refactor: dataset.Sync() = fetch+conditional-swap, no public Swap()

Callers only need Init() + Run() + Load(). Sync() handles the full
fetch→swap cycle internally when the source reports a change.

net/dataset/dataset.go

@@ -2,7 +2,7 @@
 // atomic.Pointer (hot-swap), providing a generic periodically-updated
 // in-memory dataset with lock-free reads.
 //
-// Standalone dataset (one syncer, one value):
+// Standalone dataset:
 //
 //	ds := dataset.New(cacher, func() (*MyType, error) {
 //	    return mytype.LoadFile(path)
@@ -11,11 +11,11 @@
 //	go ds.Run(ctx, 47*time.Minute)
 //	val := ds.Load() // *MyType, lock-free
 //
-// Group (one syncer, multiple values — e.g. inbound+outbound from one git repo):
+// Group (one syncer, multiple values):
 //
 //	g := dataset.NewGroup(repo)
-//	inbound  := dataset.Add(g, func() (*ipcohort.Cohort, error) { return ipcohort.LoadFiles(inboundPaths...) })
-//	outbound := dataset.Add(g, func() (*ipcohort.Cohort, error) { return ipcohort.LoadFiles(outboundPaths...) })
+//	inbound  := dataset.Add(g, func() (*ipcohort.Cohort, error) { ... })
+//	outbound := dataset.Add(g, func() (*ipcohort.Cohort, error) { ... })
 //	if err := g.Init(); err != nil { ... }
 //	go g.Run(ctx, 47*time.Minute)
 package dataset
@@ -28,8 +28,7 @@ import (
 	"time"
 )
 
-// Syncer is implemented by any value that can fetch a remote resource and
-// report whether it changed.
+// Syncer reports whether a remote resource has changed.
 type Syncer interface {
 	Fetch() (updated bool, err error)
 }
@@ -50,8 +49,7 @@ func (ms MultiSyncer) Fetch() (bool, error) {
 	return anyUpdated, nil
 }
 
-// NopSyncer is a Syncer that always reports no update and no error.
-// Use for datasets backed by local files with no remote source.
+// NopSyncer always reports no update. Use for local-file datasets.
 type NopSyncer struct{}
 
 func (NopSyncer) Fetch() (bool, error) { return false, nil }
@@ -59,10 +57,9 @@ func (NopSyncer) Fetch() (bool, error) { return false, nil }
 // Dataset couples a Syncer, a load function, and an atomic.Pointer[T].
 // Load is safe for concurrent use without locks.
 type Dataset[T any] struct {
-	// Name is used in error messages. Optional.
+	// Name is used in error messages.
 	Name string
-	// Close is called with the previous value after each successful swap.
-	// Use this for values that hold resources, e.g. func(r *geoip2.Reader) { r.Close() }.
+	// Close is called with the old value after each successful swap.
 	Close func(*T)
 
 	syncer Syncer
@@ -70,8 +67,7 @@ type Dataset[T any] struct {
 	ptr    atomic.Pointer[T]
 }
 
-// New creates a Dataset. The syncer fetches updates; load produces the value.
-// load is a closure — it captures whatever paths or config it needs.
+// New creates a Dataset. The syncer reports changes; load produces the value.
 func New[T any](syncer Syncer, load func() (*T, error)) *Dataset[T] {
 	return &Dataset[T]{syncer: syncer, load: load}
 }
@@ -81,26 +77,41 @@ func (d *Dataset[T]) Load() *T {
 	return d.ptr.Load()
 }
 
-// Init fetches (if needed) then always loads, ensuring the dataset is
-// populated on startup from an existing local file even if nothing changed.
+func (d *Dataset[T]) swap() error {
+	val, err := d.load()
+	if err != nil {
+		return err
+	}
+	if old := d.ptr.Swap(val); old != nil && d.Close != nil {
+		d.Close(old)
+	}
+	return nil
+}
+
+// Sync calls the syncer and, if updated, reloads and atomically installs the
+// new value. Returns whether the source changed.
+func (d *Dataset[T]) Sync() (bool, error) {
+	updated, err := d.syncer.Fetch()
+	if err != nil {
+		return false, err
+	}
+	if !updated {
+		return false, nil
+	}
+	return true, d.swap()
+}
+
+// Init syncs and always loads, ensuring the dataset is populated from an
+// existing local file even if nothing changed upstream.
 func (d *Dataset[T]) Init() error {
 	if _, err := d.syncer.Fetch(); err != nil {
 		return err
 	}
-	return d.reload()
+	return d.swap()
 }
 
