add multi delta enumeration to collection tree

tests coming in follow-up PR

src/internal/m365/collection/drive/collections_tree.go

@@ -277,8 +277,20 @@ func (c *Collections) populateTree(
 	ctx = clues.Add(ctx, "invalid_prev_delta", len(prevDeltaLink) == 0)
 
 	var (
+		currDeltaLink = prevDeltaLink
 		driveID       = ptr.Val(drv.GetId())
 		el            = errs.Local()
+		du            pagers.DeltaUpdate
+		finished      bool
+		hitLimit      bool
+	)
+
+	for !hitLimit && !finished && el.Failure() == nil {
+		var (
+			pageCount     int
+			pageItemCount int
+			hadReset      bool
+			err           error
 		)
 
 		// TODO(keepers): to end in a correct state, we'll eventually need to run this
@@ -286,23 +298,27 @@ func (c *Collections) populateTree(
 		pager := c.handler.EnumerateDriveItemsDelta(
 			ctx,
 			driveID,
-		prevDeltaLink,
+			currDeltaLink,
 			api.CallConfig{
 				Select: api.DefaultDriveItemProps(),
 			})
 
 		for page, reset, done := pager.NextPage(); !done; page, reset, done = pager.NextPage() {
 			if el.Failure() != nil {
-			break
+				return du, el.Failure()
 			}
 
 			if reset {
 				counter.Inc(count.PagerResets)
 				tree.reset()
 				c.resetStats()
+
+				pageCount = 0
+				pageItemCount = 0
+				hadReset = true
 			}
 
-		err := c.enumeratePageOfItems(
+			err = c.enumeratePageOfItems(
 				ctx,
 				tree,
 				drv,
@@ -312,6 +328,7 @@ func (c *Collections) populateTree(
 				errs)
 			if err != nil {
 				if errors.Is(err, errHitLimit) {
+					hitLimit = true
 					break
 				}
 
@@ -324,8 +341,13 @@ func (c *Collections) populateTree(
 			// at the end of the loop so we don't request another page (pager.NextPage)
 			// before seeing we've passed the limit.
 			if limiter.hitPageLimit(int(counter.Get(count.PagesEnumerated))) {
+				hitLimit = true
 				break
 			}
+
+			pageCount++
+
+			pageItemCount += len(page)
 		}
 
 		// Always cancel the pager so that even if we exit early from the loop above
@@ -333,11 +355,18 @@ func (c *Collections) populateTree(
 		// essentially a noop.
 		pager.Cancel()
 
-	du, err := pager.Results()
+		du, err = pager.Results()
 		if err != nil {
 			return du, clues.Stack(err)
 		}
 
+		currDeltaLink = du.URL
+
+		// 0 pages is never expected.  We should at least have one (empty) page to
+		// consume.  But checking pageCount == 1 is brittle in a non-helpful way.
+		finished = pageCount < 2 && pageItemCount == 0 && !hadReset
+	}
+
 	logger.Ctx(ctx).Infow("enumerated collection delta", "stats", counter.Values())
 
 	return du, el.Failure()