Repository: mlange-42/arche (go)

Issue:
Lock world during Query iteration
Arche does not (and will not) support adding or removing components od entire entities during query iteration. However, nothing prevents the user from trying it, with undefined behaviour.

As a first measure, lock the world when a query is created, and hand it back to unlock.

Later, add/remove operations could be buffered and executed at unlock.

Pull request:
Querys lock the World
Queries lock the world, and unlock it when iteration is done.
Each query has a lock bit to track individual query locks.

Fixes #12

Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 30d0d39..55cbc06 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 ### Other
 
 * Use aligned item size in component storage, for approx. 20% faster query iteration (#25)
+* Queries lock the World, and automatically unlock it after iteration (#26)
 
 ## [[v0.1.1]](https://github.com/mlange-42/arche/compare/v0.1.0...v0.1.1)
 
diff --git a/ecs/bit_pool.go b/ecs/bit_pool.go
new file mode 100644
index 0000000..5152ace
--- /dev/null
+++ b/ecs/bit_pool.go
@@ -0,0 +1,43 @@
+package ecs
+
+// newEntityPool creates a new, initialized bit pool
+func newBitPool() bitPool {
+	return bitPool{
+		bits:      [MaskTotalBits]uint8{},
+		next:      0,
+		length:    0,
+		available: 0,
+	}
+}
+
+// bitPool is an entityPool implementation using implicit linked lists.
+// Implements https://skypjack.github.io/2019-05-06-ecs-baf-part-3/
+type bitPool struct {
+	bits      [MaskTotalBits]uint8
+	next      uint8
+	length    uint8
+	available uint8
+}
+
+// Get returns a fresh or recycled bit
+func (p *bitPool) Get() uint8 {
+	if p.available == 0 {
+		if p.length >= MaskTotalBits {
+			panic("run out of the maximum of 64 bits")
+		}
+		b := p.length
+		p.bits[p.length] = b
+		p.length++
+		return b
+	}
+	curr := p.next
+	p.next, p.bits[p.next] = p.bits[p.next], p.next
+	p.available--
+	return p.bits[curr]
+}
+
+// Recycle hands a bit back for recycling
+func (p *bitPool) Recycle(b uint8) {
+	p.next, p.bits[b] = b, p.next
+	p.available++
+}
diff --git a/ecs/entity_pool.go b/ecs/entity_pool.go
index 1bd3f51..03b33a0 100644
--- a/ecs/entity_pool.go
+++ b/ecs/entity_pool.go
@@ -2,7 +2,7 @@ package ecs
 
 import "math"
 
-// newEntityPool creates a new, initialized EntityPool
+// newEntityPool creates a new, initialized Entity pool
 func newEntityPool() entityPool {
 	return entityPool{
 		entities:  []Entity{{0, math.MaxUint16}},
@@ -11,7 +11,7 @@ func newEntityPool() entityPool {
 	}
 }
 
-// entityPool is an entityPool implementation using implicit linked lists.
+// entityPool is an implementation using implicit linked lists.
 // Implements https://skypjack.github.io/2019-05-06-ecs-baf-part-3/
 type entityPool struct {
 	entities  []Entity
diff --git a/ecs/query.go b/ecs/query.go
index bb31a37..b7972d9 100644
--- a/ecs/query.go
+++ b/ecs/query.go
@@ -6,17 +6,21 @@ import (
 
 // Query is an iterator to iterate entities
 type Query struct {
+	world      *World
 	archetypes []archetypeIter
 	index      int
 	done       bool
+	lockBit    uint8
 }
 
 // newQuery creates a new QueryIter
-func newQuery(arches []archetypeIter) Query {
+func newQuery(world *World, arches []archetypeIter, lockBit uint8) Query {
 	return Query{
+		world:      world,
 		archetypes: arches,
 		index:      0,
 		done:       false,
+		lockBit:    lockBit,
 	}
 }
 
@@ -32,6 +36,7 @@ func (q *Query) Next() bool {
 		q.index++
 		if q.index >= len(q.archetypes) {
 			q.done = true
+			q.world.closeQuery(q)
 			return false
 		}
 	}
@@ -61,6 +66,15 @@ func (q *Query) Entity() Entity {
 	return q.archetypes[q.index].Entity()
 }
 
+// Close closes the query and unlocks the world.
+//
+// Automatically called when iteration finishes.
+// Needs to be called only if breaking out of the query iteration.
+func (q *Query) Close() {
+	q.done = true
+	q.world.closeQuery(q)
+}
+
 type archetypeIter struct {
 	Archetype *archetype
 	Length    int
diff --git a/ecs/world.go b/ecs/world.go
index bfcd214..1007fa2 100644
--- a/ecs/world.go
+++ b/ecs/world.go
@@ -11,8 +11,10 @@ func NewWorld() World {
 	return World{
 		entities:   []entityIndex{{-1, 0}},
 		entityPool: newEntityPool(),
+		bitPool:    newBitPool(),
 		registry:   newComponentRegistry(),
 		archetypes: []archetype{newArchetype()},
+		locks:      Mask(0),
 	}
 }
 
@@ -21,13 +23,17 @@ type World struct {
 	entities   []entityIndex
 	archetypes []archetype
 	entityPool entityPool
+	bitPool    bitPool
 	registry   componentRegistry
+	locks      Mask
 }
 
