@@ -0,0 +1,387 @@
+// Package checkpoint lets state clients order their checkpoints after the resources they summarize.
+package checkpoint
+
+import (
+	"context"
+	"errors"
+	"slices"
+	"sync"
+	"sync/atomic"
+
+	"github.com/cloudquery/plugin-sdk/v4/message"
+	"github.com/cloudquery/plugin-sdk/v4/schema"
+)
+
+// ErrSyncAborted is returned for checkpoints that were dropped because the sync was cancelled.
+var ErrSyncAborted = errors.New("sync aborted before the checkpoint could be sent")
+
+// Scope is the part of a sync that a table resolver drives: the resources it produces and their relations.
+type Scope interface {
+	// Checkpoint passes msg to Deliver after every resource the scope produced before the call is sent, or to Drop if
+	// the sync is cancelled first. It may return before that.
+	Checkpoint(ctx context.Context, msg message.SyncMessage) error
+}
+
+// Ordering is implemented by scopes that never send a checkpoint before an unsent one they already hold.
+type Ordering interface {
+	// SendsAfter reports whether a checkpoint passed to the scope now will be sent no earlier than delivery.
+	SendsAfter(delivery *Delivery) bool
+}
+
+// Collapsing is implemented by scopes that can become equivalent to a scope they share with others, such as a table
+// resolver's scope once the resolver ends.
+type Collapsing interface {
+	// Collapsed returns the shared scope, or the scope itself.
+	Collapsed() Scope
+}
+
+type scopeKey struct{}
+
+func WithScope(ctx context.Context, scope Scope) context.Context {
+	return context.WithValue(ctx, scopeKey{}, scope)
+}
+
+// FromContext returns the scope ctx belongs to, or nil outside table resolvers.
+func FromContext(ctx context.Context) Scope {
+	scope, _ := ctx.Value(scopeKey{}).(Scope)
+	return scope
+}
+
+// Deliver sends msg on res. A *Barrier is marked as reached instead, and a *Tracked message is sent and marked as sent.
+func Deliver(ctx context.Context, res chan<- message.SyncMessage, msg message.SyncMessage) error {
+	switch m := msg.(type) {
+	case *Barrier:
+		return m.reached(ctx, res)
+	case *Tracked:
+		if m.Delivery.Aborted() {
+			return nil
+		}
+		if err := Deliver(ctx, res, m.Message); err != nil {
+			m.Delivery.Abort()
+			return err
+		}
+		return m.Delivery.Finish(ctx, res)
+	default:
+		select {
+		case res <- msg:
+			return nil
+		case <-ctx.Done():
+			return context.Cause(ctx)
+		}
+	}
+}
+
+// Drop marks msg as never to be sent: a dropped *Tracked message, or the message a dropped *Barrier leads to, is
+// aborted, and so are the checkpoints waiting for it.
+func Drop(msg message.SyncMessage) {
+	switch m := msg.(type) {
+	case *Barrier:
+		m.abort()
+	case *Tracked:
+		m.Delivery.Abort()
+	}
+}
+
+// Barrier travels through scopes and sent checkpoints, and calls reached each time one of them gets to it.
+type Barrier struct {
+	reached func(ctx context.Context, res chan<- message.SyncMessage) error
+	abort   func()
+}
+
+func (*Barrier) GetTable() *schema.Table { return nil }
+func (*Barrier) IsSyncMessage() bool     { return true }
+
+// Tracked is a message that marks Delivery once Deliver sends it.
+type Tracked struct {
+	Message  message.SyncMessage
+	Delivery *Delivery
+}
+
+func (*Tracked) GetTable() *schema.Table { return nil }
+func (*Tracked) IsSyncMessage() bool     { return true }
+
+// Delivery tracks whether a checkpoint was sent or aborted, so that later checkpoints of the same keys can wait for it.
+type Delivery struct {
+	mu      sync.Mutex
+	done    bool
+	aborted bool
+	waiting []*Barrier
+	// set when the checkpoint was merged into another one, which is then sent in its place
+	mergedInto *Delivery
+	// for a checkpoint that others are merged into: the scope holding it
+	heldBy Scope
+	// set when the checkpoint is aborted
+	abortFlags map[*atomic.Bool]struct{}
+}
+
+// NewHeld returns a Delivery for a checkpoint that holder merges other checkpoints into.
