internal/store: export new interface

This exposes store operations safe for direct access, and enables #19 to be implemented in internal/outcome.

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

internal/store/segment.go

@@ -13,49 +13,52 @@ import (
 	"hakurei.app/internal/lockedfile"
 )
 
-// stateEntryHandle is a handle on a state entry retrieved from a storeHandle.
-// Must only be used while its parent storeHandle.fileMu is held.
-type stateEntryHandle struct {
+// EntryHandle is a handle on a state entry retrieved from a [Handle].
+// Must only be used while its parent [Handle.Lock] is held.
+type EntryHandle struct {
 	// Error returned while decoding pathname.
-	// A non-nil value disables stateEntryHandle.
-	decodeErr error
+	// A non-nil value disables EntryHandle.
+	DecodeErr error
 
-	// Checked path to entry file.
-	pathname *check.Absolute
+	// Checked pathname to entry file.
+	Pathname *check.Absolute
 
 	hst.ID
 }
 
-// open opens the underlying state entry file, returning [hst.AppError] for a non-nil error.
-func (eh *stateEntryHandle) open(flag int, perm os.FileMode) (*os.File, error) {
-	if eh.decodeErr != nil {
-		return nil, eh.decodeErr
+// open opens the underlying state entry file.
+// A non-nil error returned by open is of type [hst.AppError].
+func (eh *EntryHandle) open(flag int, perm os.FileMode) (*os.File, error) {
+	if eh.DecodeErr != nil {
+		return nil, eh.DecodeErr
 	}
 
-	if f, err := os.OpenFile(eh.pathname.String(), flag, perm); err != nil {
+	if f, err := os.OpenFile(eh.Pathname.String(), flag, perm); err != nil {
 		return nil, &hst.AppError{Step: "open state entry", Err: err}
 	} else {
 		return f, nil
 	}
 }
 
-// destroy removes the underlying state entry file, returning [hst.AppError] for a non-nil error.
-func (eh *stateEntryHandle) destroy() error {
+// Destroy removes the underlying state entry.
+// A non-nil error returned by Destroy is of type [hst.AppError].
+func (eh *EntryHandle) Destroy() error {
 	// destroy does not go through open
-	if eh.decodeErr != nil {
-		return eh.decodeErr
+	if eh.DecodeErr != nil {
+		return eh.DecodeErr
 	}
 
-	if err := os.Remove(eh.pathname.String()); err != nil {
+	if err := os.Remove(eh.Pathname.String()); err != nil {
 		return &hst.AppError{Step: "destroy state entry", Err: err}
 	}
 	return nil
 }
 
-// save encodes [hst.State] and writes it to the underlying file.
+// Save encodes [hst.State] and writes it to the underlying file.
 // An error is returned if a file already exists with the same identifier.
-// save does not validate the embedded [hst.Config].
-func (eh *stateEntryHandle) save(state *hst.State) error {
+// Save does not validate the embedded [hst.Config].
+// A non-nil error returned by Save is of type [hst.AppError].
+func (eh *EntryHandle) Save(state *hst.State) error {
 	f, err := eh.open(os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
 	if err != nil {
 		return err
@@ -68,10 +71,11 @@ func (eh *stateEntryHandle) save(state *hst.State) error {
 	return err
 }
 
-// load loads and validates the state entry header, and returns the [hst.Enablement] byte.
+// Load loads and validates the state entry header, and returns the [hst.Enablement] byte.
 // for a non-nil v, the full state payload is decoded and stored in the value pointed to by v.
-// load validates the embedded hst.Config value.
-func (eh *stateEntryHandle) load(v *hst.State) (hst.Enablement, error) {
+// Load validates the embedded [hst.Config] value.
+// A non-nil error returned by Load is of type [hst.AppError].
+func (eh *EntryHandle) Load(v *hst.State) (hst.Enablement, error) {
 	f, err := eh.open(os.O_RDONLY, 0)
 	if err != nil {
 		return 0, err
@@ -94,31 +98,42 @@ func (eh *stateEntryHandle) load(v *hst.State) (hst.Enablement, error) {
 	return et, err
 }
 