 // NewEntity creates a new or recycled entity.
 //
 // Do not use during Query iteration!
 func (w *World) NewEntity() Entity {
+	w.checkLocked()
+
 	entity := w.entityPool.Get()
 	idx := w.archetypes[0].Add(entity)
 	if int(entity.id) == len(w.entities) {
@@ -42,6 +48,8 @@ func (w *World) NewEntity() Entity {
 //
 // Do not use during Query iteration!
 func (w *World) RemEntity(entity Entity) {
+	w.checkLocked()
+
 	index := w.entities[entity.id]
 	oldArch := &w.archetypes[index.arch]
 	swapped := oldArch.Remove(int(index.index))
@@ -79,6 +87,8 @@ func (w *World) Has(entity Entity, comp ID) bool {
 //
 // Do not use during Query iteration!
 func (w *World) Add(entity Entity, comps ...ID) {
+	w.checkLocked()
+
 	if len(comps) == 0 {
 		return
 	}
@@ -123,6 +133,8 @@ func (w *World) Add(entity Entity, comps ...ID) {
 //
 // Do not use during Query iteration!
 func (w *World) Remove(entity Entity, comps ...ID) {
+	w.checkLocked()
+
 	if len(comps) == 0 {
 		return
 	}
@@ -196,6 +208,8 @@ func (w *World) ComponentID(tp reflect.Type) ID {
 }
 
 // Query creates a query iterator for the given components
+//
+// Locks the world to prevent changes to component compositions
 func (w *World) Query(comps ...ID) Query {
 	mask := NewMask(comps...)
 	arches := []archetypeIter{}
@@ -206,7 +220,30 @@ func (w *World) Query(comps ...ID) Query {
 			arches = append(arches, newArchetypeIter(arch))
 		}
 	}
-	return newQuery(arches)
+	lock := w.bitPool.Get()
+	w.locks.Set(ID(lock), true)
+	return newQuery(w, arches, lock)
+}
+
+// closeQuery closes a query and unlocks the world
+func (w *World) closeQuery(query *Query) {
+	l := query.lockBit
+	if !w.locks.Get(ID(l)) {
+		panic("unbalanced query unlock")
+	}
+	w.locks.Set(ID(l), false)
+	w.bitPool.Recycle(l)
+}
+
+// IsLocked returns whether the world is locked by any queries
+func (w *World) IsLocked() bool {
+	return w.locks != 0
+}
+
+func (w *World) checkLocked() {
+	if w.locks != 0 {
+		panic("attempt to modify a locked world")
+	}
 }
 
 // ComponentID provides a way to get a component's ID via generics


Test patch:
diff --git a/ecs/bit_pool_test.go b/ecs/bit_pool_test.go
new file mode 100644
index 0000000..abaa204
--- /dev/null
+++ b/ecs/bit_pool_test.go
@@ -0,0 +1,26 @@
+package ecs
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBitPool(t *testing.T) {
+	p := newBitPool()
+
+	for i := 0; i < MaskTotalBits; i++ {
+		assert.Equal(t, i, int(p.Get()))
+	}
+
+	assert.Panics(t, func() { p.Get() })
+
+	for i := 0; i < 10; i++ {
+		p.Recycle(uint8(i))
+	}
+	for i := 9; i >= 0; i-- {
+		assert.Equal(t, i, int(p.Get()))
+	}
+
+	assert.Panics(t, func() { p.Get() })
+}
diff --git a/ecs/world_test.go b/ecs/world_test.go
index b7c9048..4cd008e 100644
--- a/ecs/world_test.go
+++ b/ecs/world_test.go
@@ -143,11 +143,57 @@ func TestWorldIter(t *testing.T) {
 		world.Add(entity, posID, rotID)
 	}
 
+	for i := 0; i < 100; i++ {
+		query := world.Query(posID, rotID)
+		for query.Next() {
+			pos := (*position)(query.Get(posID))
+			_ = pos
+		}
+		assert.Panics(t, func() { query.Next() })
+	}
+
+	for i := 0; i < MaskTotalBits-1; i++ {
+		query := world.Query(posID, rotID)
+		for query.Next() {
+			pos := (*position)(query.Get(posID))
+			_ = pos
+			break
+		}
+	}
 	query := world.Query(posID, rotID)
-	for query.Next() {
-		pos := (*position)(query.Get(posID))
-		_ = pos
+
+	assert.Panics(t, func() { world.Query(posID, rotID) })
+
+	query.Close()
+	assert.Panics(t, func() { query.Close() })
+}
+
+func TestWorldLock(t *testing.T) {
+	world := NewWorld()
+
+	posID := ComponentID[position](&world)
+	rotID := ComponentID[rotation](&world)
+
+	var entity Entity
+	for i := 0; i < 100; i++ {
+		entity = world.NewEntity()
+		world.Add(entity, posID)
 	}
+
+	query1 := world.Query(posID)
+	query2 := world.Query(posID)
+	assert.True(t, world.IsLocked())
+	query1.Close()
+	assert.True(t, world.IsLocked())
+	query2.Close()
+	assert.False(t, world.IsLocked())
+
+	query1 = world.Query(posID)
+
+	assert.Panics(t, func() { world.NewEntity() })
+	assert.Panics(t, func() { world.RemEntity(entity) })
+	assert.Panics(t, func() { world.Add(entity, rotID) })
+	assert.Panics(t, func() { world.Remove(entity, posID) })
 }
 
 func TestRegisterComponents(t *testing.T) {