+func NewHeld(holder Scope) *Delivery {
+	return &Delivery{heldBy: holder}
+}
+
+// root returns the checkpoint s is sent with.
+func (s *Delivery) root() *Delivery {
+	for {
+		s.mu.Lock()
+		into := s.mergedInto
+		s.mu.Unlock()
+		if into == nil {
+			return s
+		}
+		s = into
+	}
+}
+
+// lockRoot returns the checkpoint s is sent with, locked. It is still the root once locked, as MergeInto runs under the
+// lock of the checkpoint it merges.
+func (s *Delivery) lockRoot() *Delivery {
+	for {
+		r := s.root()
+		r.mu.Lock()
+		if r.mergedInto == nil {
+			return r
+		}
+		r.mu.Unlock()
+	}
+}
+
+// Delivered reports whether the checkpoint was sent.
+func (s *Delivery) Delivered() bool {
+	r := s.lockRoot()
+	defer r.mu.Unlock()
+	return r.done
+}
+
+// Aborted reports whether the checkpoint was dropped.
+func (s *Delivery) Aborted() bool {
+	r := s.lockRoot()
+	defer r.mu.Unlock()
+	return r.aborted
+}
+
+func (s *Delivery) holder() Scope {
+	r := s.lockRoot()
+	defer r.mu.Unlock()
+	return r.heldBy
+}
+
+// NotifyAbort sets flag if the checkpoint is aborted, or now if it already was.
+func (s *Delivery) NotifyAbort(flag *atomic.Bool) {
+	r := s.lockRoot()
+	defer r.mu.Unlock()
+	if r.aborted {
+		flag.Store(true)
+		return
+	}
+	if r.abortFlags == nil {
+		r.abortFlags = make(map[*atomic.Bool]struct{})
+	}
+	r.abortFlags[flag] = struct{}{}
+}
+
+// MergedInto reports whether s is sent with into.
+func (s *Delivery) MergedInto(into *Delivery) bool {
+	return s != into && s.root() == into
+}
+
+// MergeInto marks s as sent once into is sent, and aborted if into is aborted. into must not be sent yet.
+func (s *Delivery) MergeInto(into *Delivery) {
+	s.mu.Lock()
+	waiting, flags := s.waiting, s.abortFlags
+	s.waiting, s.abortFlags = nil, nil
+	s.mergedInto = into
+	s.mu.Unlock()
+	for _, barrier := range waiting {
+		into.wait(barrier)
+	}
+	for flag := range flags {
+		into.NotifyAbort(flag)
+	}
+}
+
+// wait makes barrier wait for s. It reports true, without waiting, if s was sent.
+func (s *Delivery) wait(barrier *Barrier) bool {
+	r := s.lockRoot()
+	switch {
+	case r.done:
+		r.mu.Unlock()
+		return true
+	case r.aborted:
+		r.mu.Unlock()
+		barrier.abort()
+		return false
+	default:
+		r.waiting = append(r.waiting, barrier)
+		r.mu.Unlock()
+		return false
+	}
+}
+
+// Finish marks s as sent, and releases the checkpoints waiting for it.
+func (s *Delivery) Finish(ctx context.Context, res chan<- message.SyncMessage) error {
+	s.mu.Lock()
+	if s.done || s.aborted {
+		s.mu.Unlock()
+		return nil
+	}
+	s.done = true
+	waiting := s.waiting
+	s.waiting, s.abortFlags = nil, nil
+	s.mu.Unlock()
+	var errs []error
+	for _, barrier := range waiting {
+		if err := barrier.reached(ctx, res); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	return errors.Join(errs...)
+}
+
+// Abort marks s as dropped, and aborts the checkpoints waiting for it.
