Track and print stats about merging (#3930)

Track and print information about the number
of directories that were moved, deleted,
recursively moved or deleted, and that
didn't move at all

This can help shed light on what's happening
and generally help with debugging issues
that may crop up

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [x] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #3929

#### Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/upload.go

@@ -28,6 +28,7 @@ import (
 	"github.com/alcionai/corso/src/internal/m365/graph"
 	"github.com/alcionai/corso/src/internal/m365/graph/metadata"
 	"github.com/alcionai/corso/src/pkg/backup/details"
+	"github.com/alcionai/corso/src/pkg/count"
 	"github.com/alcionai/corso/src/pkg/fault"
 	"github.com/alcionai/corso/src/pkg/logger"
 	"github.com/alcionai/corso/src/pkg/path"
@@ -852,6 +853,7 @@ func traverseBaseDir(
 	expectedDirPath *path.Builder,
 	dir fs.Directory,
 	roots map[string]*treeMap,
+	stats *count.Bus,
 ) error {
 	ctx = clues.Add(ctx,
 		"old_dir_path", oldDirPath,
@@ -889,9 +891,27 @@ func traverseBaseDir(
 		// This directory was deleted.
 		if upb == nil {
 			currentPath = nil
+
+			stats.Inc(statDel)
 		} else {
-			// This directory was moved/renamed and the new location is in upb.
+			// This directory was explicitly mentioned and the new (possibly
+			// unchanged) location is in upb.
 			currentPath = upb.ToBuilder()
+
+			if oldDirPath.String() == currentPath.String() {
+				stats.Inc(statNoMove)
+			} else {
+				stats.Inc(statMove)
+			}
+		}
+	} else {
+		// Just stats tracking stuff.
+		if currentPath == nil {
+			stats.Inc(statRecursiveDel)
+		} else if oldDirPath.String() == currentPath.String() {
+			stats.Inc(statNoMove)
+		} else {
+			stats.Inc(statRecursiveMove)
 		}
 	}
 
@@ -920,7 +940,8 @@ func traverseBaseDir(
 			oldDirPath,
 			currentPath,
 			dEntry,
-			roots)
+			roots,
+			stats)
 	})
 	if err != nil {
 		return clues.Wrap(err, "traversing base directory")
@@ -993,6 +1014,21 @@ func logBaseInfo(ctx context.Context, m ManifestEntry) {
 		"base_backup_id", mbID)
 }
 
+const (
+	// statNoMove denotes an directory that wasn't moved at all.
+	statNoMove = "directories_not_moved"
+	// statMove denotes an directory that was explicitly moved.
+	statMove = "directories_explicitly_moved"
+	// statRecursiveMove denotes an directory that moved because one or more or
+	// its ancestors moved and it wasn't explicitly mentioned.
+	statRecursiveMove = "directories_recursively_moved"
+	// statDel denotes a directory that was explicitly deleted.
+	statDel = "directories_explicitly_deleted"
+	// statRecursiveDel denotes a directory that was deleted because one or more
+	// of its ancestors was deleted and it wasn't explicitly mentioned.
+	statRecursiveDel = "directories_recursively_deleted"
+)
+
 func inflateBaseTree(
 	ctx context.Context,
 	loader snapshotLoader,
@@ -1058,10 +1094,13 @@ func inflateBaseTree(
 		// The prefix is the tenant/service/owner/category set, which remains
 		// otherwise unchecked in tree inflation below this point.
 		newSubtreePath := subtreePath.ToBuilder()
+
 		if p, ok := updatedPaths[subtreePath.String()]; ok {
 			newSubtreePath = p.ToBuilder()
 		}
 
+		stats := count.New()
+
 		if err = traverseBaseDir(
 			ictx,
 			0,
@@ -1070,9 +1109,18 @@ func inflateBaseTree(
 			newSubtreePath.Dir(),
 			subtreeDir,
 			roots,
+			stats,
 		); err != nil {
 			return clues.Wrap(err, "traversing base snapshot").WithClues(ictx)
 		}
+
+		logger.Ctx(ctx).Infow(
+			"merge subtree stats",
+			statNoMove, stats.Get(statNoMove),
+			statMove, stats.Get(statMove),
+			statRecursiveMove, stats.Get(statRecursiveMove),
+			statDel, stats.Get(statDel),
+			statRecursiveDel, stats.Get(statRecursiveDel))
 	}
 
 	return nil