internal/store: rename from state

This reduces collision with local variable names, and generally makes sense for the new store package, since it no longer specifies the state struct.

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

11 files changed, 1617 insertions, 0 deletions

diff --git a/internal/store/compat.go b/internal/store/compat.go
new file mode 100644
index 00000000..c0ef6dd7
--- /dev/null
+++ b/internal/store/compat.go
@@ -0,0 +1,130 @@
+package store
+
+import (
+	"strconv"
+
+	"hakurei.app/container/check"
+	"hakurei.app/hst"
+	"hakurei.app/message"
+)
+
+/* this provides an implementation of Store on top of the improved state tracking to ease in the changes */
+
+type Store interface {
+	// Do calls f exactly once and ensures store exclusivity until f returns.
+	// Returns whether f is called and any errors during the locking process.
+	// Cursor provided to f becomes invalid as soon as f returns.
+	Do(identity int, f func(c Cursor)) (ok bool, err error)
+
+	// List queries the store and returns a list of identities known to the store.
+	// Note that some or all returned identities might not have any active apps.
+	List() (identities []int, err error)
+}
+
+func (s *stateStore) Do(identity int, f func(c Cursor)) (bool, error) {
+	if h, err := s.identityHandle(identity); err != nil {
+		return false, err
+	} else {
+		return h.do(f)
+	}
+}
+
+// storeAdapter satisfies [Store] via stateStore.
+type storeAdapter struct {
+	msg message.Msg
+	*stateStore
+}
+
+func (s storeAdapter) List() ([]int, error) {
+	segments, n, err := s.segments()
+	if err != nil {
+		return nil, err
+	}
+
+	identities := make([]int, 0, n)
+	for si := range segments {
+		if si.err != nil {
+			if m, ok := message.GetMessage(err); ok {
+				s.msg.Verbose(m)
+			} else {
+				// unreachable
+				return nil, err
+			}
+			continue
+		}
+		identities = append(identities, si.identity)
+	}
+	return identities, nil
+}
+
+// NewMulti returns an instance of the multi-file store.
+func NewMulti(msg message.Msg, prefix *check.Absolute) Store {
+	return storeAdapter{msg, newStore(prefix.Append("state"))}
+}
+
+// Cursor provides access to the store of an identity.
+type Cursor interface {
+	Save(state *hst.State) error
+	Destroy(id hst.ID) error
+	Load() (map[hst.ID]*hst.State, error)
+	Len() (int, error)
+}
+
+// do implements stateStore.Do on storeHandle.
+func (h *storeHandle) do(f func(c Cursor)) (bool, error) {
+	if unlock, err := h.fileMu.Lock(); err != nil {
+		return false, &hst.AppError{Step: "acquire lock on store segment " + strconv.Itoa(h.identity), Err: err}
+	} else {
+		defer unlock()
+	}
+
+	f(h)
+	return true, nil
+}
+
+/* these compatibility methods must only be called while fileMu is held */
+
+func (h *storeHandle) Save(state *hst.State) error {
+	return (&stateEntryHandle{nil, h.path.Append(state.ID.String()), state.ID}).save(state)
+}
+
+func (h *storeHandle) Destroy(id hst.ID) error {
+	return (&stateEntryHandle{nil, h.path.Append(id.String()), id}).destroy()
+}
+
+func (h *storeHandle) Load() (map[hst.ID]*hst.State, error) {
+	entries, n, err := h.entries()
+	if err != nil {
+		return nil, err
+	}
+
+	r := make(map[hst.ID]*hst.State, n)
+	for eh := range entries {
+		if eh.decodeErr != nil {
+			err = eh.decodeErr
+			break
+		}
+		var s hst.State
+		if _, err = eh.load(&s); err != nil {
+			break
+		}
+		r[eh.ID] = &s
+	}
+	return r, err
+}
+
+func (h *storeHandle) Len() (int, error) {
+	entries, _, err := h.entries()
+	if err != nil {
+		return -1, err
+	}
+
+	var n int
+	for eh := range entries {
+		if eh.decodeErr != nil {
+			err = eh.decodeErr
+		}
+		n++
+	}
+	return n, err
+}
diff --git a/internal/store/compat_test.go b/internal/store/compat_test.go
new file mode 100644
index 00000000..c8e7fa44
--- /dev/null
+++ b/internal/store/compat_test.go
@@ -0,0 +1,120 @@
+package store_test
+
+import (
+	"log"
+	"math/rand"
+	"reflect"
+	"slices"
+	"testing"
+	"time"
+
+	"hakurei.app/container/check"
+	"hakurei.app/hst"
+	"hakurei.app/internal/store"
+	"hakurei.app/message"
+)
+
+func TestMulti(t *testing.T) {
+	s := store.NewMulti(message.NewMsg(log.New(log.Writer(), "multi: ", 0)), check.MustAbs(t.TempDir()))
+
+	t.Run("list empty store", func(t *testing.T) {
+		if identities, err := s.List(); err != nil {
+			t.Fatalf("List: error = %v", err)
+		} else if len(identities) != 0 {
+			t.Fatalf("List: identities = %#v", identities)
+		}
+	})
+
+	const (
+		insertEntryChecked = iota
+		insertEntryNoCheck
+		insertEntryOtherApp
+
+		tl
+	)
+
+	var tc [tl]hst.State
+	for i := 0; i < tl; i++ {
+		if err := hst.NewInstanceID(&tc[i].ID); err != nil {
+			t.Fatalf("cannot create dummy state: %v", err)
+		}
+		tc[i].PID = rand.Int()
+		tc[i].Config = hst.Template()
+		tc[i].Time = time.Now()
+	}
+
+	do := func(identity int, f func(c store.Cursor)) {
+		if ok, err := s.Do(identity, f); err != nil {
+			t.Fatalf("Do: ok = %v, error = %v", ok, err)
+		}
+	}
+
+	insert := func(i, identity int) {
+		do(identity, func(c store.Cursor) {
+			if err := c.Save(&tc[i]); err != nil {
+				t.Fatalf("Save: error = %v", err)
+			}
+		})
+	}
+
+	check := func(i, identity int) {
+		do(identity, func(c store.Cursor) {
+			if entries, err := c.Load(); err != nil {
+				t.Fatalf("Load: error = %v", err)
+			} else if got, ok := entries[tc[i].ID]; !ok {
+				t.Fatalf("Load: entry %s missing", &tc[i].ID)
+			} else {
+				got.Time = tc[i].Time
+				if !reflect.DeepEqual(got, &tc[i]) {
+					t.Fatalf("Load: entry %s got %#v, want %#v", &tc[i].ID, got, &tc[i])
+				}
+			}
+		})
+	}
+
+	// insert entry checked
+	insert(insertEntryChecked, 0)
+	check(insertEntryChecked, 0)
+
+	// insert entry unchecked
+	insert(insertEntryNoCheck, 0)
+
+	// insert entry different identity
+	insert(insertEntryOtherApp, 1)
+	check(insertEntryOtherApp, 1)
+
+	// check previous insertion
+	check(insertEntryNoCheck, 0)
+
+	// list identities
+	if identities, err := s.List(); err != nil {
+		t.Fatalf("List: error = %v", err)
+	} else {
+		slices.Sort(identities)
+		want := []int{0, 1}
+		if !slices.Equal(identities, want) {
+			t.Fatalf("List() = %#v, want %#v", identities, want)
+		}
+	}
+
+	// join store
+	if entries, err := store.Join(s); err != nil {
+		t.Fatalf("Join: error = %v", err)
+	} else if len(entries) != 3 {
+		t.Fatalf("Join(s) = %#v", entries)
+	}
+
+	// clear identity 1
+	do(1, func(c store.Cursor) {
+		if err := c.Destroy(tc[insertEntryOtherApp].ID); err != nil {
+			t.Fatalf("Destroy: error = %v", err)
+		}
+	})
+	do(1, func(c store.Cursor) {
+		if l, err := c.Len(); err != nil {
+			t.Fatalf("Len: error = %v", err)
+		} else if l != 0 {
+			t.Fatalf("Len: %d, want 0", l)
+		}
+	})
+}
diff --git a/internal/store/data.go b/internal/store/data.go
new file mode 100644
index 00000000..d8f2bc6d
--- /dev/null
+++ b/internal/store/data.go
@@ -0,0 +1,52 @@
+package store
+
+import (
+	"encoding/gob"
+	"fmt"
+	"io"
+	"os"
+
+	"hakurei.app/hst"
+)
+
+// entryEncode encodes [hst.State] into [io.Writer] with the state entry header.