+func (s *Delivery) Abort() {
+	r := s.lockRoot()
+	if r.done || r.aborted {
+		r.mu.Unlock()
+		return
+	}
+	r.aborted = true
+	waiting, flags := r.waiting, r.abortFlags
+	r.waiting, r.abortFlags = nil, nil
+	r.mu.Unlock()
+	for flag := range flags {
+		flag.Store(true)
+	}
+	for _, barrier := range waiting {
+		barrier.abort()
+	}
+}
+
+// Schedule passes msg to Deliver once every one of scope's resources produced before the call is sent, and every
+// checkpoint in after is sent, which may be after it returns. Without a scope or pending checkpoints to wait for, it
+// sends msg on res. The returned Delivery tracks msg. If a checkpoint in after was aborted, Schedule returns
+// ErrSyncAborted.
+func Schedule(ctx context.Context, res chan<- message.SyncMessage, scope Scope, after []*Delivery, msg message.SyncMessage) (*Delivery, error) {
+	pending := make([]*Delivery, 0, len(after))
+	for _, previous := range after {
+		switch {
+		case previous.Aborted():
+			return nil, ErrSyncAborted
+		case !previous.Delivered():
+			pending = append(pending, previous)
+		}
+	}
+	if scope == nil {
+		// waiting only for checkpoints one scope holds: going through that scope sends no earlier, and lets it merge msg
+		scope = commonHolder(pending)
+	}
+	if ordering, ok := scope.(Ordering); ok {
+		pending = slices.DeleteFunc(pending, ordering.SendsAfter)
+	}
+
+	sent := &Delivery{}
+	tracked := &Tracked{Message: msg, Delivery: sent}
+	scopes := flatten(scope)
+	if len(pending) == 0 {
+		switch len(scopes) {
+		case 0:
+			return sent, Deliver(ctx, res, tracked)
+		case 1:
+			return sent, scopes[0].Checkpoint(ctx, tracked)
+		}
+	}
+
+	var remaining atomic.Int64
+	remaining.Store(int64(len(scopes) + len(pending)))
+	barrier := &Barrier{
+		reached: func(ctx context.Context, res chan<- message.SyncMessage) error {
+			if remaining.Add(-1) > 0 {
+				return nil
+			}
+			return Deliver(ctx, res, tracked)
+		},
+		abort: sent.Abort,
+	}
+	var errs []error
+	for _, previous := range pending {
+		if previous.wait(barrier) {
+			if err := barrier.reached(ctx, res); err != nil {
+				errs = append(errs, err)
+			}
+		}
+	}
+	for _, s := range scopes {
+		if err := s.Checkpoint(ctx, barrier); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	if err := errors.Join(errs...); err != nil {
+		sent.Abort()
+		return nil, err
+	}
+	return sent, nil
+}
+
+func commonHolder(pending []*Delivery) Scope {
+	var holder Scope
+	for _, sent := range pending {
+		h := sent.holder()
+		if h == nil || (holder != nil && h != holder) {
+			return nil
+		}
+		holder = h
+	}
+	return holder
+}
+
+// Collapse returns the shared scope a Collapsing scope collapsed into, or scope itself.
+func Collapse(scope Scope) Scope {
+	if c, ok := scope.(Collapsing); ok {
+		return c.Collapsed()
+	}
+	return scope
+}
+
+// Join returns a scope whose checkpoints follow the resources of all the given scopes. It skips nil scopes, merges
+// scopes that collapsed into the same one, and returns nil if all are nil.
+func Join(scopes ...Scope) Scope {
+	seen := make(map[Scope]struct{}, len(scopes))
+	var unique []Scope
+	for _, scope := range scopes {
+		for _, s := range flatten(scope) {
+			s = Collapse(s)
+			if _, ok := seen[s]; ok {
+				continue
+			}
+			seen[s] = struct{}{}
+			unique = append(unique, s)
+		}
+	}
+	switch len(unique) {
+	case 0:
+		return nil
+	case 1:
+		return unique[0]
+	default:
+		return &joined{scopes: unique}
+	}
+}
+
+type joined struct {
+	scopes []Scope
+}
+
+func (j *joined) Checkpoint(ctx context.Context, msg message.SyncMessage) error {
+	_, err := Schedule(ctx, nil, j, nil, msg)
+	return err
+}
+
+func flatten(scope Scope) []Scope {
+	switch s := scope.(type) {
+	case nil:
+		return nil
+	case *joined:
+		return s.scopes
+	default:
+		return []Scope{s}
+	}
+}