CBG-4218 [3.1.11 backport] Move duplicate checks for pending sequence…

…s to read side (#7105)

Reduces complexity and avoids loop bug when duplicates are present when running _addPendingLogs.

When popping an unused sequence range from pendingLogs, checks for overlap with the next entry in pending.  When found, truncates range at next pending entry.

Adds check in addPendingLogs for the case where the pending includes sequences earlier than nextSequence.

Info logging emitted for any unexpected duplication.

db/change_cache.go

@@ -16,7 +16,6 @@ import (
 	"errors"
 	"fmt"
 	"math"
-	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -126,6 +125,10 @@ func (entry *LogEntry) SetDeleted() {
 	entry.Flags |= channels.Deleted
 }
 
+func (entry *LogEntry) IsUnusedRange() bool {
+	return entry.DocID == "" && entry.EndSequence > 0
+}
+
 type LogEntries []*LogEntry
 
 // A priority-queue of LogEntries, kept ordered by increasing sequence #.
@@ -633,71 +636,23 @@ func (c *changeCache) processUnusedRange(ctx context.Context, fromSequence, toSe
 		// isn't possible under normal processing - unused sequence ranges will normally be moved
 		// from pending to skipped in their entirety, as it's the processing of the pending sequence
 		// *after* the range that triggers the range to be skipped.  A partial range in skipped means
-		// an duplicate entry with a sequence within the bounds of the range was previously present
+		// a duplicate entry with a sequence within the bounds of the range was previously present
 		// in pending.
-		base.WarnfCtx(ctx, "unused sequence range of #%d to %d contains duplicate sequences", fromSequence, toSequence)
+		base.WarnfCtx(ctx, "unused sequence range of #%d to %d contains duplicate sequences, will be ignored", fromSequence, toSequence)
 	}
 	return allChangedChannels
 }
 
-// _pushRangeToPending will push a sequence range to pending logs. If pending has entries in it, we will check if
-// those entries are in the range and handle it, so we don't push duplicate sequences to pending
+// _pushRangeToPending will push a sequence range to pendingLogs
 func (c *changeCache) _pushRangeToPending(ctx context.Context, startSeq, endSeq uint64, timeReceived time.Time) {
-	if c.pendingLogs.Len() == 0 {
-		// push whole range & return early to avoid duplicate checks
-		entry := &LogEntry{
-			TimeReceived: timeReceived,
-			Sequence:     startSeq,
-			EndSequence:  endSeq,
-		}
-		heap.Push(&c.pendingLogs, entry)
-		return
-	}
 
-	// check for duplicate sequences between range and pending logs
-	// loop till we have processed unused sequence range (or until we
-	// have range of sequences that aren't present in pending list)
-	for startSeq <= endSeq {
-		i, found := sort.Find(c.pendingLogs.Len(), func(i int) int {
-			value := c.pendingLogs[i]
-			if value.Sequence > endSeq {
-				// range is less than current pending entry
-				return -1
-			}
-			if startSeq <= value.Sequence && endSeq >= value.Sequence {
-				// found pending entry that has duplicate entry between itself and unused range
-				return 0
-			}
-			// range is larger then current element
-			return 1
-		})
-		if found {
-			// grab pending entry at that index and process unused range between startSeq and pending entry.Sequence - 1
-			pendingEntry := c.pendingLogs[i]
-			base.DebugfCtx(ctx, base.KeyCache, "Ignoring duplicate of #%d (unusedSequence)", pendingEntry.Sequence)
-			entry := &LogEntry{
-				TimeReceived: timeReceived,
-				Sequence:     startSeq,
-				EndSequence:  pendingEntry.Sequence - 1,
-			}
-			heap.Push(&c.pendingLogs, entry)
-			// update start seq on range
-			startSeq = pendingEntry.Sequence + 1
-		} else {
-			// if range not found in pending then break from loop early
-			break
-		}
+	entry := &LogEntry{
+		TimeReceived: timeReceived,
+		Sequence:     startSeq,
+		EndSequence:  endSeq,
 	}
+	heap.Push(&c.pendingLogs, entry)
 
-	// push what's left of seq range
-	if startSeq <= endSeq {
-		entry := &LogEntry{
-			TimeReceived: timeReceived,
-			Sequence:     startSeq,
-			EndSequence:  endSeq,
-		}
-		heap.Push(&c.pendingLogs, entry)
-	}
 }
 