+// entryEncode does not validate the embedded [hst.Config] value.
+//
+// A non-nil error returned by entryEncode is of type [hst.AppError].
+func entryEncode(w io.Writer, s *hst.State) error {
+	if err := entryWriteHeader(w, s.Enablements.Unwrap()); err != nil {
+		return &hst.AppError{Step: "encode state header", Err: err}
+	} else if err = gob.NewEncoder(w).Encode(s); err != nil {
+		return &hst.AppError{Step: "encode state body", Err: err}
+	} else {
+		return nil
+	}
+}
+
+// entryDecodeHeader calls entryReadHeader, returning [hst.AppError] for a non-nil error.
+func entryDecodeHeader(r io.Reader) (hst.Enablement, error) {
+	if et, err := entryReadHeader(r); err != nil {
+		return 0, &hst.AppError{Step: "decode state header", Err: err}
+	} else {
+		return et, nil
+	}
+}
+
+// entryDecode decodes [hst.State] from [io.Reader] and stores the result in the value pointed to by p.
+// entryDecode validates the embedded [hst.Config] value.
+//
+// A non-nil error returned by entryDecode is of type [hst.AppError].
+func entryDecode(r io.Reader, p *hst.State) (hst.Enablement, error) {
+	if et, err := entryDecodeHeader(r); err != nil {
+		return et, err
+	} else if err = gob.NewDecoder(r).Decode(&p); err != nil {
+		return et, &hst.AppError{Step: "decode state body", Err: err}
+	} else if err = p.Config.Validate(); err != nil {
+		return et, err
+	} else if p.Enablements.Unwrap() != et {
+		return et, &hst.AppError{Step: "validate state enablement", Err: os.ErrInvalid,
+			Msg: fmt.Sprintf("state entry %s has unexpected enablement byte %#x, %#x", p.ID.String(), byte(p.Enablements.Unwrap()), byte(et))}
+	} else {
+		return et, nil
+	}
+}
diff --git a/internal/store/data_test.go b/internal/store/data_test.go
new file mode 100644
index 00000000..79faebae
--- /dev/null
+++ b/internal/store/data_test.go
@@ -0,0 +1,145 @@
+package store
+
+import (
+	"bytes"
+	"encoding/gob"
+	"errors"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"hakurei.app/container/stub"
+	"hakurei.app/hst"
+)
+
+func TestEntryData(t *testing.T) {
+	t.Parallel()
+
+	mustEncodeGob := func(e any) string {
+		var buf bytes.Buffer
+		if err := gob.NewEncoder(&buf).Encode(e); err != nil {
+			t.Fatalf("cannot encode invalid state: %v", err)
+			return "\x00" // not reached
+		} else {
+			return buf.String()
+		}
+	}
+	templateStateGob := mustEncodeGob(newTemplateState())
+
+	testCases := []struct {
+		name string
+		data string
+		s    *hst.State
+		err  error
+	}{
+		{"invalid header", "\x00\xff\xca\xfe\xff\xff\xff\x00", nil, &hst.AppError{
+			Step: "decode state header", Err: errors.New("unexpected revision ffff")}},
+
+		{"invalid gob", "\x00\xff\xca\xfe\x00\x00\xff\x00", nil, &hst.AppError{
+			Step: "decode state body", Err: io.EOF}},
+
+		{"invalid config", "\x00\xff\xca\xfe\x00\x00\xff\x00" + mustEncodeGob(new(hst.State)), new(hst.State), &hst.AppError{
+			Step: "validate configuration", Err: hst.ErrConfigNull,
+			Msg: "invalid configuration"}},
+
+		{"inconsistent enablement", "\x00\xff\xca\xfe\x00\x00\xff\x00" + templateStateGob, newTemplateState(), &hst.AppError{
+			Step: "validate state enablement", Err: os.ErrInvalid,
+			Msg: "state entry aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa has unexpected enablement byte 0xd, 0xff"}},
+
+		{"template", "\x00\xff\xca\xfe\x00\x00\x0d\xf2" + templateStateGob, newTemplateState(), nil},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			t.Run("encode", func(t *testing.T) {
+				if tc.s == nil || tc.s.Config == nil {
+					return
+				}
+				t.Parallel()
+
+				var buf bytes.Buffer
+				if err := entryEncode(&buf, tc.s); err != nil {
+					t.Fatalf("entryEncode: error = %v", err)
+				}
+
+				if tc.err == nil {
+					// Gob encoding is not guaranteed to be deterministic.
+					// While the current implementation mostly is, it has randomised order
+					// for iterating over maps, and hst.Config holds a map for environ.
