core/state/snapshot: simplify snapshot rebuild (#30772)

This PR is purely for improved readability; I was doing work involving
the file and think this may help others who are trying to understand
what's going on.

1. `snapshot.Tree.Rebuild()` now returns a function that blocks until
regeneration is complete, allowing `Tree.waitBuild()` to be removed
entirely as all it did was search for the `done` channel behind this new
function.
2. Its usage inside `New()` is also simplified by (a) only waiting if
`!AsyncBuild`; and (b) avoiding the double negative of `if !NoBuild`.

---------

Co-authored-by: Martin HS <martin@swende.se>

core/state/snapshot/snapshot.go

@@ -206,47 +206,24 @@ func New(config Config, diskdb ethdb.KeyValueStore, triedb *triedb.Database, roo
 		log.Warn("Snapshot maintenance disabled (syncing)")
 		return snap, nil
 	}
-	// Create the building waiter iff the background generation is allowed
-	if !config.NoBuild && !config.AsyncBuild {
-		defer snap.waitBuild()
-	}
 	if err != nil {
 		log.Warn("Failed to load snapshot", "err", err)
-		if !config.NoBuild {
+		if config.NoBuild {
-			snap.Rebuild(root)
+			return nil, err
-			return snap, nil
 		}
-		return nil, err // Bail out the error, don't rebuild automatically.
+		wait := snap.Rebuild(root)
+		if !config.AsyncBuild {
+			wait()
+		}
+		return snap, nil
 	}
 	// Existing snapshot loaded, seed all the layers
-	for head != nil {
+	for ; head != nil; head = head.Parent() {
 		snap.layers[head.Root()] = head
-		head = head.Parent()
 	}
 	return snap, nil
 }
 
-// waitBuild blocks until the snapshot finishes rebuilding. This method is meant
-// to be used by tests to ensure we're testing what we believe we are.
-func (t *Tree) waitBuild() {
-	// Find the rebuild termination channel
-	var done chan struct{}
-
-	t.lock.RLock()
-	for _, layer := range t.layers {
-		if layer, ok := layer.(*diskLayer); ok {
-			done = layer.genPending
-			break
-		}
-	}
-	t.lock.RUnlock()
-
-	// Wait until the snapshot is generated
-	if done != nil {
-		<-done
-	}
-}
-
 // Disable interrupts any pending snapshot generator, deletes all the snapshot
 // layers in memory and marks snapshots disabled globally. In order to resume
 // the snapshot functionality, the caller must invoke Rebuild.
@@ -688,8 +665,9 @@ func (t *Tree) Journal(root common.Hash) (common.Hash, error) {
 
 // Rebuild wipes all available snapshot data from the persistent database and
 // discard all caches and diff layers. Afterwards, it starts a new snapshot
-// generator with the given root hash.
+// generator with the given root hash. The returned function blocks until
-func (t *Tree) Rebuild(root common.Hash) {
+// regeneration is complete.
+func (t *Tree) Rebuild(root common.Hash) (wait func()) {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
@@ -721,9 +699,11 @@ func (t *Tree) Rebuild(root common.Hash) {
 	// Start generating a new snapshot from scratch on a background thread. The
 	// generator will run a wiper first if there's not one running right now.
 	log.Info("Rebuilding state snapshot")
+	disk := generateSnapshot(t.diskdb, t.triedb, t.config.CacheSize, root)
 	t.layers = map[common.Hash]snapshot{
-		root: generateSnapshot(t.diskdb, t.triedb, t.config.CacheSize, root),
+		root: disk,
 	}
+	return func() { <-disk.genPending }
 }
 
 // AccountIterator creates a new account iterator for the specified root hash and