 // Process unused sequence notification.  Extracts sequence from docID and sends to cache for buffering
@@ -885,16 +840,23 @@ func (c *changeCache) _addPendingLogs(ctx context.Context) channels.Set {
 		isNext = oldestPending.Sequence == c.nextSequence
 
 		if isNext {
-			heap.Pop(&c.pendingLogs)
+			oldestPending = c._popPendingLog(ctx)
 			changedChannels = changedChannels.UpdateWithSlice(c._addToCache(ctx, oldestPending))
+		} else if oldestPending.Sequence < c.nextSequence {
+			// oldest pending is lower than next sequence, should be ignored
+			base.InfofCtx(ctx, base.KeyCache, "Oldest entry in pending logs %v (%d, %d) is earlier than cache next sequence (%d), ignoring as sequence has already been cached", base.UD(oldestPending.DocID), oldestPending.Sequence, oldestPending.EndSequence, c.nextSequence)
+			oldestPending = c._popPendingLog(ctx)
+
+			// If the oldestPending was a range that extended past nextSequence, update nextSequence
+			if oldestPending.IsUnusedRange() && oldestPending.EndSequence >= c.nextSequence {
+				c.nextSequence = oldestPending.EndSequence + 1
+			}
 		} else if len(c.pendingLogs) > c.options.CachePendingSeqMaxNum || time.Since(c.pendingLogs[0].TimeReceived) >= c.options.CachePendingSeqMaxWait {
 			//  Skip all sequences up to the oldest Pending
 			c.PushSkipped(ctx, c.nextSequence, oldestPending.Sequence-1)
-			// disallow c.nextSequence decreasing
-			if c.nextSequence < oldestPending.Sequence {
-				c.nextSequence = oldestPending.Sequence
-			}
+			c.nextSequence = oldestPending.Sequence
 		} else {
+			// nextSequence is not in pending logs, and pending logs size/age doesn't trigger skipped sequences
 			break
 		}
 	}
@@ -905,6 +867,41 @@ func (c *changeCache) _addPendingLogs(ctx context.Context) channels.Set {
 	return changedChannels
 }
 
+// _popPendingLog pops the next pending LogEntry from the c.pendingLogs heap.  When the popped entry is an unused range,
+// performs a defensive check for duplicates with the next entry in pending.  If unused range overlaps with next entry,
+// reduces the unused range to stop at the next pending entry.
+func (c *changeCache) _popPendingLog(ctx context.Context) *LogEntry {
+	poppedEntry := heap.Pop(&c.pendingLogs).(*LogEntry)
+	// If it's not a range, no additional handling needed
+	if !poppedEntry.IsUnusedRange() {
+		return poppedEntry
+	}
+	// If there are no more pending logs, no additional handling needed
+	if len(c.pendingLogs) == 0 {
+		return poppedEntry
+	}
+
+	nextPendingEntry := c.pendingLogs[0]
+	// If popped entry range does not overlap with next pending entry, no additional handling needed
+	//  e.g. popped [15-20], nextPendingEntry is [25]
+	if poppedEntry.EndSequence < nextPendingEntry.Sequence {
+		return poppedEntry
+	}
+
+	// If nextPendingEntry's sequence duplicates the start of the unused range, ignored popped entry and return next entry instead
+	//   e.g. popped [15-20], nextPendingEntry is [15]
+	if poppedEntry.Sequence == nextPendingEntry.Sequence {
+		base.InfofCtx(ctx, base.KeyCache, "Unused sequence range in pendingLogs (%d, %d) has start equal to next pending sequence (%s, %d) - unused range will be ignored", poppedEntry.Sequence, poppedEntry.EndSequence, nextPendingEntry.DocID, nextPendingEntry.Sequence)
+		return c._popPendingLog(ctx)
+	}
+
+	// Otherwise, reduce the popped unused range to end before the next pending sequence
+	//  e.g. popped [15-20], nextPendingEntry is [18]
+	base.InfofCtx(ctx, base.KeyCache, "Unused sequence range in pendingLogs (%d, %d) overlaps with next pending sequence (%s, %d) - unused range will be truncated", poppedEntry.Sequence, poppedEntry.EndSequence, nextPendingEntry.DocID, nextPendingEntry.Sequence)
+	poppedEntry.EndSequence = nextPendingEntry.Sequence - 1
+	return poppedEntry
+}
+
 func (c *changeCache) GetStableSequence(docID string) SequenceID {
 	// Stable sequence is independent of docID in changeCache
 	return SequenceID{Seq: c.LastSequence()}