+					var got hst.State
+					if et, err := entryDecode(&buf, &got); err != nil {
+						t.Fatalf("entryDecode: error = %v", err)
+					} else if stateEt := got.Enablements.Unwrap(); et != stateEt {
+						t.Fatalf("entryDecode: et = %x, state %x", et, stateEt)
+					}
+					if !reflect.DeepEqual(&got, tc.s) {
+						t.Errorf("entryEncode: %x", buf.Bytes())
+					}
+				} else if testing.Verbose() {
+					t.Logf("%x", buf.String())
+				}
+			})
+
+			t.Run("decode", func(t *testing.T) {
+				t.Parallel()
+
+				var got hst.State
+				if et, err := entryDecode(strings.NewReader(tc.data), &got); !reflect.DeepEqual(err, tc.err) {
+					t.Fatalf("entryDecode: error = %#v, want %#v", err, tc.err)
+				} else if err != nil {
+					return
+				} else if stateEt := got.Enablements.Unwrap(); et != stateEt {
+					t.Fatalf("entryDecode: et = %x, state %x", et, stateEt)
+				}
+
+				if !reflect.DeepEqual(&got, tc.s) {
+					t.Errorf("entryDecode: %#v, want %#v", &got, tc.s)
+				}
+			})
+		})
+	}
+
+	t.Run("encode fault", func(t *testing.T) {
+		t.Parallel()
+		s := newTemplateState()
+
+		t.Run("gob", func(t *testing.T) {
+			var want = &hst.AppError{Step: "encode state body", Err: stub.UniqueError(0xcafe)}
+			if err := entryEncode(stubNErrorWriter(entryHeaderSize), s); !reflect.DeepEqual(err, want) {
+				t.Errorf("entryEncode: error = %#v, want %#v", err, want)
+			}
+		})
+
+		t.Run("header", func(t *testing.T) {
+			var want = &hst.AppError{Step: "encode state header", Err: stub.UniqueError(0xcafe)}
+			if err := entryEncode(stubNErrorWriter(entryHeaderSize-1), s); !reflect.DeepEqual(err, want) {
+				t.Errorf("entryEncode: error = %#v, want %#v", err, want)
+			}
+		})
+	})
+}
+
+// newTemplateState returns the address of a new template [hst.State] struct.
+func newTemplateState() *hst.State {
+	return &hst.State{
+		ID:      hst.ID(bytes.Repeat([]byte{0xaa}, len(hst.ID{}))),
+		PID:     0xcafebabe,
+		ShimPID: 0xdeadbeef,
+		Config:  hst.Template(),
+		Time:    time.Unix(0, 0),
+	}
+}
+
+// stubNErrorWriter returns an error for writes above a certain size.
+type stubNErrorWriter int
+
+func (w stubNErrorWriter) Write(p []byte) (n int, err error) {
+	if len(p) > int(w) {
+		return int(w), stub.UniqueError(0xcafe)
+	}
+	return io.Discard.Write(p)
+}
diff --git a/internal/store/header.go b/internal/store/header.go
new file mode 100644
index 00000000..a6e756f9
--- /dev/null
+++ b/internal/store/header.go
@@ -0,0 +1,86 @@
+package store
+
+import (
+	"encoding/hex"
+	"errors"
+	"io"
+	"os"
+	"strconv"
+	"syscall"
+
+	"hakurei.app/hst"
+)
+
+const (
+	// entryHeaderMagic are magic bytes at the beginning of the state entry file.
+	entryHeaderMagic = "\x00\xff\xca\xfe"
+	// entryHeaderRevision follows entryHeaderMagic and is incremented for revisions of the format.
+	entryHeaderRevision = "\x00\x00"
+	// entryHeaderSize is the fixed size of the header in bytes, including the enablement byte and its complement.
+	entryHeaderSize = len(entryHeaderMagic+entryHeaderRevision) + 2
+)
+
+// entryHeaderEncode encodes a state entry header for a [hst.Enablement] byte.
+func entryHeaderEncode(et hst.Enablement) *[entryHeaderSize]byte {
+	data := [entryHeaderSize]byte([]byte(
+		entryHeaderMagic + entryHeaderRevision + string([]hst.Enablement{et, ^et}),
+	))
+	return &data
+}
+
+// entryHeaderDecode validates a state entry header and returns the [hst.Enablement] byte.
+func entryHeaderDecode(data *[entryHeaderSize]byte) (hst.Enablement, error) {
+	if magic := data[:len(entryHeaderMagic)]; string(magic) != entryHeaderMagic {
+		return 0, errors.New("invalid header " + hex.EncodeToString(magic))
+	}
+	if revision := data[len(entryHeaderMagic):len(entryHeaderMagic+entryHeaderRevision)]; string(revision) != entryHeaderRevision {
+		return 0, errors.New("unexpected revision " + hex.EncodeToString(revision))
+	}
+
+	et := data[len(entryHeaderMagic+entryHeaderRevision)]
+	if et != ^data[len(entryHeaderMagic+entryHeaderRevision)+1] {
+		return 0, errors.New("header enablement value is inconsistent")
+	}
+	return hst.Enablement(et), nil
+}
+
+// EntrySizeError is returned for a file too small to hold a state entry header.
+type EntrySizeError struct {
+	Name string
+	Size int64
+}
+
+func (e *EntrySizeError) Error() string {
+	if e.Name == "" {
+		return "state entry file is too short"
+	}
+	return "state entry file " + strconv.Quote(e.Name) + " is too short"
+}
+
+// entryCheckFile checks whether [os.FileInfo] refers to a file that might hold [hst.State].
+func entryCheckFile(fi os.FileInfo) error {
+	if fi.IsDir() {
+		return syscall.EISDIR
+	}
+	if s := fi.Size(); s <= int64(entryHeaderSize) {
+		return &EntrySizeError{Name: fi.Name(), Size: s}
+	}
+	return nil
+}
+
+// entryReadHeader reads [hst.Enablement] from an [io.Reader].
+func entryReadHeader(r io.Reader) (hst.Enablement, error) {
+	var data [entryHeaderSize]byte
+	if n, err := r.Read(data[:]); err != nil {
+		return 0, err
+	} else if n != entryHeaderSize {
+		return 0, &EntrySizeError{Size: int64(n)}
+	}
+	return entryHeaderDecode(&data)
+}
+
+// entryWriteHeader writes [hst.Enablement] header to an [io.Writer].
