state: store config in separate gob stream

This enables early serialisation of config.

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

internal/state/multi.go

@@ -1,9 +1,11 @@
 package state
 
 import (
+	"encoding/binary"
 	"encoding/gob"
 	"errors"
 	"fmt"
+	"io"
 	"io/fs"
 	"os"
 	"path"
@@ -208,12 +210,11 @@ func (b *multiBackend) load(decode bool) (Entries, error) {
 				s := new(State)
 				r[*id] = s
 
-				// append regardless, but only parse if required, used to implement Len
+				// append regardless, but only parse if required, implements Len
 				if decode {
-					if err = gob.NewDecoder(f).Decode(s); err != nil {
+					if err = b.decodeState(f, s); err != nil {
 						return err
 					}
-
 					if s.ID != *id {
 						return fmt.Errorf("state entry %s has unexpected id %s", id, &s.ID)
 					}
@@ -229,18 +230,65 @@ func (b *multiBackend) load(decode bool) (Entries, error) {
 	return r, nil
 }
 
+// state file consists of an eight byte header, followed by concatenated gobs
+// of [fst.Config] and [State], if [State.Config] is not nil or offset < 0,
+// the first gob is skipped
+func (b *multiBackend) decodeState(r io.ReadSeeker, state *State) error {
+	offset := make([]byte, 8)
+	if l, err := r.Read(offset); err != nil {
+		if errors.Is(err, io.EOF) {
+			return fmt.Errorf("state file too short: %d bytes", l)
+		}
+		return err
+	}
+
+	// decode volatile state first
+	var skipConfig bool
+	{
+		o := int64(binary.LittleEndian.Uint64(offset))
+		skipConfig = o < 0
+
+		if !skipConfig {
+			if l, err := r.Seek(o, io.SeekCurrent); err != nil {
+				return err
+			} else if l != 8+o {
+				return fmt.Errorf("invalid seek offset %d", l)
+			}
+		}
+	}
+	if err := gob.NewDecoder(r).Decode(state); err != nil {
+		return err
+	}
+
+	// decode sealed config
+	if state.Config == nil {
+		// config must be provided either as part of volatile state,
+		// or in the config segment
+		if skipConfig {
+			return ErrNoConfig
+		}
+
+		state.Config = new(fst.Config)
+		if _, err := r.Seek(8, io.SeekStart); err != nil {
+			return err
+		}
+		return gob.NewDecoder(r).Decode(state.Config)
+	} else {
+		return nil
+	}
+}
+
 // Save writes process state to filesystem
-func (b *multiBackend) Save(state *State) error {
+func (b *multiBackend) Save(state *State, configWriter io.WriterTo) error {
 	b.lock.Lock()
 	defer b.lock.Unlock()
 
-	if state.Config == nil {
-		return errors.New("state does not contain config")
+	if configWriter == nil && state.Config == nil {
+		return ErrNoConfig
 	}
 
 	statePath := b.filename(&state.ID)
 
-	// create and open state data file
 	if f, err := os.OpenFile(statePath, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600); err != nil {
 		return err
 	} else {
@@ -250,11 +298,43 @@ func (b *multiBackend) Save(state *State) error {
 				panic("state file closed prematurely")
 			}
 		}()
-		// encode into state file
-		return gob.NewEncoder(f).Encode(state)
+		return b.encodeState(f, state, configWriter)
 	}
 }
 
+func (b *multiBackend) encodeState(w io.WriteSeeker, state *State, configWriter io.WriterTo) error {
+	offset := make([]byte, 8)
+
+	// skip header bytes
+	if _, err := w.Seek(8, io.SeekStart); err != nil {
+		return err
+	}
+
+	if configWriter != nil {
+		// write config gob and encode header
+		if l, err := configWriter.WriteTo(w); err != nil {
+			return err
+		} else {
+			binary.LittleEndian.PutUint64(offset, uint64(l))
+		}
+	} else {
+		// offset == -1 indicates absence of config gob
+		binary.LittleEndian.PutUint64(offset, 0xffffffffffffffff)
+	}
+
+	// encode volatile state
+	if err := gob.NewEncoder(w).Encode(state); err != nil {
+		return err
+	}
+
+	// write header
+	if _, err := w.Seek(0, io.SeekStart); err != nil {
+		return err
+	}
+	_, err := w.Write(offset)
+	return err
+}
+
 func (b *multiBackend) Destroy(id fst.ID) error {
 	b.lock.Lock()
 	defer b.lock.Unlock()