add distinct type for cache paths

go.mod

@@ -76,6 +76,7 @@ require (
 	github.com/hashicorp/errwrap v1.1.0 // indirect
 	github.com/jessevdk/go-flags v1.4.0 // indirect
 	github.com/klauspost/compress v1.15.0 // indirect
+	github.com/klauspost/cpuid/v2 v2.0.9 // indirect
 	github.com/klauspost/pgzip v1.2.4 // indirect
 	github.com/lucasb-eyer/go-colorful v1.2.0 // indirect
 	github.com/mattn/go-runewidth v0.0.13 // indirect
@@ -96,6 +97,7 @@ require (
 	github.com/tonistiigi/fsutil v0.0.0-20220115021204-b19f7f9cb274 // indirect
 	github.com/tonistiigi/vt100 v0.0.0-20210615222946-8066bb97264f // indirect
 	github.com/vbatts/go-mtree v0.5.0 // indirect
+	github.com/zeebo/xxh3 v1.0.2 // indirect
 	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.29.0 // indirect
 	go.opentelemetry.io/otel v1.4.1 // indirect
 	go.opentelemetry.io/otel/exporters/otlp/otlptrace v1.4.1 // indirect

go.sum

@@ -628,6 +628,8 @@ github.com/klauspost/compress v1.11.3/go.mod h1:aoV0uJVorq1K+umq18yTdKaF57EivdYs
 github.com/klauspost/compress v1.11.13/go.mod h1:aoV0uJVorq1K+umq18yTdKaF57EivdYsUV+/s2qKfXs=
 github.com/klauspost/compress v1.15.0 h1:xqfchp4whNFxn5A4XFyyYtitiWI8Hy5EW59jEwcyL6U=
 github.com/klauspost/compress v1.15.0/go.mod h1:/3/Vjq9QcHkK5uEr5lBEmyoZ1iFhe47etQ6QUkpK6sk=
+github.com/klauspost/cpuid/v2 v2.0.9 h1:lgaqFMSdTdQYdZ04uHyN2d/eKdOMyi2YLSvlQIBFYa4=
+github.com/klauspost/cpuid/v2 v2.0.9/go.mod h1:FInQzS24/EEf25PyTYn52gqo7WaD8xa0213Md/qVLRg=
 github.com/klauspost/pgzip v1.2.4 h1:TQ7CNpYKovDOmqzRHKxJh0BeaBI7UdQZYc6p7pMQh1A=
 github.com/klauspost/pgzip v1.2.4/go.mod h1:Ch1tH69qFZu15pkjo5kYi6mth2Zzwzt50oCQKQE9RUs=
 github.com/konsorten/go-windows-terminal-sequences v1.0.1/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
@@ -1014,6 +1016,8 @@ github.com/yuin/goldmark v1.4.1/go.mod h1:mwnBkeHKe2W/ZEtQ+71ViKU8L12m81fl3OWwC1
 github.com/yvasiyarov/go-metrics v0.0.0-20140926110328-57bccd1ccd43/go.mod h1:aX5oPXxHm3bOH+xeAttToC8pqch2ScQN/JoXYupl6xs=
 github.com/yvasiyarov/gorelic v0.0.0-20141212073537-a9bba5b9ab50/go.mod h1:NUSPSUX/bi6SeDMUh6brw0nXpxHnc96TguQh0+r/ssA=
 github.com/yvasiyarov/newrelic_platform_go v0.0.0-20140908184405-b21fdbd4370f/go.mod h1:GlGEuHIJweS1mbCqG+7vt2nvWLzLLnRHbXz5JKd/Qbg=
+github.com/zeebo/xxh3 v1.0.2 h1:xZmwmqxHZA8AI603jOQ0tMqmBr9lPeFwGg6d+xy9DC0=
+github.com/zeebo/xxh3 v1.0.2/go.mod h1:5NWz9Sef7zIDm2JHfFlcQvNekmcEl9ekUZQQKCYaDcA=
 go.etcd.io/bbolt v1.3.2/go.mod h1:IbVyRI1SCnLcuJnV2u8VeU0CEYM7e686BmAb1XKL+uU=
 go.etcd.io/bbolt v1.3.3/go.mod h1:IbVyRI1SCnLcuJnV2u8VeU0CEYM7e686BmAb1XKL+uU=
 go.etcd.io/bbolt v1.3.5/go.mod h1:G5EMThwa9y8QZGBClrRx5EY+Yw9kAhnjy3bSjsnlVTQ=

nix/vendorSha256.txt

@@ -1 +1 @@
-sha256-YDpyencMX3tg1slH3D3nhcazpF/oPfRJFgu/vsm9LD8=
+sha256-ee675i1cCL78hHCQM6zQXy16rygOcco1lW7puvJdRSY=