+func entryWriteHeader(w io.Writer, et hst.Enablement) error {
+	_, err := w.Write(entryHeaderEncode(et)[:])
+	return err
+}
diff --git a/internal/store/header_test.go b/internal/store/header_test.go
new file mode 100644
index 00000000..51a6939a
--- /dev/null
+++ b/internal/store/header_test.go
@@ -0,0 +1,184 @@
+package store
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"io/fs"
+	"os"
+	"reflect"
+	"syscall"
+	"testing"
+	"time"
+
+	"hakurei.app/hst"
+)
+
+func TestEntryHeader(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		data [entryHeaderSize]byte
+		et   hst.Enablement
+		err  error
+	}{
+		{"complement mismatch", [entryHeaderSize]byte{0x00, 0xff, 0xca, 0xfe, 0x00, 0x00,
+			0x0a, 0xf6}, 0,
+			errors.New("header enablement value is inconsistent")},
+		{"unexpected revision", [entryHeaderSize]byte{0x00, 0xff, 0xca, 0xfe, 0xff, 0xff}, 0,
+			errors.New("unexpected revision ffff")},
+		{"invalid header", [entryHeaderSize]byte{0x00, 0xfe, 0xca, 0xfe}, 0,
+			errors.New("invalid header 00fecafe")},
+
+		{"success high", [entryHeaderSize]byte{0x00, 0xff, 0xca, 0xfe, 0x00, 0x00,
+			0xff, 0x00}, 0xff, nil},
+		{"success", [entryHeaderSize]byte{0x00, 0xff, 0xca, 0xfe, 0x00, 0x00,
+			0x09, 0xf6}, hst.EWayland | hst.EPulse, nil},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			t.Run("encode", func(t *testing.T) {
+				if tc.err != nil {
+					return
+				}
+				t.Parallel()
+
+				if got := entryHeaderEncode(tc.et); *got != tc.data {
+					t.Errorf("entryHeaderEncode: %x, want %x", *got, tc.data)
+				}
+
+				t.Run("write", func(t *testing.T) {
+					var buf bytes.Buffer
+					if err := entryWriteHeader(&buf, tc.et); err != nil {
+						t.Fatalf("entryWriteHeader: error = %v", err)
+					}
+					if got := ([entryHeaderSize]byte)(buf.Bytes()); got != tc.data {
+						t.Errorf("entryWriteHeader: %x, want %x", got, tc.data)
+					}
+				})
+			})
+
+			t.Run("decode", func(t *testing.T) {
+				t.Parallel()
+
+				got, err := entryHeaderDecode(&tc.data)
+				if !reflect.DeepEqual(err, tc.err) {
+					t.Fatalf("entryHeaderDecode: error = %#v, want %#v", err, tc.err)
+				}
+				if err != nil {
+					return
+				}
+				if got != tc.et {
+					t.Errorf("entryHeaderDecode: et = %q, want %q", got, tc.et)
+				}
+
+				if got, err = entryReadHeader(bytes.NewReader(tc.data[:])); err != nil {
+					t.Fatalf("entryReadHeader: error = %#v", err)
+				} else if got != tc.et {
+					t.Errorf("entryReadHeader: et = %q, want %q", got, tc.et)
+				}
+			})
+		})
+	}
+}
+
+func TestEntrySizeError(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		err  error
+		want string
+	}{
+		{"size only", &EntrySizeError{Size: 0xdeadbeef},
+			`state entry file is too short`},
+		{"full", &EntrySizeError{Name: "nonexistent", Size: 0xdeadbeef},
+			`state entry file "nonexistent" is too short`},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			if got := tc.err.Error(); got != tc.want {
+				t.Errorf("Error: %s, want %s", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestEntryCheckFile(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		fi   os.FileInfo
+		err  error
+	}{
+		{"dir", &stubFi{name: "dir", isDir: true},
+			syscall.EISDIR},
+		{"short", stubFi{name: "short", size: 8},
+			&EntrySizeError{Name: "short", Size: 8}},
+		{"success", stubFi{size: 9}, nil},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			if err := entryCheckFile(tc.fi); !reflect.DeepEqual(err, tc.err) {
+				t.Errorf("entryCheckFile: error = %#v, want %#v", err, tc.err)
+			}
+		})
+	}
+}
+
+func TestEntryReadHeader(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		newR func() io.Reader
+		err  error
+	}{
+		{"eof", func() io.Reader { return bytes.NewReader([]byte{}) }, io.EOF},
+		{"short", func() io.Reader { return bytes.NewReader([]byte{0}) }, &EntrySizeError{Size: 1}},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			if _, err := entryReadHeader(tc.newR()); !reflect.DeepEqual(err, tc.err) {
+				t.Errorf("entryReadHeader: error = %#v, want %#v", err, tc.err)
+			}
+		})
+	}
+}
+
+// stubFi partially implements [os.FileInfo] using hardcoded values.
+type stubFi struct {
+	name  string
+	size  int64
+	isDir bool
+}
+
+func (fi stubFi) Name() string {
+	if fi.name == "" {
+		panic("unreachable")
+	}
+	return fi.name
+}
+
+func (fi stubFi) Size() int64 {
+	if fi.size < 0 {
+		panic("unreachable")
+	}
+	return fi.size
+}
+
+func (fi stubFi) IsDir() bool { return fi.isDir }
+
+func (fi stubFi) Mode() fs.FileMode  { panic("unreachable") }
+func (fi stubFi) ModTime() time.Time { panic("unreachable") }
+func (fi stubFi) Sys() any           { panic("unreachable") }
diff --git a/internal/store/join.go b/internal/store/join.go
new file mode 100644
index 00000000..0bb69df7
--- /dev/null
+++ b/internal/store/join.go
@@ -0,0 +1,64 @@
+package store
+
+import (
+	"errors"
+	"maps"
+
+	"hakurei.app/hst"
+)
+
+var (
+	ErrDuplicate = errors.New("store contains duplicates")
+)
+
+/*
+Joiner is the interface that wraps the Join method.
+
+The Join function uses Joiner if available.
+*/
+type Joiner interface {
+	Join() (map[hst.ID]*hst.State, error)
+}
+
+// Join returns joined state entries of all active identities.
+func Join(s Store) (map[hst.ID]*hst.State, error) {
+	if j, ok := s.(Joiner); ok {
+		return j.Join()
+	}
+
+	var (
+		aids    []int
+		entries = make(map[hst.ID]*hst.State)
+
+		el      int
+		res     map[hst.ID]*hst.State
+		loadErr error
+	)
+
+	if ln, err := s.List(); err != nil {
+		return nil, err
+	} else {
+		aids = ln
+	}
+
+	for _, aid := range aids {
+		if _, err := s.Do(aid, func(c Cursor) {
+			res, loadErr = c.Load()
+		}); err != nil {
+			return nil, err
+		}
+
+		if loadErr != nil {
+			return nil, loadErr
+		}
+
+		// save expected length
+		el = len(entries) + len(res)
+		maps.Copy(entries, res)
+		if len(entries) != el {
+			return nil, ErrDuplicate
+		}
+	}
+
+	return entries, nil
+}
diff --git a/internal/store/segment.go b/internal/store/segment.go
new file mode 100644
index 00000000..ef3e20c7
--- /dev/null
+++ b/internal/store/segment.go
@@ -0,0 +1,161 @@
+package store
+
+import (
+	"errors"
+	"fmt"
+	"iter"
+	"os"
+	"strconv"
+	"sync"
+
+	"hakurei.app/container/check"
+	"hakurei.app/hst"
+	"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 {
+	// Error returned while decoding pathname.
