hst/fs: interface filesystem config

This allows mount points to be represented by different underlying structs.

Signed-off-by: Ophestra <cat@gensokyo.uk>

hst/fs.go

@@ -0,0 +1,121 @@
+package hst
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"reflect"
+
+	"hakurei.app/container"
+)
+
+// FilesystemConfig is an abstract representation of a mount point.
+type FilesystemConfig interface {
+	// Type returns the type of this mount point.
+	Type() string
+	// Target returns the pathname of the mount point in the container.
+	Target() *container.Absolute
+	// Host returns a slice of all host paths used by this mount point.
+	Host() []*container.Absolute
+	// Apply appends the [container.Op] implementing this mount point.
+	Apply(ops *container.Ops)
+
+	fmt.Stringer
+}
+
+var (
+	ErrFSNull = errors.New("unexpected null in mount point")
+)
+
+// FSTypeError is returned when [ContainerConfig.Filesystem] contains an entry with invalid type.
+type FSTypeError string
+
+func (f FSTypeError) Error() string { return fmt.Sprintf("invalid filesystem type %q", string(f)) }
+
+// FSImplError is returned when the underlying struct of [FilesystemConfig] does not match
+// what [FilesystemConfig.Type] claims to be.
+type FSImplError struct {
+	Type  string
+	Value FilesystemConfig
+}
+
+func (f FSImplError) Error() string {
+	implType := reflect.TypeOf(f.Value)
+	var name string
+	for implType != nil && implType.Kind() == reflect.Ptr {
+		name += "*"
+		implType = implType.Elem()
+	}
+	if implType != nil {
+		name += implType.Name()
+	} else {
+		name += "nil"
+	}
+	return fmt.Sprintf("implementation %s is not %s", name, f.Type)
+}
+
+// FilesystemConfigJSON is the [json] adapter for [FilesystemConfig].
+type FilesystemConfigJSON struct {
+	FilesystemConfig
+}
+
+// Valid returns whether the [FilesystemConfigJSON] is valid.
+func (f *FilesystemConfigJSON) Valid() bool { return f != nil && f.FilesystemConfig != nil }
+
+func (f *FilesystemConfigJSON) MarshalJSON() ([]byte, error) {
+	if f == nil || f.FilesystemConfig == nil {
+		return nil, ErrFSNull
+	}
+	var v any
+	t := f.Type()
+	switch t {
+	case FilesystemBind:
+		if ct, ok := f.FilesystemConfig.(*FSBind); !ok {
+			return nil, FSImplError{t, f.FilesystemConfig}
+		} else {
+			v = &struct {
+				Type string `json:"type"`
+				*FSBind
+			}{FilesystemBind, ct}
+		}
+
+	case FilesystemEphemeral:
+		if ct, ok := f.FilesystemConfig.(*FSEphemeral); !ok {
+			return nil, FSImplError{t, f.FilesystemConfig}
+		} else {
+			v = &struct {
+				Type string `json:"type"`
+				*FSEphemeral
+			}{FilesystemEphemeral, ct}
+		}
+
+	default:
+		return nil, FSTypeError(t)
+	}
+
+	return json.Marshal(v)
+}
+
+func (f *FilesystemConfigJSON) UnmarshalJSON(data []byte) error {
+	t := new(struct {
+		Type string `json:"type"`
+	})
+	if err := json.Unmarshal(data, &t); err != nil {
+		return err
+	}
+	if t == nil {
+		return ErrFSNull
+	}
+	switch t.Type {
+	case FilesystemBind:
+		*f = FilesystemConfigJSON{new(FSBind)}
+
+	case FilesystemEphemeral:
+		*f = FilesystemConfigJSON{new(FSEphemeral)}
+
+	default:
+		return FSTypeError(t.Type)
+	}
+
+	return json.Unmarshal(data, f.FilesystemConfig)
+}