pkg/bass/cache_path.go

@@ -0,0 +1,137 @@
+package bass
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/binary"
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"github.com/vito/bass/pkg/proto"
+	"github.com/zeebo/xxh3"
+)
+
+// CachePath is a Path within an ephemeral directory managed by the runtime.
+type CachePath struct {
+	ID   string
+	Path FileOrDirPath
+}
+
+var _ Value = CachePath{}
+
+func NewCacheDir(id string) CachePath {
+	return NewCachePath(id, ParseFileOrDirPath("."))
+}
+
+func NewCachePath(id string, path FileOrDirPath) CachePath {
+	return CachePath{
+		ID:   id,
+		Path: path,
+	}
+}
+
+func ParseCachePath(path string) CachePath {
+	return NewCachePath(
+		filepath.Dir(path),
+		ParseFileOrDirPath(filepath.Base(path)),
+	)
+}
+
+func (value CachePath) String() string {
+	return fmt.Sprintf("<cache: %s>/%s", value.ID, strings.TrimPrefix(value.Path.Slash(), "./"))
+}
+
+func (value CachePath) Hash() string {
+	var tmp [8]byte
+	binary.BigEndian.PutUint64(tmp[:], xxh3.HashString(value.ID))
+	return base64.URLEncoding.EncodeToString(tmp[:])
+}
+
+func (value CachePath) Equal(other Value) bool {
+	var o CachePath
+	return other.Decode(&o) == nil &&
+		value.ID == o.ID &&
+		value.Path.FilesystemPath().Equal(o.Path.FilesystemPath())
+}
+
+func (value CachePath) Decode(dest any) error {
+	switch x := dest.(type) {
+	case *CachePath:
+		*x = value
+		return nil
+	case *Path:
+		*x = value
+		return nil
+	case *Value:
+		*x = value
+		return nil
+	case *Applicative:
+		*x = value
+		return nil
+	case *Combiner:
+		*x = value
+		return nil
+	case Decodable:
+		return x.FromValue(value)
+	default:
+		return DecodeError{
+			Source:      value,
+			Destination: dest,
+		}
+	}
+}
+
+func (path *CachePath) UnmarshalProto(msg proto.Message) error {
+	p, ok := msg.(*proto.CachePath)
+	if !ok {
+		return fmt.Errorf("unmarshal proto: %w", DecodeError{msg, path})
+	}
+
+	path.ID = p.Id
+
+	return path.Path.UnmarshalProto(p.Path)
+}
+
+// Eval returns the value.
+func (value CachePath) Eval(_ context.Context, _ *Scope, cont Cont) ReadyCont {
+	return cont.Call(value, nil)
+}
+
+var _ Applicative = ThunkPath{}
+
+func (app CachePath) Unwrap() Combiner {
+	if app.Path.File != nil {
+		return ThunkOperative{
+			Cmd: ThunkCmd{
+				Cache: &app,
+			},
+		}
+	} else {
+		return ExtendOperative{app}
+	}
+}
+
+var _ Combiner = CachePath{}
+
+func (combiner CachePath) Call(ctx context.Context, val Value, scope *Scope, cont Cont) ReadyCont {
+	return Wrap(combiner.Unwrap()).Call(ctx, val, scope, cont)
+}
+
+var _ Path = CachePath{}
+
+func (path CachePath) Name() string {
+	return path.Path.FilesystemPath().Name()
+}
+
+func (path CachePath) Extend(ext Path) (Path, error) {
+	extended := path
+
+	var err error
+	extended.Path, err = path.Path.Extend(ext)
+	if err != nil {
+		return nil, err
+	}
+
+	return extended, nil
+}

pkg/bass/ground.go

@@ -769,6 +769,11 @@ func init() {
 		`=> (def file-thunk (from (linux/alpine) ($ sh -c "echo 42 > file")))`,
 		`=> (next (read file-thunk/file :json))`,
 	)
+
+	Ground.Set("cache-dir",
+		Func("cache-dir", "[id]", NewCacheDir),
+		`returns a cache directory corresponding to the string identifier`,
+		`Cache directories may be mounted to thunks. Their content persists across thunk runs.`)
 }
 
 type primPred struct {

pkg/bass/json_test.go

@@ -288,8 +288,11 @@ var validThunkMountSources = []bass.ThunkMountSource{
 		FSPath: bass.NewInMemoryFile("fs/mount-dir/file", "hello").Dir(),
 	},
 	{
-		Cache: &bass.FileOrDirPath{
-			Dir: &bass.DirPath{"cache/dir"},
+		Cache: &bass.CachePath{
+			ID: "some-cache",
+			Path: bass.FileOrDirPath{
+				Dir: &bass.DirPath{"cache/dir"},
+			},
 		},
 	},
 	{

pkg/bass/proto.go

@@ -293,6 +293,21 @@ func (value *FSPath) MarshalProto() (proto.Message, error) {
 	return lp, nil
 }
 
+func (value CachePath) MarshalProto() (proto.Message, error) {
+	pv := &proto.CachePath{
+		Id: value.ID,
+	}
+
+	pathp, err := value.Path.MarshalProto()
+	if err != nil {
+		return nil, err
+	}
+
+	pv.Path = pathp.(*proto.FilesystemPath)
+
+	return pv, nil
+}
+
 func (value Thunk) MarshalProto() (proto.Message, error) {
 	thunk := &proto.Thunk{
 		Insecure: value.Insecure,