.OCIManifest.References()
+		digests = make([]digest.Digest, 0, len(refs))
+		for _, descriptor := range refs {
 			digests = append(digests, descriptor.Digest)
 		}
 	}

opts/throttledevice.go

@@ -62,9 +62,8 @@ type ThrottledeviceOpt struct {
 
 // NewThrottledeviceOpt creates a new ThrottledeviceOpt
 func NewThrottledeviceOpt(validator ValidatorThrottleFctType) ThrottledeviceOpt {
-	values := []*blkiodev.ThrottleDevice{}
 	return ThrottledeviceOpt{
-		values:    values,
+		values:    []*blkiodev.ThrottleDevice{},
 		validator: validator,
 	}
 }
@@ -85,7 +84,7 @@ func (opt *ThrottledeviceOpt) Set(val string) error {
 
 // String returns ThrottledeviceOpt values as a string.
 func (opt *ThrottledeviceOpt) String() string {
-	var out []string
+	out := make([]string, 0, len(opt.values))
 	for _, v := range opt.values {
 		out = append(out, v.String())
 	}
@@ -95,7 +94,9 @@ func (opt *ThrottledeviceOpt) String() string {
 
 // GetList returns a slice of pointers to ThrottleDevices.
 func (opt *ThrottledeviceOpt) GetList() []*blkiodev.ThrottleDevice {
-	return append([]*blkiodev.ThrottleDevice{}, opt.values...)
+	out := make([]*blkiodev.ThrottleDevice, 0, len(opt.values))
+	copy(out, opt.values)
+	return out
 }
 
 // Type returns the option type

opts/ulimit.go

@@ -34,7 +34,7 @@ func (o *UlimitOpt) Set(val string) error {
 
 // String returns Ulimit values as a string. Values are sorted by name.
 func (o *UlimitOpt) String() string {
-	var out []string
+	out := make([]string, 0, len(*o.values))
 	for _, v := range *o.values {
 		out = append(out, v.String())
 	}
@@ -44,7 +44,7 @@ func (o *UlimitOpt) String() string {
 
 // GetList returns a slice of pointers to Ulimits. Values are sorted by name.
 func (o *UlimitOpt) GetList() []*units.Ulimit {
-	var ulimits []*units.Ulimit
+	ulimits := make([]*units.Ulimit, 0, len(*o.values))
 	for _, v := range *o.values {
 		ulimits = append(ulimits, v)
 	}