+	// A non-nil value disables stateEntryHandle.
+	decodeErr error
+
+	// Checked path 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
+	}
+
+	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 does not go through open
+	if eh.decodeErr != nil {
+		return eh.decodeErr
+	}
+
+	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.
+// 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 {
+	f, err := eh.open(os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
+	if err != nil {
+		return err
+	}
+
+	err = entryEncode(f, state)
+	if closeErr := f.Close(); closeErr != nil && err == nil {
+		err = &hst.AppError{Step: "close state file", Err: closeErr}
+	}
+	return err
+}
+
+// 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) {
+	f, err := eh.open(os.O_RDONLY, 0)
+	if err != nil {
+		return 0, err
+	}
+
+	var et hst.Enablement
+	if v != nil {
+		et, err = entryDecode(f, v)
+		if err == nil && v.ID != eh.ID {
+			err = &hst.AppError{Step: "validate state identifier", Err: os.ErrInvalid,
+				Msg: fmt.Sprintf("state entry %s has unexpected id %s", eh.ID.String(), v.ID.String())}
+		}
+	} else {
+		et, err = entryDecodeHeader(f)
+	}
+
+	if closeErr := f.Close(); closeErr != nil && err == nil {
+		err = &hst.AppError{Step: "close state file", Err: closeErr}
+	}
+	return et, err
+}
+
+// storeHandle is a handle on a stateStore segment.
+// Initialised by stateStore.identityHandle.
+type storeHandle 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
+
+	// 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) {
+	// 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 {
+		return nil, -1, &hst.AppError{Step: step, Err: err}
+	} else {
+		entries = pl
+	}
+
+	// expects lock file
+	l := len(entries)
+	if l > 0 {
+		l--
+	}
+
+	return func(yield func(*stateEntryHandle) bool) {
+		for _, ent := range entries {
+			var eh = stateEntryHandle{pathname: h.path.Append(ent.Name())}
+
+			// this should never happen
+			if ent.IsDir() {
+				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 {
+				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}
+				goto out
+			}
+
+		out:
+			if !yield(&eh) {
+				break
+			}
+		}
+	}, l, nil
+}
diff --git a/internal/store/segment_test.go b/internal/store/segment_test.go
new file mode 100644
index 00000000..dab019ae
--- /dev/null
+++ b/internal/store/segment_test.go
@@ -0,0 +1,259 @@
+package store
+
+import (
+	"errors"
+	"iter"
+	"os"
+	"reflect"
+	"slices"
+	"strings"
+	"syscall"
+	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
+	"hakurei.app/hst"
+	"hakurei.app/internal/lockedfile"
+)
+
+func TestStateEntryHandle(t *testing.T) {
+	t.Parallel()
+
+	t.Run("lockout", func(t *testing.T) {
+		t.Parallel()
+		wantErr := func() error { return stub.UniqueError(0) }
+		eh := stateEntryHandle{decodeErr: wantErr(), pathname: check.MustAbs("/proc/nonexistent")}
+
+		if _, err := eh.open(-1, 0); !reflect.DeepEqual(err, wantErr()) {
+			t.Errorf("open: error = %v, want %v", err, wantErr())
+		}
+		if err := eh.destroy(); !reflect.DeepEqual(err, wantErr()) {
+			t.Errorf("destroy: error = %v, want %v", err, wantErr())
+		}
+		if err := eh.save(nil); !reflect.DeepEqual(err, wantErr()) {
+			t.Errorf("save: error = %v, want %v", err, wantErr())
+		}
+		if _, err := eh.load(nil); !reflect.DeepEqual(err, wantErr()) {
+			t.Errorf("load: error = %v, want %v", err, wantErr())
+		}
+	})
+
+	t.Run("od", func(t *testing.T) {
+		t.Parallel()
+
+		{
+			eh := stateEntryHandle{pathname: check.MustAbs(t.TempDir()).Append("entry")}
+			if f, err := eh.open(os.O_CREATE|syscall.O_EXCL, 0); err != nil {
+				t.Fatalf("open: error = %v", err)
+			} else if err = f.Close(); err != nil {
+				t.Errorf("Close: error = %v", err)
+			}
+			if err := eh.destroy(); err != nil {
+				t.Fatalf("destroy: error = %v", err)
+			}
+		}
+
+		t.Run("nonexistent", func(t *testing.T) {
+			t.Parallel()
+			eh := stateEntryHandle{pathname: check.MustAbs("/proc/nonexistent")}
+
+			wantErrOpen := &hst.AppError{Step: "open state entry",
+				Err: &os.PathError{Op: "open", Path: "/proc/nonexistent", Err: syscall.ENOENT}}
+			if _, err := eh.open(os.O_CREATE|syscall.O_EXCL, 0); !reflect.DeepEqual(err, wantErrOpen) {
+				t.Errorf("open: error = %#v, want %#v", err, wantErrOpen)
+			}
+
+			wantErrDestroy := &hst.AppError{Step: "destroy state entry",
+				Err: &os.PathError{Op: "remove", Path: "/proc/nonexistent", Err: syscall.ENOENT}}
+			if err := eh.destroy(); !reflect.DeepEqual(err, wantErrDestroy) {
+				t.Errorf("destroy: error = %#v, want %#v", err, wantErrDestroy)
+			}
+		})
+	})
+
+	t.Run("saveload", func(t *testing.T) {
+		t.Parallel()
+		eh := stateEntryHandle{pathname: check.MustAbs(t.TempDir()).Append("entry"),
+			ID: newTemplateState().ID}
+
+		if err := eh.save(newTemplateState()); err != nil {
+			t.Fatalf("save: error = %v", err)
+		}
+
+		t.Run("validate", func(t *testing.T) {
+			t.Parallel()
+
+			t.Run("internal", func(t *testing.T) {
+				t.Parallel()
+
+				var got hst.State
+				if f, err := os.Open(eh.pathname.String()); err != nil {
+					t.Fatal(err.Error())
+				} else if _, err = entryDecode(f, &got); err != nil {
+					t.Fatalf("entryDecode: error = %v", err)
+				} else if err = f.Close(); err != nil {
+					t.Fatal(f.Close())
+				}
+
+				if want := newTemplateState(); !reflect.DeepEqual(&got, want) {
+					t.Errorf("entryDecode: %#v, want %#v", &got, want)
+				}
+			})
+
+			t.Run("load header only", func(t *testing.T) {
+				t.Parallel()
+
+				if et, err := eh.load(nil); err != nil {
+					t.Fatalf("load: error = %v", err)