-// Sync fetches and reloads if the content changed. Returns whether updated.
-func (d *Dataset[T]) Sync() (bool, error) {
-	updated, err := d.syncer.Fetch()
-	if err != nil || !updated {
-		return updated, err
-	}
-	return true, d.reload()
-}
-
-// Run calls Sync on every interval until ctx is done.
-// Errors are written to stderr and do not stop the loop.
+// Run calls Sync on every interval. Errors are written to stderr and do not
+// stop the loop.
 func (d *Dataset[T]) Run(ctx context.Context, interval time.Duration) {
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()
@@ -120,26 +131,15 @@ func (d *Dataset[T]) Run(ctx context.Context, interval time.Duration) {
 	}
 }
 
-func (d *Dataset[T]) reload() error {
-	val, err := d.load()
-	if err != nil {
-		return err
-	}
-	if old := d.ptr.Swap(val); old != nil && d.Close != nil {
-		d.Close(old)
-	}
-	return nil
-}
-
 // -- Group: one Syncer driving multiple datasets ---------------------------
 
-// member is the type-erased reload handle stored in a Group.
+// member is the type-erased swap handle stored in a Group.
 type member interface {
-	reload() error
+	swap() error
 }
 
 // Group ties one Syncer to multiple datasets so a single Fetch drives all
-// reloads — no redundant network calls when datasets share a source.
+// swaps — no redundant network calls when datasets share a source.
 type Group struct {
 	syncer  Syncer
 	members []member
@@ -150,44 +150,38 @@ func NewGroup(syncer Syncer) *Group {
 	return &Group{syncer: syncer}
 }
 
-// View is the read-only handle returned by Add. It exposes only Load —
-// fetch and reload are driven by the owning Group.
-type View[T any] struct {
-	d *Dataset[T]
+func (g *Group) swapAll() error {
+	for _, m := range g.members {
+		if err := m.swap(); err != nil {
+			return err
+		}
+	}
+	return nil
 }
 
-// Load returns the current value. Returns nil before the Group is initialised.
-func (v *View[T]) Load() *T { return v.d.ptr.Load() }
-
-func (v *View[T]) reload() error { return v.d.reload() }
-
-// Add registers a new dataset in g and returns a View. Call Load to read the
-// current value. Drive updates by calling Init/Sync/Run on the Group.
-// load is a closure capturing whatever paths or config it needs.
-func Add[T any](g *Group, load func() (*T, error)) *View[T] {
-	v := &View[T]{d: &Dataset[T]{load: load}}
-	g.members = append(g.members, v)
-	return v
+// Sync calls the syncer and, if updated, reloads all member datasets.
+// Returns whether the source changed.
+func (g *Group) Sync() (bool, error) {
+	updated, err := g.syncer.Fetch()
+	if err != nil {
+		return false, err
+	}
+	if !updated {
+		return false, nil
+	}
+	return true, g.swapAll()
 }
 
-// Init fetches once then reloads all registered datasets.
+// Init syncs and always loads all datasets.
 func (g *Group) Init() error {
 	if _, err := g.syncer.Fetch(); err != nil {
 		return err
 	}
-	return g.reloadAll()
+	return g.swapAll()
 }
 
-// Sync fetches and reloads all datasets if the syncer reports an update.
-func (g *Group) Sync() (bool, error) {
-	updated, err := g.syncer.Fetch()
-	if err != nil || !updated {
-		return updated, err
-	}
-	return true, g.reloadAll()
-}
-
-// Run calls Sync on every interval until ctx is done.
+// Run calls Sync on every interval; reloads all datasets only when the source
+// reports a change.
 func (g *Group) Run(ctx context.Context, interval time.Duration) {
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()
@@ -203,11 +197,20 @@ func (g *Group) Run(ctx context.Context, interval time.Duration) {
 	}
 }
 
-func (g *Group) reloadAll() error {
-	for _, m := range g.members {
-		if err := m.reload(); err != nil {
-			return err
-		}
-	}
-	return nil
+// View is the read-only handle returned by Add. Sync is driven by the owning
+// Group.
+type View[T any] struct {
+	d *Dataset[T]
+}
+
+// Load returns the current value. Returns nil before the Group is initialised.
+func (v *View[T]) Load() *T { return v.d.ptr.Load() }
+
+func (v *View[T]) swap() error { return v.d.swap() }
+
+// Add registers a new dataset in g and returns a View for reading.
+func Add[T any](g *Group, load func() (*T, error)) *View[T] {
+	v := &View[T]{d: &Dataset[T]{load: load}}
+	g.members = append(g.members, v)
+	return v
 }