-// storeHandle is a handle on a stateStore segment.
-// Initialised by stateStore.identityHandle.
-type storeHandle struct {
+// Handle is a handle on a [Store] segment.
+// Initialised by [Store.Handle].
+type Handle struct {
 	// Identity of instances tracked by this segment.
-	identity int
-	// Pathname of directory that the segment referred to by storeHandle is rooted in.
-	path *check.Absolute
-	// Inter-process mutex to synchronise operations against resources in this segment.
-	fileMu *lockedfile.Mutex
+	Identity int
+	// Pathname of directory that the segment referred to by Handle is rooted in.
+	Path *check.Absolute
 
+	// Inter-process mutex to synchronise operations against resources in this segment.
+	// Must not be held directly, callers should use [Handle.Lock] instead.
+	fileMu *lockedfile.Mutex
 	// Must be held alongside fileMu.
 	mu sync.Mutex
 }
 
-// entries returns an iterator over all stateEntryHandle held in this segment.
-// Must be called while holding a lock on mu and fileMu.
-// A non-nil error attached to a stateEntryHandle indicates a malformed identifier and is of type [hst.AppError].
-// A non-nil error returned by entries is of type [hst.AppError].
-func (h *storeHandle) entries() (iter.Seq[*stateEntryHandle], int, error) {
+// Lock attempts to acquire a lock on [Handle].
+// If successful, Lock returns a non-nil unlock function.
+// A non-nil error returned by Lock is of type [hst.AppError].
+func (h *Handle) Lock() (unlock func(), err error) {
+	if unlock, err = h.fileMu.Lock(); err != nil {
+		return nil, &hst.AppError{Step: "acquire lock on store segment " + strconv.Itoa(h.Identity), Err: err}
+	}
+	return
+}
+
+// Entries returns an iterator over all [EntryHandle] held in this segment.
+// Must be called while holding [Handle.Lock].
+// A non-nil error attached to a [EntryHandle] indicates a malformed identifier and is of type [hst.AppError].
+// A non-nil error returned by Entries is of type [hst.AppError].
+func (h *Handle) Entries() (iter.Seq[*EntryHandle], int, error) {
 	// for error reporting
 	const step = "read store segment entries"
 
 	// read directory contents, should only contain storeMutexName and identifier
 	var entries []os.DirEntry
-	if pl, err := os.ReadDir(h.path.String()); err != nil {
+	if pl, err := os.ReadDir(h.Path.String()); err != nil {
 		return nil, -1, &hst.AppError{Step: step, Err: err}
 	} else {
 		entries = pl
@@ -130,25 +145,25 @@ func (h *storeHandle) entries() (iter.Seq[*stateEntryHandle], int, error) {
 		l--
 	}
 
-	return func(yield func(*stateEntryHandle) bool) {
+	return func(yield func(*EntryHandle) bool) {
 		for _, ent := range entries {
-			var eh = stateEntryHandle{pathname: h.path.Append(ent.Name())}
+			var eh = EntryHandle{Pathname: h.Path.Append(ent.Name())}
 
 			// this should never happen
 			if ent.IsDir() {
-				eh.decodeErr = &hst.AppError{Step: step,
+				eh.DecodeErr = &hst.AppError{Step: step,
 					Err: errors.New("unexpected directory " + strconv.Quote(ent.Name()) + " in store")}
 				goto out
 			}
 
 			// silently skip lock file
-			if ent.Name() == storeMutexName {
+			if ent.Name() == MutexName {
 				continue
 			}
 
 			// this either indicates a serious bug or external interference
 			if err := eh.ID.UnmarshalText([]byte(ent.Name())); err != nil {
-				eh.decodeErr = &hst.AppError{Step: "decode store segment entry", Err: err}
+				eh.DecodeErr = &hst.AppError{Step: "decode store segment entry", Err: err}
 				goto out
 			}
 
@@ -159,3 +174,10 @@ func (h *storeHandle) entries() (iter.Seq[*stateEntryHandle], int, error) {
 		}
 	}, l, nil
 }
+
+// newHandle returns the address of a new segment [Handle] rooted in base.
+func newHandle(base *check.Absolute, identity int) *Handle {
+	h := Handle{Identity: identity, Path: base.Append(strconv.Itoa(identity))}
+	h.fileMu = lockedfile.MutexAt(h.Path.Append(MutexName).String())
+	return &h
+}