+				} else if want := newTemplateState().Enablements.Unwrap(); et != want {
+					t.Errorf("load: et = %x, want %x", et, want)
+				}
+			})
+
+			t.Run("load", func(t *testing.T) {
+				t.Parallel()
+
+				var got hst.State
+				if _, err := eh.load(&got); err != nil {
+					t.Fatalf("load: error = %v", err)
+				} else if want := newTemplateState(); !reflect.DeepEqual(&got, want) {
+					t.Errorf("load: %#v, want %#v", &got, want)
+				}
+			})
+
+			t.Run("load inconsistent", func(t *testing.T) {
+				t.Parallel()
+				wantErr := &hst.AppError{Step: "validate state identifier", Err: os.ErrInvalid,
+					Msg: "state entry 00000000000000000000000000000000 has unexpected id aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}
+
+				ehi := stateEntryHandle{pathname: eh.pathname}
+				if _, err := ehi.load(new(hst.State)); !reflect.DeepEqual(err, wantErr) {
+					t.Errorf("load: error = %#v, want %#v", err, wantErr)
+				}
+			})
+		})
+	})
+}
+
+func TestStoreHandle(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		ents [2][]string
+		want func(newEh func(err error, name string) *stateEntryHandle) []*stateEntryHandle
+		ext  func(t *testing.T, entries iter.Seq[*stateEntryHandle], n int)
+	}{
+		{"errors", [2][]string{{
+			"e81eb203b4190ac5c3842ef44d429945",
+			"lock",
+			"f0-invalid",
+		}, {
+			"f1-directory",
+		}}, func(newEh func(err error, name string) *stateEntryHandle) []*stateEntryHandle {
+			return []*stateEntryHandle{
+				newEh(nil, "e81eb203b4190ac5c3842ef44d429945"),
+				newEh(&hst.AppError{Step: "decode store segment entry",
+					Err: hst.IdentifierDecodeError{Err: hst.ErrIdentifierLength}}, "f0-invalid"),
+				newEh(&hst.AppError{Step: "read store segment entries",
+					Err: errors.New(`unexpected directory "f1-directory" in store`)}, "f1-directory"),
+			}
+		}, nil},
+
+		{"success", [2][]string{{
+			"e81eb203b4190ac5c3842ef44d429945",
+			"7958cfbb9272d9cf9cfd61c85afa13f1",
+			"d0b5f7446dd5bd3424ff2f7ac9cace1e",
+			"c8c8e2c4aea5c32fe47240ff8caa874e",
+			"fa0d30b249d80f155a1f80ceddcc32f2",
+			"lock",
+		}}, func(newEh func(err error, name string) *stateEntryHandle) []*stateEntryHandle {
+			return []*stateEntryHandle{
+				newEh(nil, "7958cfbb9272d9cf9cfd61c85afa13f1"),
+				newEh(nil, "c8c8e2c4aea5c32fe47240ff8caa874e"),
+				newEh(nil, "d0b5f7446dd5bd3424ff2f7ac9cace1e"),
+				newEh(nil, "e81eb203b4190ac5c3842ef44d429945"),
+				newEh(nil, "fa0d30b249d80f155a1f80ceddcc32f2"),
+			}
+		}, func(t *testing.T, entries iter.Seq[*stateEntryHandle], n int) {
+			if n != 5 {
+				t.Fatalf("entries: n = %d", n)
+			}
+
+			// check partial drain
+			for range entries {
+				break
+			}
+		}},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			p := check.MustAbs(t.TempDir()).Append("segment")
+			if err := os.Mkdir(p.String(), 0700); err != nil {
+				t.Fatal(err.Error())
+			}
+			createEntries(t, p, tc.ents)
+
+			var got []*stateEntryHandle
+			if entries, n, err := (&storeHandle{
+				identity: -0xbad,
+				path:     p,
+				fileMu:   lockedfile.MutexAt(p.Append("lock").String()),
+			}).entries(); err != nil {
+				t.Fatalf("entries: error = %v", err)
+			} else {
+				got = slices.AppendSeq(make([]*stateEntryHandle, 0, n), entries)
+				if tc.ext != nil {
+					tc.ext(t, entries, n)
+				}
+			}
+
+			slices.SortFunc(got, func(a, b *stateEntryHandle) int { return strings.Compare(a.pathname.String(), b.pathname.String()) })
+			want := tc.want(func(err error, name string) *stateEntryHandle {
+				eh := stateEntryHandle{decodeErr: err, pathname: p.Append(name)}
+				if err == nil {
+					if err = eh.UnmarshalText([]byte(name)); err != nil {
+						t.Fatalf("UnmarshalText: error = %v", err)
+					}
+				}
+				return &eh
+			})
+
+			if !reflect.DeepEqual(got, want) {
+				t.Errorf("entries: %q, want %q", got, want)
+			}
+		})
+	}
+
+	t.Run("nonexistent", func(t *testing.T) {
+		var wantErr = &hst.AppError{Step: "read store segment entries", Err: &os.PathError{
+			Op:   "open",
+			Path: "/proc/nonexistent",
+			Err:  syscall.ENOENT,
+		}}
+		if _, _, err := (&storeHandle{
+			identity: -0xbad,
+			path:     check.MustAbs("/proc/nonexistent"),
+		}).entries(); !reflect.DeepEqual(err, wantErr) {
+			t.Fatalf("entries: error = %#v, want %#v", err, wantErr)
+		}
+	})
+}
+
+// createEntries creates file and directory entries in the specified prefix.
+func createEntries(t *testing.T, prefix *check.Absolute, ents [2][]string) {
+	for _, s := range ents[0] {
+		if f, err := os.OpenFile(prefix.Append(s).String(), os.O_CREATE|os.O_EXCL, 0600); err != nil {
+			t.Fatal(err.Error())
+		} else if err = f.Close(); err != nil {
+			t.Fatal(err.Error())
+		}
+	}
+	for _, s := range ents[1] {
+		if err := os.Mkdir(prefix.Append(s).String(), 0700); err != nil {
+			t.Fatal(err.Error())
+		}
+	}
+}
diff --git a/internal/store/store.go b/internal/store/store.go
new file mode 100644
index 00000000..942eeef5
--- /dev/null
+++ b/internal/store/store.go
@@ -0,0 +1,162 @@
+package store
+
+import (
+	"errors"
+	"io/fs"
+	"iter"
+	"os"
+	"strconv"
+	"sync"
+	"syscall"
+
+	"hakurei.app/container/check"
+	"hakurei.app/hst"
+	"hakurei.app/internal/lockedfile"
+)
+
+// storeMutexName is the pathname of the file backing [lockedfile.Mutex] of a stateStore and storeHandle.
+const storeMutexName = "lock"
+
+// A stateStore keeps track of [hst.State] via a well-known filesystem accessible to all hakurei priv-side processes.
+// Access to store data and related resources are synchronised on a per-segment basis via storeHandle.
+type stateStore struct {
+	// Pathname of directory that the store is rooted in.
+	base *check.Absolute
+
+	// All currently known instances of storeHandle, keyed by their identity.
+	handles sync.Map
+
+	// Inter-process mutex to synchronise operations against the entire store.
+	// Held during List and when initialising previously unknown identities during Do.
+	// Must not be accessed directly. Callers should use the bigLock method instead.
+	fileMu *lockedfile.Mutex
+
+	// For creating the base directory.
+	mkdirOnce sync.Once
+	// Stored error value via mkdirOnce.
+	mkdirErr error
+}
+
+// bigLock acquires fileMu on stateStore.
+// A non-nil error returned by bigLock is of type [hst.AppError].
+func (s *stateStore) bigLock() (unlock func(), err error) {
+	s.mkdirOnce.Do(func() { s.mkdirErr = os.MkdirAll(s.base.String(), 0700) })
+	if s.mkdirErr != nil {
+		return nil, &hst.AppError{Step: "create state store directory", Err: s.mkdirErr}
+	}
+
+	if unlock, err = s.fileMu.Lock(); err != nil {
+		return nil, &hst.AppError{Step: "acquire lock on the state store", Err: err}
+	}
+	return
+}
+
+// identityHandle loads or initialises a storeHandle for identity.
+// A non-nil error returned by identityHandle is of type [hst.AppError].
+func (s *stateStore) identityHandle(identity int) (*storeHandle, error) {
+	h := new(storeHandle)
+	h.mu.Lock()
+
+	if v, ok := s.handles.LoadOrStore(identity, h); ok {
+		h = v.(*storeHandle)
+	} else {
+		// acquire big lock to initialise previously unknown segment handle
+		if unlock, err := s.bigLock(); err != nil {
+			return nil, err
+		} else {
+			defer unlock()
+		}
+
+		h.identity = identity
+		h.path = s.base.Append(strconv.Itoa(identity))
+		h.fileMu = lockedfile.MutexAt(h.path.Append(storeMutexName).String())
+
+		err := os.MkdirAll(h.path.String(), 0700)
+		h.mu.Unlock()
+		if err != nil && !errors.Is(err, fs.ErrExist) {
+			// handle methods will likely return ENOENT
+			s.handles.CompareAndDelete(identity, h)
+			return nil, &hst.AppError{Step: "create store segment directory", Err: err}
+		}
+	}
+	return h, nil
+}
+
+// segmentIdentity is produced by the iterator returned by stateStore.segments.
+type segmentIdentity struct {
+	// Identity of the current segment.
+	identity int
+	// Error encountered while processing this segment.
+	err error
+}
+
+// segments returns an iterator over all segmentIdentity known to the store.
+// To obtain a storeHandle on a segment, caller must then call identityHandle.
+// A non-nil error returned by segments is of type [hst.AppError].
+func (s *stateStore) segments() (iter.Seq[segmentIdentity], int, error) {
+	// read directory contents, should only contain storeMutexName and identity
+	var entries []os.DirEntry
+
+	// acquire big lock to read store segment list
+	if unlock, err := s.bigLock(); err != nil {
+		return nil, -1, err
+	} else {
+		entries, err = os.ReadDir(s.base.String())
+		unlock()
+
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
+			return nil, -1, &hst.AppError{Step: "read store segments", Err: err}
+		}
+	}
+
+	// expects lock file
+	l := len(entries)
+	if l > 0 {
+		l--
+	}
+
+	return func(yield func(segmentIdentity) bool) {
+		// for error reporting
+		const step = "process store segment"
+
+		for _, ent := range entries {
+			si := segmentIdentity{identity: -1}
+
+			// should only be the big lock
+			if !ent.IsDir() {
+				if ent.Name() == storeMutexName {
+					continue
+				}
+
+				// this should never happen
+				si.err = &hst.AppError{Step: step, Err: syscall.EISDIR,
+					Msg: "skipped non-directory entry " + strconv.Quote(ent.Name())}
+				goto out
+			}
+
+			// failure paths either indicates a serious bug or external interference
+			if v, err := strconv.Atoi(ent.Name()); err != nil {
+				si.err = &hst.AppError{Step: step, Err: err,
+					Msg: "skipped non-identity entry " + strconv.Quote(ent.Name())}
+				goto out
+			} else if v < hst.IdentityMin || v > hst.IdentityMax {
+				si.err = &hst.AppError{Step: step, Err: syscall.ERANGE,
+					Msg: "skipped out of bounds entry " + strconv.Itoa(v)}
+				goto out
+			} else {
+				si.identity = v
+			}
+
+		out:
+			if !yield(si) {
+				break
+			}
+		}
+	}, l, nil
+}
+
+// newStore returns the address of a new instance of stateStore.
+// Multiple instances of stateStore rooted in the same directory is supported, but discouraged.
+func newStore(base *check.Absolute) *stateStore {
+	return &stateStore{base: base, fileMu: lockedfile.MutexAt(base.Append(storeMutexName).String())}
+}
diff --git a/internal/store/store_test.go b/internal/store/store_test.go
new file mode 100644
index 00000000..db1a4c71
--- /dev/null
+++ b/internal/store/store_test.go
@@ -0,0 +1,254 @@
+package store
+
+import (
+	"cmp"
+	"iter"
+	"os"
+	"reflect"
+	"slices"
+	"strconv"
+	"strings"
+	"syscall"
+	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/hst"
+)
+
+func TestStateStoreBigLock(t *testing.T) {
+	t.Parallel()
+
+	{
+		s := newStore(check.MustAbs(t.TempDir()).Append("state"))
+		for i := 0; i < 2; i++ { // check once behaviour
+			if unlock, err := s.bigLock(); err != nil {
+				t.Fatalf("bigLock: error = %v", err)
+			} else {
+				unlock()
+			}
+		}
+	}
+
+	t.Run("mkdir", func(t *testing.T) {
+		t.Parallel()
+
+		wantErr := &hst.AppError{Step: "create state store directory",
+			Err: &os.PathError{Op: "mkdir", Path: "/proc/nonexistent", Err: syscall.ENOENT}}
+		for i := 0; i < 2; i++ { // check once behaviour
+			if _, err := newStore(check.MustAbs("/proc/nonexistent")).bigLock(); !reflect.DeepEqual(err, wantErr) {
+				t.Errorf("bigLock: error = %#v, want %#v", err, wantErr)
+			}
+		}
+	})
+
+	t.Run("access", func(t *testing.T) {
+		t.Parallel()
+
+		base := check.MustAbs(t.TempDir()).Append("inaccessible")
+		if err := os.MkdirAll(base.String(), 0); err != nil {
+			t.Fatal(err.Error())
+		}
+
+		wantErr := &hst.AppError{Step: "acquire lock on the state store",
+			Err: &os.PathError{Op: "open", Path: base.Append(storeMutexName).String(), Err: syscall.EACCES}}
+		if _, err := newStore(base).bigLock(); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("bigLock: error = %#v, want %#v", err, wantErr)
+		}
+	})
+}
+
+func TestStateStoreIdentityHandle(t *testing.T) {
+	t.Parallel()
+
+	t.Run("loadstore", func(t *testing.T) {
+		t.Parallel()
+
+		s := newStore(check.MustAbs(t.TempDir()).Append("store"))
+
+		var handleAddr [8]*storeHandle
+		checkHandle := func(identity int, load bool) {
+			if h, err := s.identityHandle(identity); err != nil {
+				t.Fatalf("identityHandle: error = %v", err)
+			} else if load != (handleAddr[identity] != nil) {
+				t.Fatalf("identityHandle: load = %v, want %v", load, handleAddr[identity] != nil)
+			} else if !load {
+				handleAddr[identity] = h
+
+				if h.identity != identity {
+					t.Errorf("identityHandle: identity = %d, want %d", h.identity, identity)
+				}
+			} else if h != handleAddr[identity] {
+				t.Fatalf("identityHandle: %p, want %p", h, handleAddr[identity])
+			}
+		}
+
+		checkHandle(0, false)
+		checkHandle(1, false)
+		checkHandle(2, false)
+		checkHandle(3, false)
+		checkHandle(7, false)
+		checkHandle(7, true)
+		checkHandle(2, true)
+		checkHandle(1, true)
+		checkHandle(2, true)
+		checkHandle(0, true)
+	})
+
+	t.Run("access", func(t *testing.T) {
+		t.Parallel()
+
+		base := check.MustAbs(t.TempDir()).Append("inaccessible")
+		if err := os.MkdirAll(base.String(), 0); err != nil {
+			t.Fatal(err.Error())
+		}
+
+		wantErr := &hst.AppError{Step: "acquire lock on the state store",
+			Err: &os.PathError{Op: "open", Path: base.Append(storeMutexName).String(), Err: syscall.EACCES}}
+		if _, err := newStore(base).identityHandle(0); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("identityHandle: error = %#v, want %#v", err, wantErr)
+		}
+	})
+
+	t.Run("access segment", func(t *testing.T) {
+		t.Parallel()
+
+		base := check.MustAbs(t.TempDir()).Append("inaccessible")
+		if err := os.MkdirAll(base.String(), 0700); err != nil {
+			t.Fatal(err.Error())
+		}
+		if f, err := os.Create(base.Append(storeMutexName).String()); err != nil {
+			t.Fatal(err.Error())
+		} else if err = f.Close(); err != nil {
+			t.Fatal(err.Error())
+		}
+		if err := os.Chmod(base.String(), 0100); err != nil {
+			t.Fatal(err.Error())
+		}
+		t.Cleanup(func() {
+			if err := os.Chmod(base.String(), 0700); err != nil {
+				t.Fatal(err.Error())
+			}
+		})
+
+		wantErr := &hst.AppError{Step: "create store segment directory",
+			Err: &os.PathError{Op: "mkdir", Path: base.Append("0").String(), Err: syscall.EACCES}}
+		if _, err := newStore(base).identityHandle(0); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("identityHandle: error = %#v, want %#v", err, wantErr)
+		}
+	})
+}
+
+func TestStateStoreSegments(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		ents [2][]string
+		want []segmentIdentity
+		ext  func(t *testing.T, segments iter.Seq[segmentIdentity], n int)
+	}{
+		{"errors", [2][]string{{
+			"f0-invalid-file",
+		}, {
+			"f1-invalid-syntax",
+			"9999",
+			"16384",
+		}}, []segmentIdentity{
+			{-1, &hst.AppError{Step: "process store segment", Err: syscall.EISDIR,
+				Msg: `skipped non-directory entry "f0-invalid-file"`}},
+			{-1, &hst.AppError{Step: "process store segment", Err: syscall.ERANGE,
+				Msg: `skipped out of bounds entry 16384`}},
+			{-1, &hst.AppError{Step: "process store segment",
+				Err: &strconv.NumError{Func: "Atoi", Num: "f1-invalid-syntax", Err: strconv.ErrSyntax},
+				Msg: `skipped non-identity entry "f1-invalid-syntax"`}},
+			{9999, nil},
+		}, nil},
+
+		{"success", [2][]string{{
+			"lock",
+		}, {
+			"0",
+			"1",
+			"2",
+			"3",
+			"4",
+			"5",
+			"6",
+			"7",
+			"9",
+			"13",
+			"20",
+			"31",
+			"197",
+		}}, []segmentIdentity{
+			{0, nil},
+			{1, nil},
+			{2, nil},
+			{3, nil},
+			{4, nil},
+			{5, nil},
+			{6, nil},
+			{7, nil},
+			{9, nil},
+			{13, nil},
+			{20, nil},
+			{31, nil},
+			{197, nil},
+		}, func(t *testing.T, segments iter.Seq[segmentIdentity], n int) {
+			if n != 13 {
+				t.Fatalf("segments: n = %d", n)
+			}
+
+			// check partial drain
+			for range segments {
+				break
+			}
+		}},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			base := check.MustAbs(t.TempDir()).Append("store")
+			if err := os.Mkdir(base.String(), 0700); err != nil {
+				t.Fatal(err.Error())
+			}
+			createEntries(t, base, tc.ents)
+
+			var got []segmentIdentity
+			if segments, n, err := newStore(base).segments(); err != nil {
+				t.Fatalf("segments: error = %v", err)
+			} else {
+				got = slices.AppendSeq(make([]segmentIdentity, 0, n), segments)
+				if tc.ext != nil {
+					tc.ext(t, segments, n)
+				}
+			}
+
+			slices.SortFunc(got, func(a, b segmentIdentity) int {
+				if a.identity == b.identity {
+					return strings.Compare(a.err.Error(), b.err.Error())
+				}
+				return cmp.Compare(a.identity, b.identity)
+			})
+			if !reflect.DeepEqual(got, tc.want) {
+				t.Errorf("segments: %#v, want %#v", got, tc.want)
+			}
+		})
+	}
+
+	t.Run("access", func(t *testing.T) {
+		t.Parallel()
+
+		base := check.MustAbs(t.TempDir()).Append("inaccessible")
+		if err := os.MkdirAll(base.String(), 0); err != nil {
+			t.Fatal(err.Error())
+		}
+
+		wantErr := &hst.AppError{Step: "acquire lock on the state store",
+			Err: &os.PathError{Op: "open", Path: base.Append(storeMutexName).String(), Err: syscall.EACCES}}
+		if _, _, err := newStore(base).segments(); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("segments: error = %#v, want %#v", err, wantErr)
+		}
+	})
+}